Dec 6, 1982 - I is modified from his book and shows the variability found so far, .... Ewart compared the metacarpals of the Io-15-mm CR embryo, where they are of ...... (b) in the number of tarsal bones (two free bones in Archaeopteryx, four ...
89
Biol. Rev. (1984), 59, pp. 89-124 Printed in Great Britain
DEVELOPMENTAL MECHANISMS UNDERLYING T H E FORMATION OF ATAVISMS BY BRIAN K. HALL
Department of Biology, Life Sciences Centre, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J1 (Received 6 December 1982, accepted 27 August 1983) CONTENTS
I. Introduction . . . . . . . . . . . . . 11. Spontaneous atavisms in natural populations . . . . . . . ( I ) Limbs of vertebrates . . . . . . . . . . . ( a ) Hind limbs in whales . . . . . . . . . . (b) T h e wingless mutant . . . . . . . . . . (c) Limbless vertebrates . . . . . . . . . . (2) Atavistic digits in modern horses . . . . . . . . (3) Atavistic muscles . . . . . . . . . . . ( a ) Birds . . . . . . . . . . . . . (b) M a m m a l s . . . . . . . . . . . . (4) Miscellaneous other atavisms . . . . . . . . . 111. The genetic basis of atavistic characters . . . . . . . . ( I ) Extra toes in guinea pigs . . . . . . . . . . (2) Atavistic dew claws in dogs . . . . . . . . . IV. The experimental production of atavisms . . . . . . . ( I ) Re-establishment of ancestral patterns in the hind limbs of embryonic chicks (2) Enamel from avian ectoderm . . . . . . . . . (3) Balancers, teeth and gills in amphibians . . . . . . . (4) Bristle pattern in Drosophila . . . . . . . . . V. Summary. . . . . . . . . . . . . . V1. Acknowledgements . . . . . . . . . . . . V l i . References . . . . . . . . . . . . . VIII. Addendum . . . . . . . . . . . . .
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I. INTRODUCTION
Atavisms (from the Latin atavus, French atavisme) are the reappearance of a’lost character (morphology or behaviour) typical of remote ancestors and not seen in the parents or recent ancestors of the organisms displaying the atavistic character. T h e phrases “tendency to reproduce the ancestral type”, “ reyersion to a previous evolutionary state”, or “throwback” all embody the essential features of the atavistic character, which are: ( I ) its persistence into adult life; ( 2 ) its absence in the parents or recent ancestors; (3) its presence in only one or a few individuals within a population, and (4) its close resemblance to (identity with?) the same character possessed by all members of an ancestral population. That atavisms resemble an ancestral character implies that they can only be recognized in an organism for which we have a good fossil record and detailed knowledge of morphological changes during evolution. Coupled with the fact that deviations from the norm will only be recognized if they are obvious (which usually means external)
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and observed by someone knowing the evolutionary history of the group, the frequency of occurrence of atavisms is almost certainly underestimated. When an atavism clearly resembles a character known to have once been possessed by all members of the animals’ ancestors (e.g. hind limbs in whales, additional digits in horses and guinea pigs), its presence in occasional recent individuals tells us that the genetic potential required to produce the character has persisted in a form capable of being expressed for very long periods of geological time, but can such characters tell us anything about the genetic basis for morphological change during evolution? I shall examine atavisms from the viewpoint of the developmental biologist in an attempt to show that, rather than being an embarrassment to the evolutionary biologist, they illustrate the hidden potential for morphological change that all organisms possess. My approach will be ( a ) to describe a variety of atavisms - both those that arise naturally and those that can be experimentally induced, ( 6 ) to discuss selection experiments which show the genetic basis underlying atavistic characters, and (c) to examine the developmental processes underlying both the acquisition of atavisms and the loss of characters, in order to show that atavisms have a ready explanation in terms of the preservation of developmental pathways for relatively long periods during evolution. Features common to the formation of divergent atavisms will be emphasized. To this end I will consider atavisms under three headings. Firstly, there are those atavisms that arise spontaneously in members of populations. Examples will be hind limbs in whales, extra digits in horses and atavistic muscles. Such atavisms tell us that they can and do occur in Nature. The second are breeding experiments with individuals that possess such a spontaneously arising atavistic character. Examples will be extra toes in guinea pigs and dew claws in dogs. Such breeding experiments tell us about the polygenic basis underlying atavisms and how they can rapidly spread from a single individual to the whole population. Thirdly, there is the ‘experimental production ’ of atavisms by manipulation of embryos. Examples will be ancestral limb patterns in embryonic chicks, enamel in avian ectoderm, balancers, teeth and gills in frog tadpoles and bristle pattern in Drosophila. T h e ability to induce atavisms tells us that they need not arise as the result of gene mutations but that epigenetic events such as timing of development, tissue interactions and/or growth and morphogenesis can activate previously quiescent portions of the genome. Throughout, I shall attempt to emphasize the developmental mechanisms that underlie the formation of atavisms by discussing either their development, if it is known, or the development of analogous structures from other species or groups. I I . SPONTANEOUS ATAVISMS IN NATURAL POPULATIONS
( I ) Limbs of vertebrates
( a ) Hind limbs in whales Although to the layman a whale is a whale is a whale, the two major groups, the toothed whales (Odontoceti) and baleen whales (Mysticeti) differ in so many features that they must either have diverged very early in their evolutionary history or have arisen independently from quadrupeds. In either case the Order Cetacea represents a diphyletic group (van Valen, 1968; Howell, 1970). Linnaeus (1758) recognized these differences when he subdivided the whales into toothed and baleen (they had previously been recognized as mammals by John Ray in 1693). Among the major differences
Atavisms
91
between the two sub-orders are the presence of teeth and absence of baleen in the toothed whales, and absence of teeth but presence of baleen in the baleen whales. Both groups contain pelvic bones (see below). No toothed whale except the sperm whale Physeter catodon has any remnant of a hind limb, while most baleen whales have a vestigial femur, either as a cartilage as in the humpback whale Megaptera novaeangliae, or as a bony element as in the blue whale Balaenoptera musculus. Fully-limbed ancestors of the modern-day whales moved from land to water around the lower Tertiary or upper Cretaceous (65-75 million years ago). In the middle Eocene, Protocetus atavus (no doubt named with atavisms in mind!), which van Valen (1968) considers could have been ancestral to both of the recent suborders, had both a pelvic girdle attached to the sacral vertebrae and the hind limbs. By the end of the Eocene the toothed whales had lost their hind limbs altogether. Most modern whales have small pelvic bones unconnected to the vertebral column. These consist of a single, horizontal bone on each side, and can be quite large, up to 42 cm long. Pelvic bones show sexual dimorphism, being twice as large in males as in females of the same size (Arvy, 1979). They are joined together by a ligament but never by a symphysis (the type of joint that unites pelvic bones in most other mammals), lack acetabular cavities, have but one ossification centre each, continue to grow throughout life and are often asymmetrical in both size and shape (Arvy, 1976, 1979).Whether these bones represent ilium, pubis or ischium (the three elements of the pelvic girdle) is uncertain. In fact, Arvy claims that they are not pelvic bones at all. They are deep seated in muscle, always separated from the vertebral column, lack a symphysis and are moved, not by segmental vertebral muscles as are pelvic bones in other vertebrates, but by cutaneous muscles which support the urogenital orifice, penis, clitoris, etc. They play a role both in mating and in birth (Berzin, 1972). Arvy (1976) concludes that their “function is to support the abdominal wall and the abdominal viscera” (p. 183), and regards them as abdominal bones, analagous to the epipubic bones of marsupials, which articulate with the pubic region of the pelvis. He argues (1979) that, because they are not seen in X-rays of late embryos, they cannot be pelvic bones (pelvic bones would arise early in embryonic development), but then goes on to affirm that they usually remain cartilaginous for a long time after birth. Their absence from X-rays of fetuses may reflect their composition as unmineralized cartilages rather than their origin after birth as neomorphs. However, their total separation from the vertebral column, activation by cutaneous muscles and function in supporting reproductive structures makes them functionally akin to penile and clitoral bones (Beresford, 1981). The forelimbs of all whales have been extensively modified as flippers. Humerus, radius and ulna are reduced and the size of the ‘hand’ increased - primarily by an increase in the number of distal phalanges (hyperphalangy). Thus, in the pilot whale Globicephala melaena, the second digit has 1 4 phalanges and the third has eleven. Whale embryos 20-30 mm in length possess well developed external hind limb-buds. In an interesting parallel, the embryos of baleen whales possess a full set of teeth which are resorbed as development proceeds. Hind limb-buds are found in later embryonic stages of baleen whales than in toothed whales, further indicating that toothed whale hind limbs lost their function earlier during evolution than did baleen whales. This variation in limb-bud development is reflected in the amount of limb skeleton produced in species belonging to the two groups. As already indicated, only the sperm whale among toothed whales retains any
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rudiment of this hind limb-bud. This consists of a femur, either separate from the pelvic bones or connected to them by a ligament (Howell, 1970; Berzin, 1972). Most baleen whales have a rudimentary cartilaginous or bony femur, again connected by ligaments to the pelvic bones. T h e most advanced hind limb rudiments are found in the right whale Eubalaena glacialis, which has a bony femur from 4 to 9 inches in length and a small cartilaginous tibia. T h e integrated nature of the development of these bones is illustrated by the presence of synovial membranes and ligaments between femur and pelvic bones and femur and tibia. Similar skeletal elements are seen in the bowhead whale Balaena mysticetus. Some remarkable examples of atavistic hind limbs are known, but only from species that possess rudimentary hind limb skeletons. Andrews (1921) described an adult, female humpback whale Megaptera novaeangliae, caught on the west coast of Vancouver Island, B.C., in July 1919. It had symmetrical left and right external hind limbs measuring 12.7 cm in length. Each contained four skeletal elements - two bones and two cartilages. T h e longest cartilage was the femur, a 3.8-cm rod lying within the body. A 3.6-cm bony tibia, a 1.2-cm cartilaginous tarsus and a 1.5-cm bony metatarsal were also present. Andrews quotes Kukenthal & Goldberg as describing hind limb-buds in embryonic Megaptera novaeangliae, Phocoena phocoena (common porpoise), Phocoenoides dalli (Dall’s porpoise) and Lagenorhynchus acutus (white-sided dolphin). Berzin (1972) reported a number of similar cases in both male and female sperm whales Physter catodon. One, a female captured near Japan, had deep-set hind limbs, each some 15 cm in diameter at the body surface and protruding 5-6 cm from it. These limbs contained both a femur and a tibia. Another specimen (see his Figure 37), an I 1.6-m male, had hind limbs that were 28 and 34 cm long, each containing a femur, tibia, fibula, 4th and 5th digits but no tarsals. All grades from limbs with single proximal elements to those complete with digits have therefore been reported. Berzin quotes Nemoto (1963) as reporting an incidence of atavistic hind limbs of 1/5ooo sperm whales, and notes that this is within the range of calculated mutation rates (obviously thinking of a single gene mutation as the mechanism for their origin). The sperm whale, like the humpback whale, has two pelvic bones. Yablokov ( I 974) describes a specimen with a rudimentary femur and cites six cases of hind limbs in adult sperm whales. Fig. I is modified from his book and shows the variability found so far, ranging from the rudimentary femur to limbs with a metatarsal and phalanges arranged into either one or two digits. (Recall that the forelimb has two elongated digits.) Embryos of all whale species possess limb buds. Rudimentary upper hind limb bones such as femur and tibia are normally seen in only a few species, such as sperm whale and humpback whale, and these are the species in which atavistic hind limbs form, for to be able to develop an atavistic character a reasonably advanced primordium of that character must normally form. T h e mechanism involved must determine continued development of the pre-existing limb bud. Differential rates of growth of a single skeletal element, which can account for atavistic digits in horses and guinea pigs (see Sections II(2) and I I I ( I ) ) , cannot account for the persistence of a whole limb, especially when that limb possesses skeletal elements not normally found in the species. Unfortunately, we lack the range of whale embryos required to study their hind limb development directly. We know that hind limbs are present in embryos up to approximately 30 mm in length and that they disappear before any cartilaginous elements arise. How they
Atavisms
I
fy
t
93 D.b.
ta
meta
Fig. I . A summary of the variability in expression of development of the hind limbs in the sperm whale Physeter catodon. I. The normal situation with a single pelvic bone in each limb bud. 11. A rudimentary femur is also present in this internally developed bud. 111. Tibia and femur are both present and limb is visible as an external swelling. IV. Tarsals are also present in this variant, organized into a single rudimentary digit. V. Tarsals organized into two rudimentary digits. V1. Metatarsals are also present in this type, which is the most complete hind-limb yet found in the species. f, femur; fi, fibula; t, tibia; ta, tarsals; meta, metatarsals; p.b., pelvic bone. (Adapted from Yablokov, 1974.)
disappear we do not know- by an active cellular process within the limb buds themselves, e.g. reduced mitotic activity, or cell death, well documented processes in vertebrate limb development (see Hinchliffe, 1982),or by increased growth in the surrounding flank leaving the limb buds behind as the body continues to grow. Even though embryonic whales are not available for study we can examine other examples of limblessness in vertebrates for mechanisms that prevent limb formation. Two well studied examples are the wingless mutant in the embryonic chick and naturally limbless vertebrates such as legless lizards and snakes.
( b ) The wingless mutant Wingless (ws) chicks form limb buds that subsequently fail to develop into wings. Two different mutants have been studied - a homozygous recessive and a sex-linked mutant. Zwilling (1949)used the homozygous recessive mutant in his classic study establishing the interactive role of the apical ectodermal ridge (AER) and the underlying limb mesoderm in controlling limb outgrowth and proximodistal patterning of the limb
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skeleton. Wingless embryos develop wing buds complete with AERs, but the AER regresses. Experimental confirmation comes from recombinations between tissues derived from wingless and wild-type embryos (Zwilling, 1956, 1974; Zwilling & Hansborough, 1956). Mesoderm from the wing bud of wingless embryos cannot maintain an AER in ectoderm derived from wild-type embryos for more than 2-3 days (normally the AER persists for 5 days). T h e mechanism of regression of the limb bud differs between the two mutants. In the homozygous recessive mutant studied by Zwilling the basal lamina produced by the AER is defective. It is discontinuous, often lacks a lamina lucida and the underlying mesenchymal cells are in much closer contact with it than is the case for normal limb buds (Sawyer, 1982). In the sex-linked mutant, precocious cell death of limb mesoderm removes the cells that produce a factor, the so-called apical ectodermal maintenance factor, required to maintain the AER (Hinchliffe Ede, 1973). ws mesoderm has not completely lost the potential for forming skeletal structures, for more skeletal elements form in recombined ws mesoderm/ ectoderm than form in wingless limb buds. What these limb buds lack is the ability to survive for long enough to form skeletal elements. Similar mechanisms, which allow increased limb growth and development of the skeletal elements, could explain the way in which hind limbs develop in whales.
++
( c ) Limbless vertebrates
Vertebrates that are normally limbless as adults include amphibians (Apoda, Caecilia) and reptiles (Squamata, Amphisbaenia). Some limbless groups such as caecilians never form limb buds at all. Most snakes lack limb buds but some form hind limb-buds and may even possess skeletal elements in those limb buds. Others, notably many limbless lizards, develop limb buds early in embryonic life. These limb buds are capped by apical ectodermal ridges, begin to grow but then regress - at about the same developmental stage as regression sets in within wing buds of ws chicks. As with ws, regression is a consequence of extensive cell death, but, unlike ws, it is not the extension of a normally occurring area of cell death that prevents further limb development but rather the death of other cells within the limb buds. This cell death begins in somitic cells which previously migrated into the limb buds (such cells form the muscle in limb buds that continue to develop), then spread to the AER and finally to the other (pre-skeletal and connective tissue) mesenchyme of the limb bud. T h e result is either complete absence of the limb skeleton, as in the slow-worm Anguisfragilis, or development of a very rudimentary skeleton (often just a rudimentary femur) as in the legless lizard Ophisaurus apodus, the python Python reticulatus, or the skinks Scelotes brevipes and S . gronopii (Raynaud, 1977). Why this cell death starts is not known. Raynaud (1972) has proposed that limb growth would continue if more somitic cells migrated into the limb buds, basing this conclusion on the observation that eight somites send processes into the limb buds of limbed lizards, while only four send processes into the limb buds of the slow-worm. No processes arise from somites adjacent to where forelimbs should develop in snakes. Too few somatic cells migrating into the limb bud or deficient maintenance of those cells that do enter the bud could explain regression of the hind limb-bud in snakes and whales. Reversal of either process could explain the occasional appearance of atavistic
Atavisms Table
I.
95
Major genera in the evolution of the modern horse with an indication of digital reduction No. of digits in Genus
Geological period
Eohippus Orohippus Epihippus Mesohippus Miohippus Parahippus Merychippus Pliohippus Equus
Lower Eocene Middle Eocene Upper Eocene Lower Oligocene Middle-upper Oligocene Lower Miocene Upper Miocene Pliocene Pleistocene-Recent
Forelimb Hindlimb
3* 3* 3* 3t
3* 3* 3* 3* 3. 3* 3t
I t
It
11
I t
4* 4* 4'
*
Weight of body borne on central weight-bearing pad; all three digits functional. Weight of body borne on hoofed toe of digit three; only digit three functional; digits two and four reduced. f Weight of body borne on hoofed toe of digit three; digits two and four vestigial.
t
hind limbs. No additional special genetic, developmental or evolutionary mechanism has to be invoked. (2) Atavistic digits in modern horses The lineage of ancestral forms leading to the modern horse Equus is one of the most complete, closely studied and highly popularized examples of morphological change during evolution (Simpson, 195I ) . Hyracotherium (Eohippus) of the lower Eocene had three functional toes on its hindfeet and four on the forelegs. In a fossil sequence through Orohippus (middle Eocene), Mesohippus (lower Oligocene), Merychippus (upper Miocene), Pliohippus (Pliocene) to Equus (Pleistocene to Recent) we can trace a progressive lengthening of the third toe and a concomitant reduction in the other digits (Table I ) . The lengthening of the third digit involved all of its elements- the metacarpals or metatarsals and hoof all elongated, the phalanges were shortened and the number of tarsal elements progressively reduced. These skeletal changes did not occur in isolation. They were accompanied by changes in the ulna (reduction and fusion to the radius), fibula (reduction to a splint -the pastern bone - attached to the tibia) and by modifications in the tendons, ligaments, muscles and joints, as the springing gait of the modern horse developed. As the third digit was elongating, digits 2 and 4 were undergoing reduction in size. I n Equus they are represented by inconspicuous, nonfunctioning splint bones carried high above the single hoof on either side of digit 3. Consequently the modern horse is functionally one-toed. This reduction in digits 2 and 4 can also be documented from the fossil record. T h e upper Miocene genus Merychippus was three-toed, but the 2nd and 4th digits were reduced in size, the weight of the body being borne on the hoofed toe of digit 3 (Table I ) . Although digits 2 and 4 are reduced to splint bones in adult horses - where these digits are shorter than the metacarpal of digit 3, differential growth of digit 3 having allowed it to form the single, functional hoof - they are well represented in the early embryo. Limb buds first appear in embryos of 5-6 mm CR length (24 days after the end of oestrous) and are well differentiated at 35-50 mm CR length ( 5 0 days after oestrous). Ewart (1894a, b) gives a detailed account of the development of the forelimbs of
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embryos over the size range 10-50 mm CR length. He describes the ro-mm embryos as having three metacarpals of equal length, but with the proximal half of metacarpals 2 and 4 both shorter and thinner than metacarpal 3, the latter having a very wide and long first phalange. At the 20-mm CR stage, metacarpal 3 is less than twice as long as it is wide. In the adult it is five times as long as it is wide. Thus, early in development, metacarpal 3 of the future single hoof can only be differentiated from metacarpals 2 and 4, the future vestigial splint bones, on the basis of its wide cartilaginous rudiment. Ewart compared the metacarpals of the Io-15-mm CR embryo, where they are of equal length, to the adult Pliocene Hipparion. However he could not equate the 20-mm stage either to the adult or to any ancestor, concluding that “This is doubtless due to abbreviation in development’’ (p. 241). He specifically looked for evidence of recapitulation, found none and concluded that any similarity to ancestral forms showed, not recapitulation, but that they were ‘genetically related’ (p. 35 I ) , an interpretation well ahead of many of his contemporaries. Ewart found a cartilaginous nodule at the distal end of metacarpal 2 in a 25-mm CR embryo. This cartilage, and similar cartilages in a 35-mm embryo (p. 346) convinced Ewart that digits 2 and 4, but especially 2, possessed cartilaginous digits, even though the adult lacked digits on the splint bones. These cartilages fused to one another and to the distal end of the metacarpals a little later in development, although Rensch (1959, p. 186) claims that small toes may be present on digits 2 and 4 at birth. Thus the modern-day horse possesses three embryonic digits, but only one, the third, persists, developing by differential growth to form the single functional hoof, while the other two are reduced to vestigial splint bones. Having said that, horses with more than one functioning digit have been known for millenia. Julius Caesar evidently possessed and greatly valued a horse “the hoofs being cleft like toes” (Gould, 1 9 8 0 ~ )Marsh . ( I 892) documents a large number of what he called polydactylous horses. Rensch (1959, Fig. 25) and Jones & Bogart ( I 973, Fig. 9.20)illustrate an individual with a well-developed second digit complete with hoof. This digit is clearly considerably shorter than the normal digit 3 and is presumably not functional. However, many of the cases described by Marsh may well have been functional. One animal is described as having two additional digits on each hindleg and one additional digit on each foreleg. These atavistic digits are accompanied by associated changes in the limbs affecting metacarpals, metatarsals, carpals, tarsals, ligaments, tendons and muscles. The third digit is often somewhat shorter than that found in a one-toed horse and results from reduced growth of the third metatarsal/metacarpal, a tendency that would enhance the likelihood of the extra digits making contact with the ground and being functional. This compensatory reduced growth of the third digit in animals with elongated 2nd and 4th digits highlights the integrative nature of the development of the limbs in such animals. Many of these specimens resemble to a striking degree, both in skeletal and in associate structures, the hindlimbs of the lower Oligocene Mesohippus or the upper Miocene Mery chippus. Not all of these supernumerary digits are atavistic. Some arise from a subdivision of the rudiment of the 3rd digit as noted by Gegenbaur as early as 1880 (cited by de Beer, 1958, p. I~o), thus: ‘‘a horse may have a sub-divided third digit, which is not at all the same thing as having a third and fourth digit. ” In such individuals, digits 2 and 4 remain as vestigial splint bones. (These are really the only types that are polydactylous, for they
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in fact have 4 digits.) In other individuals, the digits have clearly arisen from preexisting digits 2 and 4 (see e.g. Fig. 25 in Rensch, 1959) and are atavistic. An important caution then is that knowledge of developmental history, either from a longitudinal study of ontogeny or from a study of associated structures (nerves, muscles, etc) is crucial to the interpretation of a structure as atavistic. How do such atavistic digits arise? Clearly, although subdivision of a digital primordium can produce polydactyly, we cannot invoke subdivision to explain the development of atavistic digits. de Beer (1958) invoked accelerated growth as the mechanism. . . “ for some unknown reason the rudiments of the extra digits grow large instead of being reduced to vestiges” (p. 120). Rensch (1959, p. 125) started with the premise that the genetic potential for forming three toes is ‘fixed’ in horses (they develop them as embryos and retain them as splint bones as adults). He supposed that the growth of the 3rd digit was accelerated by a gene that did not act on digits 2 and 4. Competition for what Rensch called ‘enough organic material ’ would slow the growth of the lateral digits. Atavistic digits would therefore be explained by mutations in the gene that normally accelerates growth of the 3rd digit, for a mutation that slowed growth of digit 3 would allow digits 2 and 4 to continue to develop. As support, he cites the fact that the 3rd metatarsal or metacarpal is shortened in such atavistic animals. Certainly, developmental processes involving the generation of a new skeletal element do not have to be invoked. Differential growth of the preexisting three digitial rudiments is sufficient to explain formation of atavistic digits. I will return to the notion of competition for material between adjacent skeletal elements and the necessity for mutations to alter growth of one element with respect to its neighbour when discussing experimental manipulations of limb development in embryonic chicks (see Section IV(I)). When only a single extra digit develops it is usually digit 2. Ewart ( I 8943) observed cartilaginous phalanges on digit 2 in early embryos. Along with several other observations, this can be used to explain why digit 2 is more likely than digit 4 to develop atavistically. T h e other observations are: (a)presence of cartilaginous phalanges on digit 2 at the 25-35-mm CR stage; (3) metacarpal 2 larger in cross-section than metacarpal 4 at the 25-mm stage, although both are of equal lengths and widths at earlier stages; ( c ) ossification of metacarpal 2 has proceeded further than has metacarpal 4 at the So-mm stage, and (d) digit 2 was retained for longer than digit 4 during the evolution of the horse. Therefore, not only does digit 3 outgrow digits 2 and 4, but digit 2 grows faster than digit 4. If digit 3 normally dominates because of its accelerated growth, it seems reasonable that digit 2, having a start on digit 4, would be more likely to persist as an atavistic digit. (3) Ataoistic muscles There are a number of examples of atavistic muscles in birds, mammals, primates and man. Some of these occur in situations where the developmental history is well known, and so shed light on the developmental mechanisms underlying the atavistic muscle.
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B. K . HALL
( a ) Birds Perhaps the best examples come from the studies of Raikow (1975)and Raikow, Borecky & Berman (1979)on the presence of atavistic thigh muscles in a number of species of passerine birds. Included in the diagnostic characters for the Order Passeriformes is absence of two muscles from the hip. These two muscles, which are present in many other avian orders, are ( I ) the Musculus iliofemoralis externus ( M . gluteus medius et minimus, M . piriformis, muscle D in Garrod’s classification), a muscle that is present in many non-passerine orders (ducks, falcons, galliformes) but absent from kingfishers, hornbills (Coraciiformes), trogons (Trogoniformes), toucans and woodpeckers (Piciformes), and cuckoos (Cuculiformes), groups close to the ancestry of the passerines, and (2)M. caudiliofemoralis pars iliofemoralis ( M . piriformis pars iliofemoralis) a muscle that is homologous to the coccygeofemoralis muscle complex of reptiles (Romer, I 927)and which has been subject to much variation during reduction of the tail in evolution (e.g. it is absent from some, but present in other, genera of Cuculidae: George & Berger, 1966). Musculus iliofemoralis externus (M,i.e.)is a small muscle of the thigh. Raikow (1975) found an M.i.e. in the left hind limb of one out of seven specimens of common myna Acridotheses tristis (Sturnidae) from Hawaii (he was not able to examine the right hind limb in this specimen so that we do not know whether the muscle was present on both sides or not - atavistic muscles are often present on only one side - see below). He also found an M.i.e. on one side in a Rothschild’s myna (Leucopsar rothschildi; Sturnidae). M.i.e. is normally absent from genera in this family. Raikow et al. (1979)found the same muscle in several Australian passerines, viz. bowerbirds such as Chlamydera nuchalis (Ptilonorhynchidae), two species of sickle-billed birds-of-paradise (Epimachus spp. ; Paradisaeidae), Loria’s bird-of-paradise Loria loriae (Paradisaeidae), and the New Zealand thrush Turnugra capensis (Turnagridae). T h e atavistic muscle’s origin on the iliac crest, insertion on the femur, structure, size and position were all comparable to the same features in those species that possess the muscle. In the left hind limb of one out of three specimens of fox sparrow (Passerella iliaca; Fringillidae) from USA, Raikow (I 975)found a M . caudiliofemoralis pars iliofemoralis (M.c.p.i.). Its origin on the posterior iliac crest of the ilium and insertion onto the posterolateral surface of the femur was typical of the location of this muscle in other Orders. However, the atavistic muscle was narower than that seen in those Orders. An M.c.p.i. was also found on the left side of one specimen of the white-breasted woodswallow (Artamus leucorhynchus: Artamidae) by Raikow et al. (I 979). One hind limb of one of two specimens of Aechmorhynchus cancellatus (Scolopacidae), a rare shorebird of the Tuamotos Archipelago in south-central Pacific Ocean, has also been reported as having an atavistic M.c.p.i. (Zusi dz Jehl, 1970). Again its insertion, origin and size allowed it to be clearly identified as homologous to M.c.p.i. in other species. A plausible mechanism for the reappearance of these muscles can be proposed by examining their development in species, all of whose members have the muscle. Raikow (1975)has provided such a mechanism based on Romer’s (1927)description of the development of the musculature in the thigh of the domestic fowl. I have summarized three patterns in Fig. 2. This group of thigh muscles originates in a single muscle mass dorsal to the
Atavisms
dorsal muscle mass -deep
muscle
99
(61
(C)
superficial muscle mass
superficial muscle mass
D
n
1
4
n
Fig. 2. A mechanism for the atavistic development of M . iliofemoralis externus (A4.i.e.) I . The pattern seen in species that normally have M.i.e. or in which it develops atavistically. The dorsal muscle mass in the thigh ( a ) subdivides into a superficial and a deep muscle layer (b). The latter further subdivides into the iliotrochantericus (i.l.t.)and M.i.e. ( c ) around the head of the femur (F).2. The pattern in species that normally lack M.i.e., if its absence is based on failure of the deep muscle mass, to split into two. 3. The pattern in species that normally lack M.i.e., if its absence is based on progressive reduction of the size of M.i.e. in succeeding generations. Pattern a would produce an instantaneous loss of the muscle in one generation (G). Pattern 3 would lead to smaller muscle size in the first generation (GI), then to presence of M.i.e. as a vestigial muscle in subsequent generations (Gn-I), and finally to its loss (G.. .n). See text for further details.
developing femur. This mass subdivides into a superficial and deep muscle layer, with the deep layer encompassing the proximal head of the femur (Fig. 2 , I (b)). This deep muscle layer further subdivides into the iliotrochantericus muscles and the M.i.e.in those species that normally possess an M.i.e. (ducks, falcons, galliformes, etc - see George & Berger (1966) p. 393 for a complete list). Absence of M.i.e. or of M.c.p.i. could come about by either (a) failure of the deep muscle mass to split into two during its development, producing an 'instantaneous' loss of the muscle (Fig. 2 , 2 ) , or (b) formation of M.i.e. or M.c.p.i. but gradual reduction of their size by reduced growth in successive generations, producing a gradual loss of the muscle(s) as they first became 4-2
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B. K. HALL
small, then vestigial and finally absent (Fig. 2,3). Raikow favoured the first mechanism as the best explanation for the reappearance of M.i.e. His argument was that, when present atavistically, it was the same size as the homologous muscle in individuals of species that have it as a constitutive (diagnostic) character. He saw its reappearance as based on the subdivision of the deep muscle mass as normally occurs in other species (Fig. 2 , I ) . Alternative (6) was favoured when explaining the presence of an atavistic M.c.p.i.Because the atavistic muscle was small, he argued that its loss during evolution was probably because of reduced growth rather than because of the more fundamental failure of the muscle anlage to split into two. Its reappearance atavistically would then be analogous to the stage reached by generation n-1 in Fig. 2, 3,arising as a primordium which was not subsequently completely reduced and eliminated as a consequence of slower growth. Alternative explanations would require either an asymmetrical splitting of the deep muscle mass to produce a small M.c.p.i. or a two-step process of symmetrical splitting of the deep muscle mass into halves, followed by reduced growth of M.c.p.i. Parsimony does not favour either of these alternatives. As Raikow et al. (1979) emphasize, loss of the muscle in the ancestry of passerine birds was a phenotypic not a genotypic loss, the potential for its formation being retained within the genome and developmental system.
( 6 ) Mammals Evans (1959) has described anomalous hyoid muscles in pure bred beagles and in mongrel dogs from New York. These consisted of anomalous insertions of the stylohyoideus, sternohyoideus and mylohyoideus muscles (see his Fig. 3). He was able to relate these changes to patterns typical of lower mammals, regarded them as atavistic and explained them : “ in the light of phylogeny on the basis of differential development and muscle migration, which results in muscle patterns reminiscent of lower mammals ” (p. 161). T h e stylohyoideus was entirely absent or absent from one side in 25 of 78 pure bred beagles and in four of 78 mongrel dogs. These high incidences of absence prompted Evans to regard the stylohyoideus as “on the wane as a constant feature in carnivores”. Raikow et al. (1979) described the presence of M . abductor cruris caudalis in one hindlimb of one of 18 specimens of a bipedal rodent, the Egyptian jerboa (Jaculus jaculus; Dipodidae). All specimens from three other genera lacked the muscle. M . palmarus longus is a thin superficial muscle which flexes the hand and forearm in man and other primates. This muscle originates on the medial epicondyle of the humerus and inserts onto the palmar aponeurosis and transverse carpal ligament of the palm of the hand. It is absent from 85 yo of individuals of the genus Gorilla and from between 3-25 ”/o of the human population. It is often only absent from one side, usually the right. Although regarded as atavistic, the relatively high incidence of presence of this muscle perhaps indicates that we should follow Raikow (1975) in regarding it as an example of maintenance of polymorphism within the population. A similar caution may apply to the primate myological anomalies regarded as atavistic by Huntington ( 1903). (4) Miscellaneous other atavisms Listed below are further examples of atavisms occurring in natural populations. These are short-changed here, not because of their unimportance, but because very little developmental data is available to aid in understanding how they came about. Never-
Atavisms
I01
theless, many of them would be amenable to developmental analysis. The examples are the following : I. Segmental repetition of reproductive organs in insects, recalling insect origins from invertebrates with such serially repeated organs (de Beer, 1958,pp. I 30-1). 2 . Winged individuals in insect species, such as earwigs, that are otherwise wingless (Pantel, 1917; de Beer, 1958;Van Valen, 1970). 3. T h e presence of an upper incisor tooth in a newborn cyclopic lamb (Raikow, 1975). Sheep lack upper incisor or canine teeth. Its presence in this one specimen was attributed to ingestion of the plant teratogen gervine by the range-fed mother, the teratogen acting to trigger a hidden developmental pathway for incisor formation. 4. Van Valen (1970)cites examples of atavisms from fossils. Examples are the reappearance of first molar tooth cones in Pleistocene Lynx, their ancestors having lost them in the Miocene (see Kurten, 1963)and atavistic characters in Paleozoic trilobites. 5. There are a number of reports of atavistic fibulae in modern-day birds. T h e avian fibula is normally reduced to a splint attached to the tibia, although the ancestors of birds, such as Archaeopteryx, had fibulae and tibiae of equal lengths. Baur (1885)and Shufeldt (1885,1894)reported fibulae almost equal in length to the tibiae in an osprey (Pandion carolinensis: Rapaces), two boobies (Sula piscator and S. cyanops: Sulides), a shag (Phalacrocorax: Pelecaniformes) and another diver, Colymbus septentrionalis (Colymbiformes). T h e last also had a separate fibulare, a bone normally fused to the fibula in the avian limb skeleton. This ancestral condition of fibulae and tibiae of equal length can be produced by experimental manipulation of limb buds of embryonic chicks as will be discussed in Section I V ( 1 ) . 6. Riedl(r978) lists several other examples of spontaneous atavisms such as five digits on the amphibian hand, extra toes in llamas, flat-fish pigmented on both sides, and several human examples such as presence of a tail, body covered with hair, presence of four or more nipples, a divided uterus or presence of a cloaca. Arey (1954)cites other human examples, such as presence of an azygus lobe of the lung as in quadrupeds, presence of an elevator muscle of the clavicle as in climbing primates, or presence of the palmarus longus muscle in the forearm. In commenting on the evident existence of genetic instructions for producing these structures, Riedl concluded that “ In the epigenetic system there must be true jobs for these archaic instructions” (p. 207). I shall return to this point later. The next three examples conform to the definition of atavisms, except that they do not persist into adult life. 7. T h e presence of a vestigial, backwardly directed first digit in the hind limb buds of emu embryos (de Beer, 1975). 8. The presence of an ascending process of the astragalus running along the anterior surface of the tibia in many avian embryos. This element is not seen in adult modern birds but was present in Archaeopteryx and in the Theropods (Ostrom, 1976;Thulborn & Hamley, 1982). 9. T h e vestigial egg tooth and caruncle of some marsupials, a remnant of the functional egg tooth used by their reptilian ancestors to hatch from their eggs, indicates retention of the capability to produce such structures. Monotremes possess a functional egg tooth (Hill & de Beer, 1950). 10.Even if a character is one that cannot be readily related to organisms on the line
B. K . HALL
I02
of descent, it can still tell us about hidden genetic potential. 'Horns' on rabbits are one such example, for we cannot trace the evolution of the rabbit back to a horned ancestor. Rowe (1947) reported a cottontail rabbit (Sylvilagus floridanus mearnsii) from northwest Missouri with eight horns on its head, some as large as 73 mm long and 25 mm basal diameter. Six were located laterally and two immediately posteriorly to the ears, i.e. in the region of the head where horns develop in cattle, sheep, etc. Evidently their occurrence is not unusual (relatively speaking), for Rowe cites three other cases from the literature and several anecdotal reports from Missouri. One published report every 1 1 . 5 years is not so rare (although with rabbits breeding as they do, 1 1 . 5 years does represent a large number of generations). T h e repeated appearance of these structures indicates a latent potential for the formation of horn-like structures. (I know of no evidence that these structures are 'true' horns, i.e. keratinized outgrowths covering a bony spike, for no histology has been reported on them.) Are they atavisms or merely a pathological condition superficially resembling an atavism ? T h e fossil record provides few answers, for there is only a good fossil record available from the lower Tertiary (Paleocene) onwards and they share no close evolutionary links with horned or antlered groups (Van Valen, 1964). 111. T H E GENEI'IC BASIS OF ATAVISTIC CHARACTERS
I now want to consider two examples in which individuals exhibiting an atavistic character have been used in attempts to select for that character. Such characters can be selected for; their frequency can be rapidly increased through a population in several generations and the degree of development of the atavistic character can be improved as a result of the selection programme. The two examples are extra toes in guinea pigs and dew claws in dogs. (I)
Extra toes in guinea pigs
The guinea pig Cavia porcellus has four digits on the front feet and three on the hind feet, a condition typical of all members of the family Caviidae and of the closely related Hydrochoeridae. Digit I (the thumb or pollex) has been lost from the front feet, while both digit I (the big toe or hallux) and digit 5 (the little toe) have been lost from the hind feet. Embryonic guinea pigs show no sign of forming more than three digits in their hind limb buds or four in their forelimb buds (Stockard, 1930, although he did not examine early, precartilaginous condensation stages). Digit 5 in the hind foot is represented by a vestigial metatarsal but no tarsal is present. Digit I is represented b y a tarsal but no metatarsal is present. Therefore, with respect to digital formula, the hind feet are more reduced than the front feet, and within the hind feet digit I shows a greater degree of reduction than does digit 5. Evolutionarily, digit I was lost before digit 5 so that digitial loss has proceeded at a slower rate in the front feet than in the hind feet. The families Caviidae (guinea pigs, cavies and maras) and Hydrochoeridae (capybaras) are grouped with many other families in the same suborder of the rodents, the Caviomorpha. T h e two sub-families (five genera) of the Caviidae and the two modern-day species of capybaras (Hydrochoerus hydrochaeris, and H . isthmius), have known fossil histories back to the late Miocene and early Pliocene, respectively. They can be traced back to the paramyids (Purarnys), ancestral rodents of the Oligocene, through Plutypittamys to the Eocardiidae (Eocardia) of the Eocene when the two recent families
A tavisms
103
diverged from one another. Reduction in digit number has been established for a long time in these groups. Castle (1906)was the first to report a deviation from this digital pattern of four digits on the front feet and three on the hind feet. Castle had been introduced to Darwin’s writings as an undergraduate and was a confirmed evolutionist. He was one of the first, if not the first, to breed Drosophila, and convinced T. H. Morgan to enter that field. In 1900, Castle (1906)found a male guinea pig with an imperfectly developed left little toe. No muscles or tendons connected it to the rest of the hind foot so that it was not functional. Castle showed that this extra digit was digit 5 and not the result of subdivision of the adjacent digit 4. Castle crossed the individual with the spontaneously arising atavistic digit with guinea pigs with normal three-digited hind feet and was able to establish an inbred strain that bred true for presence of an additional digit 5 . By selecting both for presence and degree of development of this digit, Castle was able to fix presence of a ‘perfect’ digit 5 by the fifth generation. This digit was complete with bones, muscles, nerves, blood vessels, a nail and even a new plantar tubercle on the foot. (More about the skeletons and muscles will be reported below.) This extra digit was more likely to arise on the left hind foot (51*7yo, 630 individuals) than on the right (48.3 yo,589 individuals), a distribution that Castle could not alter by selection. In six generations, these guinea pigs had gone from absence of digit 5 , through presence of an imperfect digit unconnected to the rest of the hind foot in one specimen, to presence of the additional functioning digit in all specimens. Castle concluded that the extra digit was not inherited as a simple recessive ( I906, p. 24). Faced with the alternatives that every gamete had the gene present in an inactive form or that the condition arose de now0 in each individual, he favoured the former. Darwin (1905, vol. 11, pp. 16, 30, 39) had taken a similar view that such characters “lie latent”. Later, Castle (1911) attributed the condition to “ inheritance of several independent factors ”, foreshadowing Wright’s later but similar conclusions on the polygenic basis of their inheritance. Castle believed that selection and inbreeding could establish races with new morphologies. In a postscript to his I 906 paper he wrote : “Apparently it is only when selection is exercised for the polydactyly* character and like individuals are mated to each other that a polydactylous race can be established. In its origin, polydactylism is a discontinuous variation or mutation, but without the aid of selection it would probably never become a racial character. Is not the same thing true of a great many of the characters which serve to distinguish the various races of domesticated animals and plants” ( I 906, p. 29). Natural selection operating on small inbred populations under adverse conditions is a way of establishing a new morphological variant. Stockard (1930) found a few individuals with an additional digit 5 on their hind feet during his work on eight inbred lines of guinea pigs. Breeding from those individuals with the most well developed extra digits rapidly fixed the character so that 1007~ of Castle referred to the individual and those bred from it as polydactylous. Polydactyly can arise either by subdivision of pre-existing digits or by development of an entirely new digital primordium. According to good etymological principles, Castle’s guinea pig was polydactylous. It had an extra digit. However, polydactyly covers all those cases in which many (up to 10) digits form and it is unfortunate that Castle used the term for an individual in which, although there is an extra digit, the total number does not exceed the 5 typical of mammals. To avoid any confusion over the origin of the extra digit, I have used the more neutral term of extra toes rather than polydactylous.
I04
B. K. HALL
the offspring had four-toed hindfeet. Pictet (1932, cited by Wright, 1 9 3 4 ~performed ) a similar series of experiments with similar results. Stockard also found one guinea pig with five digits instead of the normal four on the front feet. T h e additional digit was the lost thumb, digit I . Grunenerg ( I947, p. I 84) cites Kroning & Engelmann (1934) as reporting an additional digit I (big toe) on the hindlimb of one guinea pig, and Wright (see below) performed considerable genetic experimentation on similar animals. It is clear that Stockard saw the presence of the additional digit as the reappearance of an ancestral pattern. He titled his paper “ T h e presence of a factorial basis for characters lost in evolution : the atavistic reappearance of digits in mammals” and concluded that the factors (genes) for forming five toes were present in the germ plasm, and that some factor, other than a mutation, permitting their expression had arisen in the original animal with the extra toe. De Beer (1958) saw new recombinations of genes as the mechanism of such apparent reversals in evolution, basing his interpretation on Stockard’s data. Thus: “ By particular recombinations of genes, the original conditions are reconstituted under which all the digits are developed. It looks as if the ‘loss’ of the digits was due to the acquisition of genes and genetic conditions resulting in their suppression” (de Beer, 1958, p. 120). Sewall Wright began to study the inheritance of extra digits in guinea pigs in Castle’s laboratory at Harvard in 1914. A long series of selection experiments culminated in four papers (Wright, 1934a, b, c ; 1935)which contain citations to earlier papers reporting progress along the way. These four papers are important for they establish the polygenic basis for the inheritance of the additional digit and the ability of developmental events to modify its expression. They laid the genetic framework for Gruneberg’s concept of ‘Quasi-continuous Variation’ (see Gruneberg, 1963, p. 228) which provides a mechanism for the origin of such discontinuous characters as presence or absence of digits, teeth, etc. The work of Castle, Stockard and Pictet had shown that only some inbred lines of guinea pigs develop an additional toe. Wright ( 1 9 3 4 ~reported ) that of 23 inbred lines, twelve lacked an extra digit at all, five showed very low incidences (0.3 Yo)of extra digits, and six had incidences ranging from 2 to 1 9yo. He began with Castle’s strain 35 with I 3.5 yo incidence of toe and bred for increased frequency of the character. Such four-toed guinea pigs were indistinguishable from the normal three-toed with respect to prenatal growth, number of live births, postnatal viability and reproductive success. Some 18% of the variation in expression of the little toe was genetic. About half the variation could be accounted for by ‘non-genetic’ factors -age and maturity of the mother, age of the male, litter size, season. T h e asymmetrical distribution of presence of the extra toe (more frequently on the left) was taken by Wright as indicating that the mechanism producing the extra toe was to be sought locally within the limb bud rather than systemically within the body. Wright clearly saw the extra digit as atavistic. He speaks of “the recurrence of an atavistic little toe” (1934a, p. 534). Gruneberg classed these structures as ‘restitutional polydactylism’ - a euphemism for the more loaded term, atavism. In criticizing Pictet’s (1932) interpretation, Wright states that Pictet ignored: “the fact that non-genetic factors play such an important role that normals and polydactyls may be of the same genetic constitution’’ (Wright, 1934a, p. 509). Again (p. 536) “ A recognition and evaluation of the importance of non-genetic factors in determining the
Atavisms
‘05 presence or absence of the little toe is a necessary foundation for analysis of the genetic [my emphasis] differences between different stocks. ” Castle (1906) had argued that some sort of latent potential for forming an extra digit was present in all guinea pigs. Wright echoes this view when he concludes : “ it is assumed that the presence or absence of the little toe depends on whether the combination of factors exceeds or falls below a threshold” (Wright, 1934a, p. 534). That polydactylous and normal individuals in the same strain were genetically the same led Gruneberg (1947) to conclude Hence the appearance or otherwise of extra toes within this strain must be ascribed to ‘developmental ’ conditions in the widest sense, more precisely to differences in the pre-natal environment’’ (p. 184). Clearly, the interpretation of the mechanism operating to produce guinea pigs with four-toed hind feet is seen to be one of environmental (developmental) modification of genetic activity. Wright’s subsequent work placed this interpretation on an even firmer footing (no pun intended). He crossed these now well established inbred lines of four-toed guinea pigs with strains possessing the normal three-digited hind feet (Wright, 19343)and was able to show (a) that three- and four-toed individuals in the same strain did not differ genetically, and (b) that three or four pairs of genes provided the genetic basis for inheritance of the extra toe. In some crosses, the genetic expression seemed to be recessive, in others, dominant. Wright concluded that genetic control was polygenic, that there was a physiological threshold for expression of the fourth toe and that some three-toed strains were closer to the threshold than others; “the character . . .approaches alternative expression because of physiological thresholds ” (Wright, 19343, p. 550). In Gruneberg’s estimation: “Multifactorial basis and the pre-natal environment plays a major role in determining whether the threshold to polydactylism will be crossed” (1947, p. 185). Gruneberg (1963, p. 228) emphasized that despite the discontinuous (presence or absence) nature of the character, the underlying genetic basis resembled that usually found in characters that show continuous variation. In discussing the genetic absence of the third molar in mice, a similarly discontinuous character, he notes (p. 243) : For such discontinuous characters which arise at the extremes of continuous distribution and which are determined by multiple genes despite [my emphasis] their discontinuous phenotype, I have proposed the name ‘ Quasi-Continuous Variation’. ” He cites missing third molars in mice and extra toes in guinea pigs as the best examples and uses ‘ Sewall Wright’s classical analysis’ (p. 228) of additive polygenic variation to explain much continuous and discontinuous skeletal variation. During the course of Wright’s breeding programme (in 1928, in fact), a guinea pig was found with vestigial little toes (digit 5) on the hindfeet, a left big toe (digit I ) and left and right thumbs (digit I ) . Wright began a breeding programme using this animal (Wright, 1934c, 1935). The gene involved Px (pollex) was a semi-dominant. The heterozygotes were normal except for the presence of extra digits (Table 2). Wright thought that these heterozygotes “ approached the original pentadactyl condition ” (1934c, p. 185).The homozygotes were entirely different. They were grossly deformed and died early in development. Their limb buds were twice the normal width and each had seven to twelve digits. The animals were microphthalmic with malformations in virtually every organ system. Yet the heterozygote only deviated from normal in having extra digits and never more than five per limb. Wright evidently continued to regard ‘ I
I‘
B. K. HALL
I 06
Table
2.
+
The percentage of heterozygotes (Px/ +) with extra digits one and/or five after breeding f r o m the original individual carrying the Px gene* Hindlimb Digit L
Original individual. . . Heterozygotes 53 yo
-
20 Yo
-
18 Yo 8 Yo I
I
LorR
Yo
t
Digit 5 L, R t
L,R -
L, R L,R
Forelimb Digit
I
L, R LorR LorR LorR
Based on data in Wright ( 1 g 3 4 r , 1935). L , left limb; R, right limb.
this as an atavism. He states: “ T h e same gene which restores atavistic digits in heterozygotes produces a generalized monstrosity in homozygotes” (1935, p. 107). He saw the evolutionary implications, for he states : Clearly the borderline between normal and monstrous types of morphological change is an arbitrary one from the genetic standpoint” (1934c, p. 362). He discusses P x along with otocephaly in the context of the role of thresholds and gene expression (1935, p. 1 0 3 fF) and thought that the homozygote suggested that it may not have been a genetic mechanism (mutation ?) that produced the original five-toed conditions. I will return to this below. Scott (1937, 1938) obtained nine newborn P x l P x guinea pigs and numerous heterozygotes from Wright and examined their development for his Ph.D. research project at the University of Chicago. He records that all organ systems in the homozygote recessive except the genital and circulatory are grossly abnormal. Embryos could first be detected as abnormal at 184 days of gestation, before the differentiation of the digits. Most die around 26 days of gestation. (The gestation period in the guinea pig varies between 58 and 72 days.) Scott attributed the abnormalities, except for absence of the tibia, to arrested morphogenesis and disproportionate growth. Clearly he did not regard the gene as atavistic. “ T h e gene is not atavistic although its effects in the heterozygote have that appearance” (1938, p. 299). His argument against its atavistic character (p. 312) is that some very peculiar fossil guinea pigs would have to have existed, presumably akin to the homozygotes P x l P x , for the homozygote to be regarded as resembling an ancestral condition. But P x l P x is lethal so that fossils would be expected to resemble the heterozygote (or the heterozygote would be expected to resemble the fossils) which they do. How is the development of a foot with exta toes controlled and coordinated so that the final appendage is fully functional ? Once the genetic and/or developmental conditions appropriate for producing an extra digit are available, correlated differentiation and morphogenesis, functional adaptation and growth of the foot as a functional unit would ensure that a functional appendage resulted. Skeletal growth can result in just such adjustments in the associated soft tissues (Goss, 1972, 1978; Hinchliffe & Johnson, 1980; Wolpert, 1981).Scott’s (1938) description of the skeletal morphology of the hindfeet of +/-t P x / + and P x / P x guinea pigs provides a clue to how the initial step in alteration of skeletal morphology to produce the extra digits I and
+
+,
+
Atavisms
I
II Ill I V
v
Fig. 3. The mechanism of formation of atavistic digits 1 and V in the guinea pig hindlimb. The normal hindlimb ( + / +) has three digits (11-IV) each with a tarsal, metatarsal and phalanges. A vestigial metatarsal V and tarsal I are present. No tarsal V or metatarsal I is found. Formation of digit I in the involves development of a new metatarsal I and of phalanges. Formation of digit heterozygote P x / + V in the heterozygote involves growth of the pre-existing metatarsal V and development of phalanges. No tarsal V forms. The homozygote P x / P x , which is polydactylous, contains ten to twelve metatarsals and phalanges but only five to six tarsals.
+ +
+
5 could have come about, although neither he nor subsequent authors seem to have commented on it. The normal hind limb bud has three digits consisting of tarsals and metatarsals 2-4 and their associated phalanges. A vestigial metatarsal 5 is present but tarsal 5 is absent. In the heterozygotes P x l ,metatarsal 5 has elongated and phalanges have been added to form the new digit 5. No tarsal 5 forms (Fig. 3). Thus, increased growth of the pre-existing metatarsal 5 and addition of new phalanges and horn produces digit 5 . There is an exact parallel here with the production of extra digits in the horse, and with digital reduction in some reptiles where metatarsal 5 is retained as digit 5 is lost (Russell & Rewcastle, 1979). The normal ( / ) hindlimb has a tarsal I , even though digit I does not form. No metatarsal I is present (recall that digit I was the first lost during evolution and has proceded further than digit 5 in terms of degree of reduction). The heterozygote P x l has retained tarsal I , developed a new metatarsal I , phalanges and horn to form the new digit I (Fig. 3). Development of a new element (metatarsal I ) , rather than elongation of apteexisting one (metatarsal 5) is clearly the mechanism used in forming digit I . These two quite diflerent developmental mechanisms are initiated in the same limb bud. The homozygote P x l P x has five or more (usually ten to twelve) metatarsals and phalanges but only five to six tarsals. It also lacks a tibia. Clearly, tarsal number is only altered with difficulty in these animals.
++
++ ++
++
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B. K. HALL
We have then in Px a gene that in the homozygous recessive condition produces a grossly abnormal and stillborn embryo (Gruneberg, I 963, allies it with Disorganization (Ds) in the mouse - the “most teratogenic gene described in any mammal”, p. I I I ) , but which in the heterozygote, where it is modified by the other genes and non-genetic factors involved in the production of extra toes, provides a stable, heritable mechanism for altering limb form in a fundamental way. The homozygote is almost totally removed from the population. The heterozygote can be maintained under selection in inbreeding populations. T h e response of the limb bud to these genetic and developmental factors differs preaxially from postaxially. I n both sites new phalanges form. In digit 5, these are added to a digit that forms by retention and accelerated growth of metatarsal 5 . No new elements more proximal than the phalanges form. In digit I , the new phalanges are added to a new metatarsal I , a development involving differentiation and morphogenesis in addition to accelerated growth. ( 2 ) Atavistic
dew claws in dogs
Most dogs have five digits on their front feet and four on their hindfeet. The missing digit is digit I , the big toe. Exceptions to this digital pattern are the mastiff and the St Bernard which usually have a big toe, and the African hunting dog Lycaon pictus which has four toes on both front and hindfeet. Dew claws, known in Europe as wolf claws, are rudimentary big toes or extra toes on the front feet and are found in dogs, ruminants, pigs, deer and other ungulates. Most dogs have dew claws on their front feet, usually only on one side and consisting of a horny claw which may have a bony core and be articulated by a joint to the foot. Historically, dew claws were thought to interfere with hunting by becoming entangled in long grass or underbrush. In the days when ladies of fashion kept small dogs as pets it was thought that dew claws would catch in the lace of their gowns and furnishings (antimacassars and the like). T h e habit therefore developed, and still exists, of removing dew claws at or soon after birth, except in those breeds where the ‘breed standard’ required their presence. Thus the Briand, which attained fame as the French army dog of World War I , being fitted with back packs to carry ammunition and first-aid supplies to the front lines (the latter to counteract the effect of the former), must have two dew claws on the hindfeet to meet the breed standard. Absence of both means disqualification from competition. Absence of one does not disqualify but prevents the dog from winning an award. (I am indebted to Dangerfield & Howell (1971)for these picayune details.) T h e Pyrenean Mountain dog must have two dew claws on each hind foot and one on each front foot to meet its breed standards. Clearly atavistic characters can be maintained for many generations under selective breeding. Dew claws on the front feet are not atavistic, for their presence exceeds the normal digitial number of five. Presumably they arise by subdivision of digit 2 when an extra digit is present, or by subdivision of the nail bed when only an extra claw is found. T h e similarity between non-atavistic dew claws on the front feet and atavistic dew claws on the hindfeet serves to emphasize the similarity between atavistic characters and their nonatavistic counterparts. On the hindfeet, dew claws restore the digital number to five. They do not arise by subdivision of digit 2 . Kelley (cited by Burns, 1 9 5 2 , p. 24) reports that they can arise in individuals whose parents lacked them, a feature typical of atavisms. Such extra digits can be inherited over many generations as in the breeds
Atavisms
I09
mentioned above, or in six-toed cats, a condition that Darwin refers to as having been inherited for three generations (1905, vol. I , p. 548). Some early writers saw their presence as a quite natural occurrence. Thus, Young (1852): “They are simply illustrations of the uniformity of structure which prevails in all animals, so far as is consistent with their destiny. T h e dew-claws only make up the number of toes in other animals” (p. I 12). It could be argued that as so many breeds have dew claws, dogs have not really lost digit I but rather just do not allow it to grow to a size at which it can function. Clearly, the degree of loss of this digit is considerably less than that seen in guinea pigs or horses. This is amply illustrated by the breeding experiments of Stockard (1930). He crossed St Bernards, a breed that has five digits on the hind feet, with Great Danes, which have only four, but which do possess a rudimentary first metatarsal. The offspring had five functional toes on their hind feet. T h e extra digit was produced by lengthening the preexisting first metatarsal and adding phalanges and a claw, i.e. it developed in the same way as digit 5 develops in guinea pigs. T h e pattern of inheritance that Stockard observed was dominant. Other workers, cited by Burns ( I952) and Burns & Fraser (1966), have observed patterns of inheritance of dew claws in various breeds ranging from recessive, dominant, variably dominant or incomplete penetrance of a single gene. Cases of increased expression of the character, e.g. double versus single dew claws, are recessively inherited. Clearly the pattern of inheritance is confused. Lande (1978) faults these data as being of too small a sample size to distinguish single from multiple factor inheritance, and cites Wright’s studies on the guinea pig as the paradigm for the type of study needed. Whatever the inheritance, dew claws develop by modifying already formed elements and by adding additional elements to them. IV. T H E EXPERIMENTAL PRODUCTION OF ATAVISMS
I have discussed examples that show that atavisms do arise in natural populations and that they can be selected for under the conditions of inbreeding established in the laboratory. I now want to document some examples of the production of atavistic characters by experimental manipulation of developing embryos. These show that such characters can be produced without any mutational event having modified the genome. (I)
Re-establishment of ancestral patterns in the hind limbs of embryonic chicks
An example often cited by evolutionary biologists (Frazzetta, 1970, 1975 ; Alberch et al., 1979; Hinchliffe & Johnson, 1980; Gould, 1980a, b ; Bonner, 1982) is Hampe’s (1959) production of an Archaeopteryx-like limb and ankle in the embryonic chick. Because of the importance of this study, because Hampk published six papers on the topic and not just the one cited by most authors, and because of ongoing work on the cellular mechanisms underlying tibia1 and fibular development in the chick which shed light on the mechanism responsible for the production of the atavism, I will discuss the work in some detail. In typical adult reptiles the tibia and fibula are of equal length and width. Five metatarsals are present, equal in length and with no sign of fusions to one another. At least four tarsal bones are present in the ankle. They are also separate from one another, from the metatarsals and from the tibia and fibula (Fig. 4). In Archaeopteryx, the tibia
B. K. HALL
II0 Reptiles
Archaeopteryx
Birds
~
tibiale
__ f i bu lare
Fig. 4. A comparison of portion of the hind limbs of typical reptiles, birds and of Archaeopteryx to show the variation in ( a ) development of the fibula (narrowed in Archaeopteryx) and a mere splint in birds), ( b ) in the number of tarsal bones (two free bones in Archaeopteryx, four bones, two fused to tibia and fibula and two to the metatarsals in birds), and (c) in the number and fusion of metatarsals (five free in reptiles, three free in Archaeopteryx, three fused in birds). See text for further details.
and fibula are of equal lengths but the fibula is only one third as wide as the tibia. Three metatarsals and two tarsals are present. The metatarsals are unfused. One tarsal (the tibiale) articulates with, but never fuses to, the tibia. The other (the fibulare) articulates with, but never fuses to, the fibula (Fig. 4). In birds, the fibula has been reduced to a splint. Its distal end lacks an epiphysis so that it fails to grow distally or to connect the tarsal bones of the ankle. A tibiale and a fibulare are present and fused to the distal ends of the tibia and fibula. The three metatarsals are fused side by side into a single bone (Fig. 4).Thus we see three correlated changes in the transition from reptiles to birds: ( I ) reduction of the fibula to a splint; (2) reduction of the number of metatarsals from five to three, and lateral fusions between them; and (3) reduction of the number of tarsal bones to two and their fusion with the tibia and fibula. As noted in Section 11(4 above, atavistic fibulae, equal in length to the tibiae, have been reported in five species of modern birds. One of these also had a separate fibulare, unfused to the tibia. The latter species (Colymbus septentrionalis) mimics the pattern of distal hind limb bones seen in Archaeopteryx. We can see the following stages in the distal third of the hind limb bud (Fig. 5 ) . At Hamburger-Hamilton (H.-H.) stage 25 (4#days of incubation) a cartilaginous tibia and fibula are both present and of equal lengths, although the tibia is slightly wider than the fibula (Fig. 5A). A cartilaginous fibulare is present and continuous with the fibula. This disparity in width between tibia and fibula becomes more pronounced as development continues, and by H.-H. stage 26 ( 5 days of incubation) the fibulare has separated from the fibula (Fig. sB). Three separate metatarsals are present (recall that
Atavisms
I11
Fig. 5. Major stages in development of the hind limb of embryonic chick as determined from autoradiographs prepared following administration of 80 p Ci 3 6 S 0 , . (A) Hamburger-Hamilton (H.-H.) stage 2 5 . Tibia (T) and fibula (F) are of equal lengths but the tibia is wider. T h e fibulare (f) is continuous with the fibula. (B) H.-H. stage 26. T h e fibula is now noticeably shorter than the tibia. T h e fibulare has separated from the fibula. Three separate metatarsals representing digits 2, 3 and 4 are present. (C) H.-H. stage 27. The fibula is being reduced to a splint. A tibiale (t) is now present. (D) H.-H. stage 28. T h e fibulare is now fused with the fibula and the tibiale is fusing with the tibia. Metatarsals are still free. H.-H. stages 25-28 represent 4&6 days of incubation. (Reproduced from Hinchliffe 8i Johnson (1983) with the permission of the authors and the publisher.)
they are fused in the adult). By H.-H. stages 27-28 (51-6days of incubation) the tibia is both much longer and somewhat wider than the fibula. A tibiale is present distal to but separated by connective tissue from the tibia. Three unfused metatarsals may still be seen (Fig. 5 C, D). This development of the tibia and fibula in the embryonic chick is very reminiscent of the development of metatarsals/metacarpals 2-4 in the embryonic horse (see Section II(2) above). Initially the cartilages are of equal length but the one that will predominate later in development is wider initially than the one that does not develop as fully. Is it accidental that commonality of increased growth in length of one element of the pair is associated with that element being wider initially, while pairs that grow equally begin as cartilages that are equal in width? In studies involving transplantation of mesenchyme from one limb bud to another, it has been found that the fibula and the first digit often fail to develop in a limb that is otherwise normal (Wolff & Hampi, 1954;Hampi, 1956, 1957).Why should these be the missingelements ? The experimental design involved removing the distal one-third from limb buds of embryos of H.-H. stages 19-21.T h e distal tip of this one-third (consisting of presumptive digital mesenchyme) was then grafted back to the stump of the limb bud (Fig. 6). T h e remainder of the distal third (consisting of presumptive mesenchyme from the region between kr,ee and ankle) was grafted to the chorioallantoic membrane of a host embryo as a control for the localization of the regions removed (Fig. 6). This grafted segment typically formed a tibiotarsus, showing that it only
B. K. HALL
I I2
-
Presumptive knee-ankle
CAM graft
Presumptive hip- knee
3
i
I
Fig. 6. The grafting procedure used to examine limb development in the embryonic chick involves isolating the distal third of an H.-H. stage 19-21 limb bud T h e most distal portion consisting of presumptive digital mesenchyme@is grafted to the limb stump@, which consists of presumptive hip-knee mesenchyme. Even though presumptive knee-ankle mesenchyme @ has been deleted, such grafts form hind-limbs complete with a tibiotarsus. However, the fibula or first digit is often missing. Presumptive kneeankle mesenchyme @ is grafted to the chorio-allantoic membrane where it forms a tibiotarsus, confirming that only that mesenchyme was deleted from the composite grafts @+@
0.
+
consisted of presumptive lower limb. Nine of the 16 grafts of proximal stumps distal limb-bud tips developed as hind limb buds complete with a tibiotarsus but often lacking a fibula and first digit. The limbs have regulative ability and can compensate for missing mesenchyme, but only at the expense of not forming a fibula or first digit (Wolff & HampC, 1954; Kieny, 1967). It is significant that the missing portions are those that normally undergo the greatest degree of reduction during normal limb development - the fibula is just a splint and the first digit is the smallest digit. T o explain this developmental instability of the fibula a theory of competitive interaction was developed by Wolff (1958) and elaborated by Kieny (1967). This theory postulates that the developing tibia dominates the developing fibula by acquiring more mesenchymal cells early in development (presumably between H.-H. stage 22 - when regulation can still occur - and H.-H. stage 25, by when the tibia is larger than the fibula (Fig. 7). In those grafted limb buds deficient in mesenchyme, as when distal tips were grafted onto proximal stumps, the tibia would always obtain its share of cells and so grow to normal size. The fibula would not always obtain sufficient cells and so would fail to form. Development of the fibula was also more affected when mesenchymal deficiency was created by X-irradiating limb buds (Wolff & Kieny, 1962). There is a
Atavisms
Fig. 7. Growth of tibia and fibula isolated from H.-H. stage 28 embryos (6 days of incubation) is determinate and not influenced by the growth rate of the element with which it is cultured. Thus the fibula (F) remains small and the tibia large whether co-cultured with fibula or with tibia (T). This experiment negates competitive interaction between tibia and fibula from H.-H. stage 28 onwards. (Reproduced from Hinchliffe &Johnson (1983) with the permission of the authors and the publisher.)
considerable literature indicating that condensations that fail to attain a critical size do not chondrify with the result that the corresponding skeletal element fails to form (Gruneberg, 1963;Hinchliffe & Johnson, 1980,1983;Hall, 1983;Thorogood, 1983). These early experiments and the competitive interaction model set the stage for the interpretation of HampC’s next experiment. He found that, rather than remaining as a short splint, the fibula grew to the same length as the tibia under three different experimental conditions (Hampe, 1958,1959,1960). The first experiment involved grafting an H.-H. stage 18-19limb bud onto the stump of an H.-H. stage 21-22 limb bud after removing the distal tip of the latter. The second involved insertion of a mica plate between the developing tibia and fibula at H.-H. stage 24. In two of these specimens, the fibula was longer than the tibia, partly because the tibia was slightly shorter than normal and partly because of increased growth of the fibula. T o confirm that this altered growth resulted from more than the normal number of cells going to the fibula one would need to insert mica plates between the tibia and fibula at various times during early limb development. Hampe only used H.-H. stage 24. Archer, Hornbruch & Wolpert (1983a ) and Archer, Rooney & Wolpert (19836 ) inserted tantalum foil barriers between the developing tibia and fibula in limb buds of embryos of H.-H. stages 22 and 23. The fibulae of five out of 36 embryos retained the distal epiphysis. In two embryos, the fibula was longer than the contralateral control. Both of these embryos received tantalum barriers at H.-H. stage 22. Hampb’s third experiment involved dissecting presumptive fibula mesenchyme out of the limb bud,
B. K. HALL rotating it through 180' and reimplanting it. Because the fibula lacks a distal epiphysis, 180' rotation reverses the direction of its growth so that tibia and fibula grow in opposite directions. Under these conditions both bones grew to the same length. Furthermore, ossification of the fibula, which normally begins at the proximal end, began at the mid-diaphysis (Hampe, I 960). T h e tibiale was also affected, sometimes remaining separate from the tibia rather than fusing with it as occurs in normal development. Thus, modification of the pattern of growth of the fibula in the lower limb has a secondary effect on development of the tarsal bone of the ankle. Furthermore, additional tarsal bones developed in some specimens. Modifications to the limb were not even restricted to the tibia/fibula and ankle regions. T h e three metacarpals that normally fuse during development remained separate in some of the experimental specimens. T h e parallel between this pattern of limb development and the skeleton of the hind limb of Archaeopteryx is very close (Fig. 4). Tibia and fibula are of equal length, ankle bones remain separate (the experimental embryos had more ankle bones than did Archaeopteryx but fewer than modern reptiles possess), metatarsals are unfused. Presence of additional limb mesenchyme or altered direction of growth does not just increase the size of the bone (fibula) receiving the additional cells, but leads to the development of a fundamentally different type of limb. Clearly we cannot consider development of skeletal elements in isolation from adjacent elements. HampC (1959, 1960) interpreted the basic change of increased growth of the fibula on the basis of the competitive interaction model outlined above. How good is the evidence for competitive interaction between the developing fibula and tibia ? Hinchliffe's autoradiographic data (Fig. 6) shows that differences in recruitment of mesenchymal cells must occur (or at least begin) before H.-H. stage 25 when the tibia is already somewhat larger than the fibula. Are the earlier condensations equal in size? Hinchliffe & Johnson (1983) argue that competitive interaction implies recruitment of cells throughout the growth phase of the cartilage, but that no direct evidence of such prolonged recruitment exits. Presumably their reasoning is that early recruitment (before H.-H. stage 25) would not be sufficient to explain the considerable differences in growth between tibia and fibula, unless rates of cell division were also quite different between the two. D. Wilson and M. Hicks, working in Hinchliffe's laboratory in Aberystwyth, have been testing the competitive interaction model (Hinchliffe & Johnson, 1980, 1983). Wilson placed a mica plate between the tibia and fibula at H.-H. stage 24 (the same stage as used by HampC) and found no difference in the lengths of tibia and fibula when compared with control embryos at 8 days of incubation. However, competitive interaction could have occurred earlier. There is a need for an experiment in which barriers are placed between developing tibia and fibula at various stages between H.-H. stages 1 8 and 24 - although such experiments are difficult in such young embryos. Hicks has also used organ culture to explore the cellular basis of the differential growth of tibia and fibula. H.-H. stage 28 tibiae and fibulae when cultured alone for 6 days continued to grow respectively fast and slow, with the tibia growing 2-5 times as fast as the fibula, even though the tibia could not appropriate any cells from a neighbouring fibula. Clearly competitive interaction is not taking place after H.-H. stage 28. Hicks also co-cultured all combinations of H.-H. stage 28 tibia and fibula for 6 days (e.g. tibia + tibia, fibula +fibula, tibia +fibula) and again found that growth was independent
Atavisms
1I 5
of the partner (Fig. 7). Growth of tibia and fibula is intrinsically programmed from at least H.-H. stage 28 onwards. Hinchliffe & Johnson (1980,p. 226) cite the action of the mutant Diplopodia (where the tibia is 35-45 yo shorter than normal and the fibula is the same length as the tibia) as indicating that growth of these two bones can be genetically programmed from an early age. T o explore the cellular basis of these differential patterns of growth further, Hicks (cited by Hinchliffe & Johnson, 1980, 1983) examined differential synthesis and differential cell deposition of extracellular matrix (as judged by incorporation of 35S), division (as judged by number of [3H]thymidine-labelled cells) and differential cell volume (as determined morphometrically) of the tibia and fibula at H.-H. stage 30. On a volume basis, 35S-incorporation was similar between tibia and fibula. Number of [3H]thymidine-labelled cells was also similar, so that neither differential cell division nor deposition of extracellular matrices can explain the differential growth at H.-H.stage 30. However, cell density was higher in the tibia than in the fibula, both in the epiphyses and in the mid-zone region and showed a greater decline in the hypertrophic zones of the tibiae than in the fibulae. Tibia1 cells start smaller (are more densely packed) but become larger than fibular cells during chondrocyte hypertrophy. This differential cellular hypertrophy could therefore explain differential growth of the tibia, but cannot be the complete explanation, for differential growth is seen as early as H.-H. stage 25. The size of the initial cell population must hold the key. I have argued elsewhere (Hall, 1982)that the fundamental control of growth is to be sought in the number of cells in an initial skeletogenic condensation. (2) Enamel
from avian ectoderm
Although the ancestors of modern birds (e.g. Archaeopteryx) were toothed vertebrates, modern birds neither possess teeth nor do they synthesize any of the specialized proteins associated with the production of the two dental tissues, dentine or enamel. Until recently the presumption was that birds lacked teeth because they had lost the genes required for producing ameloblasts and odontoblasts, depositing enamel and dentine and integrating the two into a tooth. I am not aware of any reports of atavistic teeth in birds. It now appears that this failure to find teeth in even one specimen does not reflect loss of genes but rather reflects their inactivation. Furthermore, the conditions required to reactivate these quiescent genes are such that experimental manipulation of tissues of the developing embryo is required before avian tissues can be made to synthesize and deposit enamel. Teeth are composite structures. Enamel is deposited by mesenchymal (neural crest-derived) odontoblasts. A complex series of inductive interactions between epithelium and mesenchyme at the various stages of differentiation of enamel organs and dental papillae, preameloblasts and preodontoblasts, ameloblasts and odontoblasts, is required before enamel and dentine can be deposited and before the three-dimensional morphology of the tooth can be generated (see Kollar, 1981; and Ruch et al., 1982,for reviews). Defective tooth development as seen in the tabby mutant in the mouse could result from a defect in the dental epithelium and/or from a defect in the dental mesenchyme. These two components can be enzymatically separated and then recombined, either with one another, or with other embryonic mesenchyme or epithelia. Such epithelial-mesenchymal recombinations have told us much about the control of tooth
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B. K. HALL
differentiation and morphogeiiesis (see above reviews). Recombinations can also be performed with tissues from different species. It was such a heterospecific recombination that Kollar & Fisher (1980) used to show that avian epithelium could be induced to synthesize and deposit enamel proteins into an enamel matrix, which together with mouse dentine formed a chimaeric chick-mouse tooth. Kollar & Fisher took epithelium from the first and second branchial arches of 5-day-old embryonic chicks. This is equivalent to the epithelia that form enamel organs and deposit enamel in toothed vertebrates such as fishes, reptiles and mammals. This buccal epithelium was recombined with dental mesenchyme from the future molar-forming region of the lower jaws of 16- to 18-day-old mouse embryos. T h e resulting tissue recombinations were grown as intraocular grafts in athymic nude mice for up to 4 weeks. Complete teeth formed in four out of 5 5 of such chimaeric recombinations. The best example (illustrated in their fig. I F) had roots and a crown in the correct configuration and consisted of ameloblasts and odontoblasts, and their extracellular products enamel and dentine. Provided that contamination of murine epithelium on the dental mesenchyme can be eliminated, the enamel formed in these teeth must have been deposited by cells of the chick epithelium under the inductive influence of the mouse dental mesenchyme. So we have the first hen’s teeth to develop for many tens of thousands of generations. What appears to have occurred during the evolution of birds from their toothed ancestors is that the buccal mesenchyme has lost the ability to exert an inductive effect on the adjacent epithelium, but the epithelium has retained the ability to respond. Providing an appropriate induction allows the quiescent genes for enamel synthesis to be expressed. (Enamel is a highly conservative protein, having changed little if at all during vertebrate evolution : Herold, Graver & Christiver, I 980.) Despite contrary current dogma genes can evidently be conserved unaltered but quiescent for very long times. Clearly, this is a remarkable example of the activation of genes by an inductive tissue interaction. Many more examples exist in the developmental literature. Recombination of avian wing-bud epithelium with mouse dental mesenchyme induces cartilage to form from the dental mesenchyme. Instead of forming the type I collagen typical of dentine, the mesenchyme forms another gene product, type I1 collagen, which is typical of cartilage (Hata & Slavkin, I 978; Silbermann et al., 1977). Heartless salamanders, mutants in which the heart fails to complete its development, can be corrected by combining endoderm, a known cardiac inducer from normal embryos, with cardiac mesoderm from embryos with the cardiac mutant (Lemanski, Paulson & Hill, 1979). The scaleless mutant in the fowl prevents the formation of scales and other structures that originate in epithelial papillae such as scleral papillae. Scales can be induced to form when mesoderm from older wild-type embryos is recombined with epithelium from scaleless embryos (McAleese & Sawyer, I 982). Recombination between epithelium and mesenchyme from wingless and wildtype embryos was discussed in Section I I ( I ) ( b ) . Such altered tissue interactions provide a developmental basis for the production of a wide variety of atavistic characters. ( 3 ) Balancers, teeth and gills in amphibians
Many larval salamanders possess a pair of balancers as lateral appendages on their heads. Balances are covered with ectoderm, have a mesodermal core and lie just behind
Atavisms
1 17
Table 3. Heterospecifc transplantation of lateral head ectoderm into gastrula embryos shows potential to form balancers* Source of ectoderm Axolotl Salamander 3 . Salamander 4. Frog I.
2.
Host gastrula Salamander Axolotl Frog Salamander
Result No balancers Balancers Balancers Adhesive glands
+ From results in Mangold ( 1 9 3 1 ) and Rotmann (1935). Most larval salamanders have balancers but lack adhesive glands. Larval anurans have adhesive glands but lack balancers. Axolotls lack balancers or adhesive glands.
the angle of the mouth under the eyes. Internally, they are attached to the quadrate by a connective tissue membrane. A mucus is produced by the cells at the distal tips of these balancers which are used to balance and orient the larva as it feeds. Once the limbs start to form, balancers degenerate. Balancers are induced to form in embryonic salamanders by the roof of the archenteron and the adjacent neural plate, which interact with the overlying ectoderm. Some salamanders, such as axolotls, lack balancers, as do all anurans. Many anuran larvae have a pair of adhesive glands instead. These are located ventrally on the head whereas balancers are lateral, but like the balancers they secrete mucus and are used for attachment. Ectoderm has been transplanted between axolotls (Ambystoma mexicanum), other salamanders (Triton taeniatus) and frogs to explore whether balancers could be induced in the axolotl and frog larvae (Mangold, 1931; Rotmann, 1935). T h e results of such transplantations are summarized in Table 3 . T h e first two experiments between axolotl and salamander embryos show clearly that axolotl ectoderm lacks the ability to respond to a balancer inducer (first experiment in Table 3), but that the axolotl embryos contain active inducers of balancers even though they do not themselves form them (second experiment). Axolotls lack balancers because their head ectoderm has lost the ability to respond to a balancer inducer, not because they lack the inducer itself. A further type of experiment showed that axolotls contain balancer inducers, for supernumerary balancers form when axolotl archenteron roof is implanted into salamander gastrulae (Mangold, 1 9 3 1 ) . The transplantations between salamander and frog embryos (experiments 3 and 4, Table 3) illustrate the similarity in inductive mechanisms producing balancers and adhesive organs. Salamander ectoderm responds to frog inducer by forming balancers. Frog ectoderm responds to salamander inducer by forming adhesive organs. T h e specificity resides within the responding ectoderm. These experiments show how the reaquisition of competence by axolotl or anuran ectoderm could immediately result in the formation of a set of balancers. The inductive tissue interactions controlling the formation of urodele and anuran amphibian teeth also illustrate how ability to respond to an inducer can be retained for long periods of time during evolution even when the organ does not form. Urodele larvae have true teeth made up of dentine and enamel with a pulp cavity, located inside the mouth, attached to the bones of the jaws and palate. Anuran larvae lack such teeth. Instead they have horny sheaths as jaws and horny teeth on an oral disk around the outside of the mouth. These teeth are keratinized and lack enamel or dentine. Both true
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B. K. HALL
teeth and horny teeth are induced by oral endoderm acting inductively on oral ectoderm. Spemann & Schotte (1932)and Rotmann ( I 935) covered the future mouth of salamander embryos with ectoderm from frog embryos. T h e resulting salamander larve had frog’s mouths complete with horny sheaths, horny teeth and adhesive organs. Despite the very long time since separation of anuran from urodele amphibian (if indeed they ever had a common ancestor), frog ectoderm can still respond to urodele inducer. T h e specificity of the response lies entirely within the responding ectoderm, the inducer acting as a releaser of that potential. A further example of how inducers could be responsible for generating new morphologies or the reappearance of old ones comes from Balinsky’s (1956) work on the South African toads. Bufo carens has three pairs of external gills, while B. regularis has only two pairs. In both species, endoderm induces ectoderm to form gills. Balinsky exchanged epidermis between larvae of the two species. When B. regularis ectoderm was grafted to B. carens, the B. carens tadpoles had three pairs of gills, i.e. specificity was to the host providing the inducer. In the reverse transplantation of B. carens ectoderm to B . regularis embryos, the B. regularis tadpoles had two pairs of gills. Again, specificity was to the host inducer. Evidently ectoderm can form two or three gills, the number formed being set by the inducer. A change in the inducer could result in the appearance of additional gills, a character that might well be atavistic. (4) Bristle pattern in Drosophila
A further example of new morphological structures (neomorphs), which can either be selected for or produced by transplantation of imaginal disks to new locations, is the production of new bristle patterns in Drosophila. In these cases the new patterns are characteristic of families closely related to the Drosophilidae, but patterns not seen in Drosophila itself. Sondhi (1962), in an experiment selecting for increasing bristle and ocelli number in the ocelliless mutant of Drosophilasubobscura, observed the development of a new pair of bristles. T h e location and size of these bristles, which are normally absent in Drosophilidae, was typical of that found in the related family Aulacigastridae. This example illustrates how hidden genetic potential can be revealed by selection. In the second approach, Loosli (1959) transplanted the dorsal metathoracic imaginal disk to a new location in Drosophila melanogaster. All the normal metathoracic bristles developed as did additional bristles. T h e latter, not normally seen in D . melanogaster, are seen in some heterozygotes with the Stunted mutant and are seen in a related family, the Sepsidae. Hidden genetic potential has been revealed not by selection but b y experimental manipulation. V. SUMMARY I . Atavisms emerge as evidence of localized modifications in development of an organ or of one of its parts. Different developmental processes can be triggered within the same organ rudiment, presumably in response to the same stimulus. We saw that that stimulus can have a genetic basis in a mutational event, which can be selected for. We also saw that atavism can be produced by experimental manipulation within developing systems increased growth of the chick fibula, enamel production from avian ectoderm, and balancer formation in amphibians. Such atavisms are not based on heritable genetic changes. They indicate the developmental plasticity that exists within embryos and the relative ease with which development can be switched from one programme to another.
Atavisms
1I9
2. Examination of mutants (wingless chicks), limbless vertebrates and experimental manipulation of embryos, shows that cell death, inductive tissue interactions and altered patterns of growth are developmental mechanisms used in the formation of atavisms. 3. Differential development mechanisms can be triggered within the same organ at the same time to produce atavisms. In the guinea pig, formation of atavistic digit V involves prolongation of growth of metatarsal V whereas formation of atavistic digit I involves development of a new metatarsal I. 4. Secondary functional modifications ensure that the atavism is integrated with the other components of the functional unit, as illustrated by extra digits in horses or guinea pigs and fibulae in birds. Atavistic 2nd and 4th digits in horses arise by continued growth of their primordia. A consequent reduction in the growth rate of digit 3, the normal single functional digit, enables all three digits to attain approximately equal lengths and so potentially to function. T h e altered functional load transmitted to the limbs results in secondary but correlated alterations in muscles and skeletal elements in other portions of the limbs. T h e fact that embryonic digit 2 normally develops to a more advanced state than digit 4 explains why digit 2 more often develops atavistically, for if variation in growth rate is the basis for the atavistic digit, digit 2 has an advantage over digit 4. 5. Atavisms should not be an embarrassment to the evolutionary biologist. They are the outward and visible sign of a hidden potential for morphology change possessed by all organisms. Neither basic capacity to form the organ nor patterning information is lost. Modification of components of inductive tissue interactions helps to explain how organs are lost during evolution (also see Regal, 1977); retention of the basic mechanism explains how structures can be revived as atavisms (also see Rachootin & Thomson, 1981).Frequency of atavisms thus provides an indication of the degree of modification or loss of the underlying developmental programme. VI. ACKNOWLEDGEMENTS The bulk of this paper was written while on sabbatical leave as a Visiting Fellow at the University of Southampton. I thank the Nuffield Foundation for the awarding of a Commonwealth Travelling Fellowship; Professor M. Sleigh and Dr P. V. Thorogood for the provision of space and facilities at Southampton; the Natural Sciences and Engineering Research Council of Canada for support of my research programme, and Dr J . Hanken for his very thoughtful comments on the manuscript. VII. REFERENCES ALBERCH, P., GOULD,S.J., OSTER,G. F. ?i WAKE,D . B. (1979). Size and shape in ontogeny and phylogeny. Paleobiology 5, 296-3 I 7. ANDREWS, R. C. ( 1 9 2 1 ) A . remarkable case of external hindlimb in a humpback whale. American Museum Novitates 9. 1-6. ARCHER, C. W . , HORNBRUCH, A. & WOLPERT, L. (1983a). Growth and morphogenesis of the fibula in the chick embryo. Journal of Embryology and Experimental Morphology, 7 5 , 1 0 1 - 1 16. ARCHER, C. W., ROONEY, P. & WOLPERT, L. (1983 b). T h e early growth and morphogenesis of limb cartilage. In Limb Development and Regeneration. Part A . (ed. J. F. Fallon and A. I. Caplan), pp. 267-278. Alan R. Liss, New York. AREY,L. B. (1954). Developmetital Anatomy. A Textbook and Laboratory Manual of Embryology, 680 pp., 6th ed. Saunders, Philadelphia. ARVY,L. (1976). Some critical remarks on the subject of the cetacean ‘girdles’. In Investigations on Cetacea (ed. G. Pilleri), volume I , 179-186. ARVY,L. (1969). The abdominal bonesofcetaceans. In Investigationson Cetacea(ed. G . Pilleri), volume 10, 2 I 5-227. BALINSKY, B. 1. (1956). Discussion. Cold Spring Harbor Symposia on Quantitative Biology 21, 354. BAUER, G . (1885). A complete fibula in an adult living carinate bird. Science 5 , 375.
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