Changing Patterns of Gene Regulation in the

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relation to two critical features of arthropod morphology: tagmatization and limb diver- sification. I begin with a brief and generic explanation of the gene networks ...
AMER. ZOOL., 38:818-828 (1998)

Changing Patterns of Gene Regulation in the Evolution of Arthropod Morphology1 LISA NAGY 2

Department of Molecular and Cellular Biology, University of Arizona, Tucson, Arizona 85745 SYNOPSIS. What can the comparative study of gene expression patterns during development contribute to the study of phytogeny? I discuss the basic properties of gene networks that function in development, using information gleaned from developmental model systems. Using examples from the analysis of anteroposterior, dorsoventral and proximodistal axis formation, I outline how the gene networks that pattern these three axes of development are linked in evolution. Finally, I discuss the types of analyses necessary to further our understanding of how gene networks function in regulating the evolution of morphology.

INTRODUCTION

1 From the symposium Evolutionary Relationships of Metazoan Phyla: Advances, Problems, and Approaches presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3—7 January 1998, at Boston, Massachusetts. 2 E-mail: [email protected]

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Remarkable similarities have been found in the gene networks that pattern basic cellular and developmental tasks in diverse metazoans. Examples from studies in development include similarity of the HOX genes in anteroposterior patterning (reviewed in McGinnis and Krumlauf, 1992; Ruddle et al, 1994; Carroll, 1995), similarity in the signaling network that patterns the dorsoventral axis of insects and vertebrates (reviewed in DeRobertis and Sasai, 1996) and similarity of the cell-signaling network that functions in proximodistal axis formation during vertebrate and insect limb development (reviewed in Shubin et al., 1997). The similarity of gene networks that pattern structures in insect and vertebrates has led to propositions like the homology of the ventral side of arthropods with the dorsal side of vertebrates (Arendt and Niibler-Jung, 1994; Peterson, 1995), and the "deep" homology of insect and vertebrate limbs (Shubin et al., 1997). To what extent are the similarities among gene networks that pattern development informative about phylogeny? Does similarity in process of subcellular design or developmental patterning imply evolutionary relationship? It is important to remember that

we have known for a long time that all animals are built from similar tissue level building blocks—neurons, muscles, epithelia. Similarly, eukaryotic organisms share biochemical pathways that perform "housekeeping" functions—transcription, translation, or arrangement of the cytoskeleton. Should it really come as any surprise that other types of cell and developmental processes are equally conservative features of metazoan evolution? Gene networks that function in development give us another character, or suite of characters for comparative analysis. By examining changes in developmental gene networks in the context of already established phylogenies, we can begin to determine how development evolves. It remains to be seen whether developmental characters will be useful in and of themselves for establishing phylogenies and maybe even more significantly, whether they will provide insight into morphological transformation. How do we distinguish between homology and convergence of morphological features patterned by similar gene networks (see Bolker and Raff, 1996)? For example, consider the gene network that patterns vertebrate and insect limbs. Should vertebrate and insect limbs be considered homologous because they are patterned by similar gene networks? Or is the similarity an example of molecular convergence, representing not an extreme conservation of limb construction throughout metazoa, but merely a consequence of a limited number of molecular

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What types of questions can the analysis of developmental gene networks address? Developmental characters have traditionally been very useful for phylogeny. Consider three classical developmental characters: 1. deuterostomy versus protostomy; 2. spiral versus radial cleavage; 3. schizocoely versus enterocoely. In spite of all the changes in our views of metazoan phylogeny over the years, all three of these, or at least certainly the first two, make rather robust single character diagnostics for metazoan clades. This may be indicative of the persistence of an early developmental ground plan since before the Cambrian. Unfortunately, we know nothing about the genetic basis of these characters. The organisms that have become model genetic systems, like the fruit-fly Drosophila or the nematode C. elegans do not fit nicely into the dichotomous categories of protostome vs. deuterostome or spiral vs. radial cleavage. And presently its not exactly clear how one could uncover the genetic basis of protostomy, or even spiral cleavage. It might be possible to mutagenize a species with spiral cleavage, looking for radialized phenotypes, but this might not be a very easy experiment. Although genetic analysis of model systems has not been yet been useful for understanding the classical developmental characters described above, the comparative study of gene expression patterns has, however, been informative concerning the questions of "How are body plans related to one another?" and "What are the origins of segmentation?" For questions of evolution within phyla, this approach has been useful for the study of tagmosis in arthropods, the loss and gain of appendages, and the evolution of pigmentation patterns (Averof and Akam, 1993, 1995; Carroll et al., 1994; Warren et al, 1994; Brakefield et al., 1996). Contributions from the developmental genetics of model systems to the understanding of the evolution of morphology. Throughout the rest of the paper, I use examples from arthropod development and evolution to illustrate some of the ideas that

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tools an organism has available to change its form? From a morphological and historical standpoint, vertebrate and insect limbs have rarely been considered homologs. There are several clades between insects and vertebrates that do not have limbs. We do not know the functions the limb patterning gene networks serve in these intermediate clades, or whether they are even conserved as a network in all intermediates. Although the molecules that function in insect and vertebrate limb development are similar in kind, i.e., some of them are TGF-3 class signaling molecules, they are not in all cases direct orthologs of one another. In addition, the "homology" of a pathway cannot be solely determined by comparing the sequence similarity of the component parts. Standards used for determining degree of homology of a DNA sequence are virtually irrelevant for determining homology of gene networks. Differences in gene sequences are interesting, and informative, in and of themselves to phylogenetics. However, developmental characters, like morphological characters, inherently have "systems" properties—not reflected in sequences. The challenge for developmental biologists is to connect differences in gene sequence to differences in function. What are the overall consequences of changes in gene sequence for the performance of the gene network as a whole? How do we relate the uni-dimensional information of gene sequences into the multi-dimensional information of development? The important task is to ask about the rules of transformation and focus on the evolution of genes as circuits or networks. We can then ask how change is enacted within gene networks. If for example, the gene networks that pattern insect and vertebrate limbs are convergent, why do these particular combinations of genes seem to have an affinity for one another? The long term goal for developmental biologists seeking to provide phylogenetic information is to ask how developmental mechanisms map onto phytogenies and to ask what this mapping tells us of evolutionary mechanism.

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viewed in Akam, 1987; Ingham, 1988). As their names imply, the mutations result in deletions of groups or pairs of segments, or alter the polarity of a segment. However, the patterns in which segments are deleted in these mutations are very different from the common variations in segment number seen in arthropod evolution. Consider also the changes in segment character that have been elicited through mutation in Drosophila. A Drosophila appendage can be changed to look like another Drosophila appendage, or something so perturbed that it can at best be called a blob-like-thing, but not to look like a lepidopteran or a hymenopteran appendage. The take home message is that mutagenesis in model systems does not undo evolution or reveal, in any direct fashion, the basis of evolutionary change. We cannot look to model systems to define the changes we see in evolution, but we can use them as a baseline of comparison. Through a comparative approach, we can begin to understand properties of developmental systems. By measuring what stays the same and what changes we can learn about both developmental possibility and constraint. What are the characteristic features of developmental gene networks? Before beginning a comparative analysis of the gene networks that pattern the three developmental axes in arthropods, I first review some of the features of developmental gene networks that have been gleaned from 50 years of the study of developmental genetics in model organisms. 1. First, development occurs through sequentially-acting cascades of genes. This initial gene activity results in the subdivision of the embryo into multiple unique domains of gene activity, or compartments of gene activity. Boundaries between these compartments of information then generate new sites of initiation for additional cascades of gene activation (reviewed in Lawrence and Struhl, 1996). 2. Second, most networks of developmental patterning genes are comprised of genes with pleiotropic functions. Rather than having genes with unique functions, most genes function at multiple times and

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have emerged from the comparative analysis of developmental gene networks. Different arthropod classes can be distinguished by unique combinations of external morphological features, including: 1. the manner in which the body is tagmatized; 2. the number of head segments; 3. the number of trunk segments; 4. the presence or absence of an appendage on a segment; and 5. the characteristics of an appendage. What do we know about the developmental genetics of these features? Genetic analyses in Drosophila have provided a great deal of inspiration for imagining what changes in development might underlie evolutionary modifications (Goldschmidt, 1940; Lewis, 1978). However, it is a common misconception that mutations in model systems retrace the evolutionary changes within an organism's genome. Drosophila mutations are unique to the developmental system that patterns Drosophila. The known genetic bases of evolutionary changes in morphology, even those of which are similar to those invoked by mutation in Drosophila are not ascribable to simple mutations that appear in Drosophila. In addition, many features that vary in evolution have never appeared as mutations in Drosophila. There are a few examples where Drosophila mutations have been characterized as atavistic, or invoking the pattern of the ancestor. For example, in homeotic mutations which remove the abdominal HOX genes from the genome limb primordia appear on every segment, reminiscent of a hypothetical homonomously-segmented arthropod ancestor, more like a present day centipede or branchiopod crustacean (Lewis, 1978). However, centipedes and crustaceans have the same component of abdominal HOX genes found in Drosophila (Averof and Akam, 1993, 1995; Grenier et al, 1997). Their homonomous segmentation is not caused by fewer homeotic genes, as suggested by the Drosophila homeotic mutations. Another example is that of Drosophila segmentation. Some mutations add or subtract the number of segments, a common variation seen in arthropod evolution. The Drosophila segmentation mutants are gap, pair-rule or segment-polarity mutants (re-

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mental pathways, it is an important property of developmental systems to keep in mind when we consider the manner in which developmental systems can be modified. How do these properties relate to the potential for evolutionary change within a developmental system? One approach to the study of this problem is to apply the comparative method and examine how a particular gene network changes within a group of related organisms. The basic method is to ask, "What are the genetic changes underlying morphological diversity?" Beginning with an integrated functioning circuit in one organism, what are the possible ways in which you could modify it to produce a viable alternative? Are there particular types of changes that occur frequently and others not seen at all? I begin with the Drosophila gene networks that build segments and position appendages and ask how the circuitry changes in another species. I will compare these networks with those of the branchiopod crustacean, Triops longicaudatus. An important goal is to be able do the same for multiple arthropods as well as other phyla. An approach to studying the evolution of gene networks I describe what has been learned about how the primary patterning networks that function in Drosophila development vary in relation to two critical features of arthropod morphology: tagmatization and limb diversification. I begin with a brief and generic explanation of the gene networks that function in segmentation and appendage development in Drosophila (Fig. 1). The Drosophila embryo is patterned by a series of gene interactions that subdivide the egg into increasingly smaller units of positional information. It is not important at this point to understand the details of all these cascades of gene activity, nor to be totally familiar with all the gene names. The essence of the process relevant to the present discussion is that the egg begins with anteroposterior and dorsoventral polarity determined by the mother. These initial polarities are detectable as gradients of information along the entire length of each respective egg axis (reviewed in Akam, 1987; Ingham,

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places. For example, in Drosophila, wingless functions first in segmentation, then in Malphigian tubule development, then later in leg and wing development. There are a few notable exceptions to this generality, including the maternal-coordinate gene bicoid, that functions at the top of the Drosophila segmentation cascade described below. 3. Third, gene networks can create dissociable units, or "modules." This property of developmental systems has been shown both through classical transplantation experiments and modern genetic experiments. For example, classical experimental embryology revealed that limb development is dissociable from anteroposterior axis formation in vertebrates (Huxley and De Beer, 1934). This ability of limb development to be dissociated from anteroposterior axis formation is even more striking because it is known that both anteroposterior and limb patterning utilize many of the same genes. In addition, there exists a class of genes, selector genes, which have the capacity to regulate a cascade of downstream gene activity. Ectopic expression of selector genes can result in the development of ectopic structures (Garcia-Bellido, 1975; Haider et ai, 1995). Alterations in the expression of these genes could be the initial trigger for dissociation in developmental processes (Garcia-Bellido, 1975). 4. Finally, it is apparent that development is amazingly robust in the face of perturbation. Embryos with up to five extra copies of the maternal coordinate gene bicoid produce a fine animal. In these flies, the primary maternal gradient that sets the entire segmentation gene cascade described below is very abnormal. But the animals recover, and become normally segmented (Frohnhoffer and Niisslein-Volhard, 1986; Berleth et ai, 1988; Namba et al, 1997). Embryos can adapt to an amazing diversity of perturbations (see Gilbert, 1997 for examples). This property of embryos has many names, including adaptability, plasticity, regulation, accommodation, self-adjustment, self-organization, tolerance, buffering, and homeostasis. While we currently have very little understanding of the genetic mechanisms responsible for the robustness of develop-

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Anterior-Posterior Axis

ed in discrete domains of the developing embryo (reviewed in McGinnis and Krumlauf, 1992). Coincident with this segment and region-specific specification along the A/P axis, the D/V axis also becomes further subdivided. Along the D/V axis each region gap Kr adopts a specific developmental fate: mesoderm, ventral nervous system and aminoserosa (reviewed in Ferguson and Anderhairy son, 1991). Thus, the activity of the A/P and D/V gene networks create stripes of positional information along both axes of the embryo. segment polarity The boundaries of these stripes are then used to activate new gene networks that function to position discrete morphological structures. Limbs, and other organs such as homeotic the salivary glands and trachea develop at these boundaries. For example, legs develop at the boundary between the lateral stripe of decapentaplegic and the segmentally reiterated stripes of wingless and engrailed (reviewed in Cohen, 1993). Limb primordia are marked by a cluster of cells expressing Distalless, a gene which encodes a homeodomain protein required for Dorsal-Ventral FIG. 1. Drosophila axis formation, simplified. Dia- proximodistal limb outgrowth (Cohen et grammatic representation of some of the gene net- ah, 1989; Fig. 1). works involved in Drosophila A/P, DA7 and P/D axis This description is, of course, an overformation. Details are described in the text, bed: bi- simplified version of embryonic patterning. coicl; Dll: Distal-less; dpp: decapentaplegic; Kr: Kriippel; wg: wingless; zen: zerknullt. Modified after Orenic Indeed, each one of these tiers of gene activity involves a complex series of gene inand Carroll, 1992; Cohen, 1993. teractions (Fig. 2). How this positional information is translated into discrete units of 1988; St. Johnston and Ntisslein-Volhard, terminal differentiation is not yet as well 1992). These gradients then activate down- understood. However, from this very genstream targets in a concentration dependent eral description we can extract some of the manner. Along the A/P axis the immediate general characteristics of how the fly emdownstream targets are the gap genes. In- bryo is patterned. Below I outline the reteractions between these large domains of sults of recent comparative studies of these the gap genes then break the egg axes into pathways from my own and other labs, in even smaller domains of gene activity: each which we have been asking how these gene future Drosophila segment is subdivided networks are conserved in evolution. into anterior and posterior (A/P) compart1. Establishing the conserved compoments. Cells in the posterior compartment nents.—Like the examples outlined in the express the transcription factor engrailed beginning of this paper, the first observation {en); cells which lie immediately anterior to the comparative method reveals is that dethe compartment boundary express the se- velopmental gene networks are composed creted signaling molecule, wingless (re- of remarkably conserved components. viewed in Martinez-Arias, 1993). As the in- HOX genes are most likely a major synadividual segments become patterned, the pomorphy for metazoans (see Duboule, homeotic genes, which confer region-spe- 1994). Distalless is also conserved in both cific segment identity, also become activat- sequence and its apparent function in limb bic

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development (Panganiban et al., 1994). In fact, it seems to be conserved in a multitude of what Greg Wray has called "sticky-outies" produced in the animal kingdom (Panganiban et al., 1997). Wingless, engrailed, hairy, decapentaplegic, nearly all the genes in the diagram in Figure 1 have been shown to be conserved at least between Drosophila and mouse, and in many cases are even much more extensively conserved. The fact that so many patterning genes are conserved throughout the metazoa leads to a view that changes in patterning predominantly involve changes in the regulation between conserved components (King and Wilson, 1975; Jacob, 1977). We are

currently somewhat limited by the fact that when we compare developmental gene pathways across phyla, the focus is on conserved features, because they are easiest to assay and recognize. But, is everything conserved? Is all of evolutionary change the result of changing regulatory interactions between conserved components? It is very difficult to assess this question as we have currently identified only a very small portion of the genes encoding proteins even within the most-studied model organisms. One of the most surprising things to come from the complete sequence of the yeast genome was that less than 20% of the identified protein coding regions represent pre-

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FIG. 2. Drosophila segmentation, less simplified. While the Drosophila segmentation cascade is frequently depicted in the simplified manner shown in Figure 1, the gene networks that pattern segmentation are much more intricate and interdependent. Most of the genes in the segmentation pathway have multiple regulatory targets as shown here. They also function at other times and places in development (not shown). This diagram is adapted from White et al., 1986; Irish et al., 1989; Gerhardt and Kirshner, 1995; and Fujioka et al., 1995.

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viously identified genes (Goffeau et al, clear in the past five years that gene dupli1996). It is likely that as more and more cations persist. In the early vertebrate linecomplete genome sequences become avail- age there is evidence for several total geable, we will get a much better handle on nome duplications. It has been suggested what percentage of genes are novel and that this increase in gene number may be unique to particular lineages and what per- responsible for the appearance of unique centage are conserved. We may even some- morphological innovations seen in the verday be able to measure degree of conser- tebrate lineage (Holland et al., 1994). As vation between non-coding regions of DNA mentioned above, many of the components as well. of the axes patterning gene networks are Understanding whether different arthro- members of gene families, wingless, decapods share components in patterning gene pentaplegic, paired, and the HOX genes are networks is important, but does not allow all members of gene families, that have at us to compare how networks operate di- least one other paralogous gene in the Drorectly. Missing components do not neces- sophila genome. It is presumed these parsarily mean altered networks. Nor do con- alogous genes all arose via gene duplicaserved components necessarily mean con- tion. In some, but not all cases, the dupliserved gene networks. How do we make cated genes have diverged in function. At what rate do their functions diverge? the transition from studying individual genes to gene networks and developmental The HOX gene family is interesting in this processes? One critical necessity is to be regard. It is presumed that the HOX genes able to isolate novel components of gene arose through a series of ancestral gene dupathways and to shift the focus from an plications. Based on the HOX genes that amazement at how similar everything is to have been surveyed in a wide range of spean emphasis on what might underlie cies, the ancestral arthropod cluster is prechange. In addition, we need to be able to dicted to have the same composition of analyze how conserved components vary in HOX genes found in Drosophila. In my their regulatory relationships with one an- preliminary survey of Triops HOX genes, I other. Below I outline two examples of ap- have found more than the expected number of HOX genes, including a Triops HOX3 proaches to these problems. 2. Finding novel components.—There are ortholog. HOX3 is found within the vertesome notable exceptions of Drosophila pat- brate HOX clusters (Holland et al, 1992), terning genes that have not yet been found but is absent from Drosophila (Kaufman et to be conserved over long-evolutionary dis- al, 1990; Falciani et al, 1996). In this potances. For example, the bicoid gene at the sition in the fly HOX cluster there are three top of the maternal hierarchy (Sander, homeobox containing genes, zen 1, zen 2 1994), has not been found outside the Dip- and bicoid with very divergent homeobox tera, nor has the wing-patterning gene ves- sequences, which either do not function at tigial. Are these examples of genes unique all in A/P patterning (zen 1, zen 2), or serve to Drosophilal What is the source of new a very different role in A/P patterning than genes like bicoidl There is accumulating all the other HOX genes (bicoid). The Droevidence to suggest that gene duplication sophila homeobox-containing genes zen 1 followed by diversification of gene function and zen 2 function in dorsal-ventral patternis a common source of new genes and gene ing (Rushlow and Levine, 1990). bicoid, as function (Thomas, 1994; Nowak et al, described above, is a maternal-coordinate 1997; Cooke et al., 1997). When originally gene, that has not yet been identified outproposed as a theoretical mechanism for side the Diptera. By contrast, in chick and change in evolution, it was argued that gene mouse embryos HOX3 has an expression duplications could never survive. Because pattern with an anterior expression boundduplicated genes would be redundant, they ary that lies between those of HOX2 and would necessarily accumulate mutation and H0X4, as would be expected from the fact drift to eventual loss (Fisher, 1935; Ohno, that the order of the HOX genes along the 1970). However, it has become abundantly chromosome parallels their spatial sequence

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among species and to ask how gene-regulatory interactions diverge. 3. Making the transition from individual components to networks.—Three of the characteristics of gene networks that pattern developmental systems described above are 1) they are composed of sequentially acting cascades of genes; 2) they can be dissociable in time; and 3) they are robust in the face of many kinds of perturbation. If a gene that acts at the top of a cascade of gene interactions that regulated a morphological structure can be modified to elicit its action at a new time or in a novel location, this change would be manifest as a shift in timing or position of the appearance of that structure from ancestor to descendent. One of the many examples of a this type of change in arthropod evolution is the appearance and/or disappearance of legs in different body regions between ancestor and descendant species. For example, adult insects have a limbless abdomen. In contrast, many other adult arthropods have limbs on every segment. The comparison of the circuitry of the segmentation and limb patterning cascade has provided evidence for the repeated addition of repressible elements into the gene circuitry (Warren et al., 1994; Panginiban et al., 1995). Regulation of the morphological appearance of limbs between crustaceans and insects and within insect larvae appears to be controlled via the modification of these repressors. We know that the abdominal HOX genes Ultrabithorax, abdominal-A, and Abdominal-B repress limb development in Drosophila. This is achieved at least in part through their direct repression of the promoter of Distalless a gene required for limb development (Cohen et al., 1989; Vachon et al., 1992). Within insects the anterior boundary of the expression of these abdominal HOX genes correlates with the morphological boundary between thorax and abdomen. However when the expression of the HOX genes and their target Distalless is examined in crustaceans, these two genes are expressed coincidentally (Panginiban et al., 1995), which would be impossible if the crustacean HOX genes repressed the crustacean Distalless gene. Somewhere in the insect lineage, the HOX genes have ac-

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of expression along the body. These data imply that the absence of a HOX3 orthologue in Drosophila is a derived condition. Falciani et al., 1996 cloned HOX3 orthologs from a beetle and a locust and identified non-homeobox sequence motifs shared between these HOX3 genes and the Drosophila zen genes. Like the Drosophila zen genes, these insect HOX3 genes are expressed only in extra-embryonic tissues. They speculate that the HOX3 genes within the insects have lost their role as a canonical HOX genes, and acquired a new function. In the Drosophila lineage, the HOX3 gene diverged rapidly, and is currently represented by zen 1 and its recent duplicate, zen 2. A comparison of the head segments of flies and crustaceans indicates that crustaceans have a head segment—the second antennal segment, which is vestigial or absent in insects. Is it possible that the loss of HOX3 correlates with the loss of this head segment? Although entirely speculative at the moment, the evidence is consistent with the idea that basally within the arthropods, HOX3 function became redundant with another HOX gene. Once redundant, HOX3 function may have diverged differentially between lineages. When analyzed in other arthropods, other modifications to HOX3 expression and presumed function may be seen. Thus, while the HOX genes represent a major metazoan synapomorphy, they may also provide the genetic resources for the evolution of new gene function. How do duplicated genes arrive at new functions? How do old genes change their patterns of regulation? On the most general level, change in gene networks can clearly be obtained through "tinkering" (Jacob, 1977). But by what means are changes in regulation accomplished and what types of changes are used? Are the bulk of the changes cw-acting regulatory changes in the promoters of duplicated genes? Or are changes in the protein coding regions of genes equally important? Is any change possible, or are particular types of change more likely to occur than others? One way to begin an analysis of how gene networks evolve is to go beyond comparing individual genes and their expression patterns

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quired the capacity to repress the Distalless 1992; Ruddle et al, 1994; Carroll, 1995; gene, thereby suppressing abdominal leg DeRobertis and Sasai, 1996; Shubin et al, development. Thus, a potential genetic 1997). The HOX genes pattern the A/P decapentaplegic-shortend-gastrulamechanism of morphological change in axis, evolution is via the addition of an inhibitory tion pattern the D/V axis, and wingless step in an ancestral gene pathway. There is hedghog-decapentaplegic work together to evidence for the repeated application of this set up the P/D axes. These genes set up a mechanism within insect evolution. In the core reaction or "module" for each of these development of larval legs in the lepidop- axes. The genes that function both upstream teran Precis coenia there is evidence for the and downstream of these core circuits have activity of an inhibitor of this inhibitor, al- not been found to be as conservative. This lowing for the evolution of abdominal legs finding forms a molecular basis for the phein the abdomen of a holometabolous insect nomenon that Waddington (1940) called larva (Warren et al., 1994). Abdominal legs "canalization." Development consists of a appear repeatedly in the evolution of holo- robust, buffered morphogenetic system. metabolous insect larvae (reviewed in Nagy Ancient, conserved genetic functions may and Grbic, 1998). It will be extremely in- be locked into very interdependent, pleioteresting to determine whether larval legs tropic gene networks. Many types of muare regulated by independent mechanisms, tations may have no consequence for the or whether the gene network is changing in phenotype, as they are masked by the buffering capacity of the system. Gene duplithe same way repeatedly. cations that provide an initially redundant SUMMARY function may provide an avenue of escape I began with the question "What can the from a multiply interdependent pathway to comparative study of gene expression con- a novel gene circuit. It will be interesting tribute to the study of phylogeny?" Be- to see what the next ten years of comparacause it is becoming increasingly clear that tive studies of the genetic basis of develmetazoans share a common molecular tool- opment reveal concerning the interplay box used during development, can we use among pleiotropy and redundancy and the this information to either infer phylogenetic evolution of developmental mechanisms. relationships, or to learn about the interface ACKNOWLEDGMENTS of developmental mechanisms and evolutionary mechanisms? If developmental patThe author thanks H. Eisthen, E. Jockterning genes, like the HOX genes, are ex- usch and T. Williams for many helpful sugtremely conserved, they are unlikely to pro- gestions on improving the clarity of this vide information about evolutionary rela- manuscript. tionships. On the other hand, if the gene sequences are conservative, but the gene REFERENCES networks are flexible and changing, studying modulations in gene network functions Arendt, D. and K. Niibler-Jung. 1994. Inversion of dorsoventral axis? Nature 371:26. may, in the future, be informative for phy- Akam, M. 1987. The molecular basis of for metameric logeny. Whereas the initial comparison of pattern in the Drosophila embryo. Development HOX gene number between crustaceans 101:1-22. and insects revealed few differences, their Averof, M. and M. Akam. 1993. HOM/Hox genes of Anemia: Implications for the origin of insect and expression patterns and changes in the regcrustacean body plans. Current Biology 3:73—78. ulatory pathways they participate in were Averof, M. and M. Akam. 1995. Hox genes and the much more informative. diversification of insect and crustacean body plans. Nature 376:420-423. It is now becoming apparent that key aspects of the genetic control of the three de- Berleth, T, M. Burri, G. Thoma, D. Bopp, S. Richstein, G. Frigerio, M. Nolland, and C. Niissleinvelopmental axes that pattern a vertebrate Volhard. 1988. The role of localization of bicoid or insect embryo—anteroposterior, dorsoRNA in organizing the anterior pattern of the Droventral and proximodistal—are conserved sophila embryo. EMBO J. 7:1749-1756. (reviewed in McGinnis and Krumlauf, Bolker, J. and R. A. Raff. 1996. Developmental ge-

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lution. Yale University Press, New Haven, Connecticut. Grenier, J., T. L. Garber, R. Warren, P. M. Whitington, and S. Carroll. 1997. Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/arthropod clade. Current Biology 7 (8):547-553. Haider, G., P. Callaerts, and W. J. Gehring. 1995. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267: 1788-1792. Holland, P. W. H., L. Z. Holland, N. A. Williams, and N. D. Holland. 1992. An amphioxus homeobox gene: Sequence conservation, spatial expression during development and insights into vertebrate evolution. Development 116:653-661. Holland, P. W. H., J. Garcia-Fernandez, N. A. Williams, and A. Sidow. 1994. Gene duplications and the origins of vertebrate development. Development (Suppl.): 135-142. Huxley, J. S. and G. R. DeBeer. 1934. The elements of experimental embryology. Cambridge University Press, Cambridge. Ingham, P. 1988. The molecular genetics of embryonic pattern formation in Drosophila. Nature 336:2534. Irish, V. F, A. Martinez-Arias, and M. Akam. 1989. Spatial regulation of the Antennapedia and Ultrabithorax genes during Drosophila early development. EMBO J. 5:1527-1537. Jacob, F. 1977. Evolution and tinkering. Science 196: 1161-1166. Kaufman, T. C, M. A. Seger, and G. Olsen. 1990. Molecular and genetic organisation of the Antennapedia complex of Drosophila melanogaster. Adv. Gen. 27:309-362. King, M. C. and A. Wilson. 1975. Evolution at two levels in humans and chimpanzees. Science 188: 107-116. Lawrence, P. and G. Struhl. 1996. Morphogens, compartments, and pattern: Lessons from Drosophila. Cell 85:951-961. Lewis, E. B. 1978. A gene complex controlling segmentation in Drosophila. Nature 276:565-570. Martinez-Arias, A. 1993. Development and patterning of the larval epidermis of Drosophila. In M. Bate and A. Martinez-Arias (eds.). The development of Drosophila melanogaster, p. 517. CSHL Press. McGinnis, W. and R. Krumlauf. 1992. Homeobox genes and axial patterning. Cell 68:283—302. Nagy, L. M. and M. Grbic. 1998. Cell lineages in larval development and evolution. In Brian K. Hall and Marvalee H. Wake (ed.), The origin and evolution of larval forms. Academic Press, San Diego. Namba, R., T. M. Pazdera, R. L. Cerroneand, and J. S. Minden. 1997. Drosophila embryonic pattern repair: How embryos respond to bicoid dosage alteration. Development 124:1393-1403. Nowak, M. A., M. Boerlijst, J. Cooke, and J. MaynardSmith. 1997. Evolution of genetic redunancy. Nature 338:167-171. Ohno, S. 1970. Evolution by gene duplication. Springer-Verlag, Heidelberg.

Downloaded from icb.oxfordjournals.org by guest on July 22, 2011

netics and traditional homology. Bioessays 18(6): 489-494. Brakefield, P. M., J. Gates, D. Keys, F. Kesbeke, P. J. Wijngaarden, A. Monteiro, V. French and S. B. Carroll. 1996. Development, plasticity and evolution of butterfly eyespot patterns. Nature 384: 236-242. Carroll, S. B. 1995. Homeotic genes and the evolution of chordates. Nature 376:479-485. Carroll, S. B., J. Gates, D. Keys, S. W. Paddock, G. F Panganiban, J. Selegueand, and J. A. Williams. 1994. Pattern formation and eyespot determination in butterfly wings. Science 265:109-114. Cohen, S. M. 1993. Imaginal disc development. In M. Bate and A. Martinez-Arias (eds.). The development of Drosophila melanogaster, p. 747. CSHL Press. Cohen, S. M., G. Bronner, F. Kuttner, G. Jurgens and H. Jackie. 1989. Distal-less encodes a homeodomain protein required for limb development in Drosophila. Nature 338:432-434. Cooke, J., M. A. Nowak, M. Boerlijst, and J. MaynardSmith. 1997. Evolutionary origins and maintenance of redundant gene expression during metazoan development. Trends in Genetics 13:360364. De Robertis, E. M. and Y. Sasai. 1996. A common plan for dorsoventral patterning in Bilateria. Nature 380:37-40. Duboule, E. 1994. A guidebook to the homeobox genes. Oxford University Press, Oxford. Falciani, F, B. Hausdorf, R. Schroder, M. Akam, D. Tautz, R. Denell, and S. Brown. 1996. Class 3 Hox genes in insects and the origin of zen. Proc. Natl. Acad. Sci. U.S.A. 93:8479-8484. Ferguson, E. L. and K. V. Anderson. 1991. Dorsalventral pattern formation in the Drosophila embryo: The role of zygotically active genes. Cur. Top. Devel. Biol. 25:17-43. Fisher, R. A. 1934. The sheltering of lethals. Am. Nat. 69:446-455. Frohnhofer, H. G. and C. Nusslein-Volhard. 1986. Organization of anterior pattern in the Drosophila embryo by the maternal gene bicoid. Nature 324: 120-125. Fujioka, M., J. B. Jaynes, and T. Goto. 1995. Early even-skipped stripes act as morphogenetic gradients at the single cell level to establish engrailed expression. Development 121:4371-82. Garcia-Bellido, A. 1975. Genetic control of wing disc development in Drosophila. CIBA Foundation Symp. 29:161-183. Gerhardt, J. and M. Kirschner. 1997. Cells, embryos, and evolution. Blackwell Science, Inc. Gilbert, S. 1997. Developmental biology. Sinauer Associates, Inc. Goffeau, A., B. G. Barrell, H. Bussey, R. W. Davis, B. Dujon, H. Feldmann, F. Galibert, J. D. Hoheisel, C. Jacq, M. Johnston, E. J. Louis, H. W. Mewes, Y. Murakami, P. Phiiippsen, H. Tettelin, and S. G. Oliver. 1996. Life with 6000 Genes. Science 274: 546-567. Goldschmidt, R. B. 1940. The material basis of evo-

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LISA NAGY

Sander, K. 1994. The evolution of insect patterning mechanisms: A survey of progress and problems in comparative molecular embryology. Development, Suppl., pp 187-191. Shubin, N., C. Tabin, and S. Carroll. 1997. Fossils, genes and the evolution of animal limbs. Nature 388:639-648. St. Johnston, R. D. and C. Niisslein-Volhard. 1992. The origin of pattern and polarity in the Drosophila embryo. Cell 68:201-219. Thomas, J. H. 1993. Thinking about genetic redundancy. Trends Gene. 9:395-399. Vachon, G., B. Cohen, C. Pfeifle, M. E. McGuffin, J. Botas, and S. Cohen. 1992. Homeotic genes of the Bithorax complex repress limb development in the abdomen of the Drosophila embryo through the target gene Distal-less. Cell 71:437-450. Waddington, C. 1940. Organisers & genes. Cambridge University Press, Cambridge. Warren, R., L. Nagy, J. Seleque, and S. Carroll. 1994. Evolution of homeotic gene regulation and function in flies and butterflies. Nature 372:458-461. White, R. A. H. and R. Lehmann. 1986. A gap gene hunchback regulates the spatial expression of Ultrabithorax. Cell 47:311-321. Corresponding Editor: Douglas H. Erwin

Downloaded from icb.oxfordjournals.org by guest on July 22, 2011

Orenic, T. V. and S. B. Carroll. 1992. The cell biology of pattern formation during Drosophila development. Inter. Rev. Cytol. 139:121-155. Panganiban, G., L. Nagy and S. Carroll. 1994. The development and evolution of insect limb types. Cur. Biol. 4:671-675. Panganiban G., A. Sebring, L. Nagy, and S. Carroll 1995. The development of crustacean limbs and the evolution of arthropods. Science 270:13631365. Panganiban G., S. M. Irvine, C. Lowe, H. Roehl, L. S. Corley, B. Sherbon, J. K. Grenier, J. F. Fallon, J. Kimble, M. Walker, G. Wray, B. J. Swalla, M. Q. Martindale, and S. B. Carroll. 1997. The origin and evolution of animal appendages. PNAS 94(10):5162-5166. Peterson, K. J. 1995. Dorsoventral axis in version. Nature 373:111-112. Ruddle, F. H., J. L. Bartels, K. L. Bentley, C. Kappen, M. T. Murtha, and J. W. Pendleton. 1994. Evolution of Hox genes. Annu. Rev. Genetics 28:423442. Rushlow, C. and M. Levine. 1990. Role of the zerknullt gene in dorsal-ventral pattern formation in Drosophila. Adv. Genet. 27:277-307.