Genetic variation and evolutionary relationships within four species of ...

Hereditas 105: 245-254 (1986)

Genetic variation and evolutionary relationships within four species of Daphnia (Crustacea: Cladocera) HELENA KORPELAINEN

Department of Genetics, University of Helsinki, Finland

KORPELAINEN.H. 1986. Genetic variation and evolutionary relationships within four species of Daphnia

(Crustacea: Cladocera). - Hereditas 105: 245-254. Lund, Sweden. ISSN 0018-0661. Received January 22, 1986

Genetic distances (D’s) within and between four Daphnia species were estimated and the genetic structures of the species were compared from frequencies of protein electromorphs at 7 loci. The average withinspecies distances varied from 0.067 in D.pulex to 0.282 in D . magna. The D values for the between-species comparisons were about 0.3 (0.271-0.373) except for the species pair D . longispinnlD. pulex, which had a D value of only 0.101. The most striking aspect of the data was the high degree of genetic differentiation within Daphnia species. The very highest value was found in D . magna, which mainly inhabits small temporary ponds. The level of heterozygosity proved lowest in populations inhabiting northern marginal habitats. The effects of cyclical parthenogenesis on the genetic structure are discussed. Helena Korpelainen, Department of Genetics, University of Helsinki, Arkadiankatu 7, SF-00100Helsinki, Finland

Cladocerans are typical of freshwater ponds and nia are predominant in the southern continents. All lakes with unpredictable environmental changes known species fit fairly clearly into one of the two and variable availability of food. Their intrinsic subgenera but there is a great difficulty in identifygrowth rates are high due to rapid reproduction by ing them at the species level within each subgenus. apomictic parthenogenesis. Thus, cladocerans can Daphnia taxonomists seem to be unable to decide effectively exploit favourable environmental condi- whether they are dealing with a very few polytypic tions. Diapausing resting eggs, produced bisexu- species or with a large number of fairly monoally, insure the survival of a Daphnia population morphic species. The variability in body shape throughout an inclement period, which may be a dry often observed in Daphnia populations: has caused spell or winter in temperate habitats, or summer in confusion in the taxonomy. Several experimental tropical habitats. Exceptions to the alternation of studies (e.g., BROOKS 1947; JACOBS 1967) have indiparthenogenesis and bisexuality have been reported cated that this phenomenon, called cyclomorphosis, for the species D . middendorffiana (EDMONDSON is affected by environmental conditions. Cyclomor1955; ZAFFAGNINI and SABELLI1972) and D . phosis includes seasonal polyphenism in head cephalata (HEBERT 1981), which reproduce by obli- shape, carapace size, eye size, tail spine length and gate parthenogenesis. Some populations of other antennule length (reviewed by KERFOOT 1980). species of Daphnia also have this ability (DEHORNE The present study examines genetic distances be1924; BANTA 1925; STROSS 1966; HEBERT and CREASE tween Daphnia populations within and between 1980). four species, and the genetic properties of those The genus Daphnia includes more than 50 species populations. The species studied are D . magna and has a world-wide distribution. A subgeneric di- Straus (Ctenoduphnia), D . pulex De Geer (Daphvision is recognized (BROOKS 1957): the subgenus niu), D . longispina 0 .F . Miiller (Daphnia) and D . Duphnia is mainly Holarctic, while the Ctenodaph- cucullata Sars (Daphnia). The taxonomy of D.

246 n. KORPELAINEN

pulex and D. longispina is especially confusing. As BROOKS (1957) states, the validity of the “species” is suspect since the members of each species have only a single character in common -the structure of the middle pecten of the postabdominal claw. The conclusion usually drawn is that D. pulex and D. longispina are extremely polymorphic.

Material and methods Populations studied Samples of 31 Daphnia magna, 19 D . longispina, 5 D. pulex and 6 D. cucullata populations were collected in 1983-1984. Sampling was done using a plankton net. Animals were stored alive at 4°C until use. The collection sites and description of the populations are given in Table 1 . Samples of D. magna populations MI-M20 were collected from six islets in Hanko, on the southern coast of Finland. Populations M21 and M22 were from Kirkkonummi, about 70 km to the east from Hanko, and populations M23-M28 were from northern Finland, over 500 km to the north from Hanko. Populations M29 was located on the southwestern coast of Sweden and populations M30 and M31 were from southern Hungary. The D. longispina sample L1 was collected from an islet in Hanko and samples L2-L9 from Kirkkonummi. Populations LlO-L1S were from southwestern Finland, about 200 km from Hanko. Sample L16 was collected from southeastern Sweden, sample L17 from southwestern Sweden, sample L18 from northern Switzerland, and sample L19 from southern Hungary. D. pulex populations P1 and P2 were from two islets in Hanko, and population P3 was from Forssa, 100km to the north from Hanko. Samples P4 and P5 were collected from southeastern Sweden. The D. cucullata population C1 was located in southeastern Sweden, and population C2, in southern Sweden, populations C3 and C4 were located in northern Switzerland, and populations CS and C6, in southern Hungary.

Hereditas 105 (1986)

(Got),leucine aminopeptidase (Lap-1 and Lap-2) and malate dehydrogenase (Mdh). Homologous loci were scored in all four Daphnia species studied. The techniques employed, electrode and gel buffers and staining methods were similar to those described by KORPELAINEN (1984). The gels were run for S-6 h at 100 V (samples in 1983) or for 4 h at 200 V (samples in 1984).

Results Electrophoretic phenotypes

The esterases, which formed the most complex pattern of enzyme bands, were also the greatest source of between-species differences. The number of isoenzyme bands varied from two to five among populations and species, but only the two densely stained bands were considered. Est-2 of Daphnia magna or corresponding loci were the most variable loci of the seven loci studied in the four Daphnia species. In always polymorphic D. magna four alleles were found segregating, but the fastest allozyme existed only in populations M19-M22. In D. longispina three alleles segregated in most populations, although populations L2, L3, L8, L13, L17 and L18 were monomorphic. Populations of D. pulex and D. cucullata possessed two or three alleles except populations P4, PS, C1 and CS, which were monomorphic. Est-1 of D . mugna or corresponding loci were polymorphic in 7 out of 31 D. noagna populations, 14out of 19 D. longispinapopulations, 3out of 5 D. pulex populations. and in 3 out of 6 D. cucullata populations. The number of alleles at Est-l varied from one to three in all species. Between-species differences in the electrophoretic speed of the allozymes were observed at both esterase loci. One or both of the two scorable Lap loci were polymorphic in 27 D. magna, 15 D. longispina, 3 D. pulex and in all 6 D. cucullata populations. Thc speed of the allozymes found at the loci Lap-1 and Lap-2 was the same in all species. Populations of D. pulex possessed two alleles and populations of other species had three alleles at both loci. The numberb of populations polymorphic for Alk-l in D. magna, Electrophoresis D. longispina, D. pulex and D. cucullata were 18.8, Using standard techniques of one-dimensional 2 and 1 , respectively. The level of variation at the starch gel electrophoresis, populations (about 100 loci Got and Mdh was low, most populations being individuals from each sample) were screened for monomorphic. The electrophoretic speed of the variation in proteins encoded by 7 loci which have predominant allozymes at Alk-1, Cot and M d h was the same in all species. been found polymorphic in D . magna (KORPELAINEN Several cases of substitutions of the major allele 1984): Alkaline phosphatase (Alk-l),esterase (Esr-1 and Est-2), glutamate oxaloacetate transaminase for another were observed: in D. magna at Alk-I (1 1


GENETIC VARIATIONWITHIN DAPHNIA SPECIES

Table I . Collection sites, dates, habitats and coding for the populations of Daphnia magna, D. longispina, D. pulex and D.cucullafa and estimated population densities at the time of collection Species

Location

D. magna

Hanko,

Finland

Kirkkonummi .,

D.longkpina

Kempele Liminka

,,

Temmes Rantsila Grebbestad Szeged,

,, ,,

,,

Sweden Hungary

Hanko, Finland Kirkkonummi ,,

Uusikaupunki ,,

D. pulex

D.cucullata

Gryt, Grebbestad, Giitighausen, Szeged. Hanko. Forssa Solstadstrom, Vastrum Figeholm. Granna Giitighausen, Ossingen Szeged,

Sweden ,,

Switzerland Hungary Finland Sweden ,, Sweden

Code

Date

Habitat

Density

(Ml) (M2) (M3) (M4) (M5) (M6) (M7) (M8) (M9) (M10) (M11) (M12) (M13) (M14) (M15) (M16) (M17) (M18) (M19) (M20) (M21) (M22) (M23) (M24) (M25) (M26) (M27) (M28) (M29) (M30) (M31) (Ll) (L2) (L3) (L4) (LS) (L6) (L7) (L8) (L9) (L10) (L11) (L12) (L13) (L14) (L15) (L16) (L17) (L18) (L19) (Pl) (P2) (P3) (P4) (P5) (Cl)

10.08.83 10.08.83 10.08.83 07.07.83 22.07.83 10.08.83 10.08.83 10.08.83 10.08.83 10.08.83 10.08.83 22.07.63 10.08.83 10.08.83 22.07.83 09.08.83 09.08.83 09.08.83 09.08.83 09.08.83 03.08.83 03.08.83 27.07.84 27.07.84 27.07.84 27.07.84 27.07.84 28.07.84 06.06.84 30.08.83 30.08.83 10.08.83 11.07.84 11.07.84 11.07.84 11.07.84 11.07.84 11.07.84 11.07.84 03.08.83 18.08.84 18.08.84 18.08.84 18.08.84 18.08.84 18.08.84 20.06.84 06.06.84 29.06.83 30.08.83 10.08.83 02.08.84 23.09.84 20.06.84 20.06.84 19.06.84 04.06.84 29.06.83 29.06.83 30.08.83 ,06.09.83

Rock pool

High High High Low Moderate High High Moderate Moderate Moderate Low Moderate Moderate Moderate Low High Low High Low Low Moderate High Moderate Moderate Moderate Moderate Moderate Moderate Moderate High Moderate High Moderate Low Moderate Moderate Low Moderate Moderate High Low High Moderate High Moderate Moderate High Moderate Moderate Moderate High Moderate Moderate Low Low Moderate Moderate Low Moderate High Moderate

(CZ)

Switzerland ,, Hungary

(C3) (C4) (C5) (C6)

Ditch in farmland

Ditch in aforest Rock pool Pond in urban area Ditch in farmland Rock pool

Ditch in a forest Pond in farmland Rock pool Pond in a forest Pond in urban area Rock pool Ditch in farmland Slowly moving stream Ditch in a forest Pond in farmland Pond in a forest Ditch in farmland Closed tributary

247

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H. KORPELAINEN

populations), Got (1population), Lap-I (3 populations), Lap-2 (1 population) and Mdh (3 populations); in D. longispina at Est-1 ( 1 population) and Est-2 (2 populations); in D . pulex at Alk-1 (1 population); and in D . cucullata at Est-2 (1 population) and Lap-I (1 population). Electrophoretic phenotypes and genetic structure of southern Finnish D. magna populations Ml-M22 are described in more detail in KORPELAINEN (1984).


D . longispina populations in southern Finland was somewhat higher than in southwestern Finland, but the mean diversity across all populations studied was greatly influenced by the highly differentiated central European populations. When all species are compared, it is noticeable that the differentiation of D . magna is clearly highest and the level of gene diversity of D . cucullata is second-highest. The differentiation of D . longispina and D . pulex is lowest and follows a similar pattern in both species. Fixation indices were computed for all samples at The amount and apportionment of heterozygosity loci with the expected heterozygosity of 0.10 or The apportionment of heterozygosity (gene diver- more. Whether the fixation indices departed signifisity) into between-population and within-popula- cantly (P 0.15 (e.g., AVISE1974; AYALA 1975). It is well known that electrophoretic techniques underestimate genetic divergence and are inadequate to measure true levels of genetic distance, but the problem can be partly alleviated with statistical corrections (NEI’SD statistic does this). D. Iongispina and D . pulex are genetically very similar (D=O.lOl) and form acluster later joined by D . cucullata and D . magna. This supports the general thought that D longispina and D . pulex are closely related species or species groups within the subgenus Daphnia. D . magna is the only member of the subgenus Ctenodaphnia among the four species and also in accordance with this study of genetic distances, it appears to be farther related than the others. However, it is somewhat surprising that its genetic distances to the species in the subgenus Daphnia are only slightly higher than the other between-species D values (except between D . longispina and D . pulex). Daphnia are morphologically difficult in their lack of large or distinct morphological characters useful in separating different species. The gradual and subtle differences of shape or form of the body, often complicated by cyclomorphosis, have turned (1981) out to be very difficult to describe. DODSON used m i - and multivariate analyses of the morphological variation of 18 characters for samples from 33 populations of D . pulex and failed to demonstrate any significant clusters of populations. A cluster analysis of the similarity of populations suggested that if distinct species do exist, they are not those presently recognized. The data supported either the view that the D . pulex species group is one widespread and variable species, or that it is comprising a much larger number of species than presently recognized. Electrophoretic studies of D . pufex indicate that populations actually consist of several clonal groups that show nearly complete reproductive isolation (HEBERT and CREASE1980, 1983; LYNCH 1983, 1984). In some cases, genetic crosses are impossible since many populations are completely unisexual. In the present study, the differentiation of D . longispina and D . pulex populations was lower than in other Daphnia species. The degree of differentiation in D . pulex might be biased because of the small number of samples examined. In any case, the conspicuously low genetic divergence between D . longispina and D . pulex, which is of the same order as the within-species diversities, and the high level of differentiation in D . magna and D . cucullata make the validity of Daphnia taxonomy doubtful. I t adds

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to the suggestion that a revision of Daphnia taxonomy is needed. Acknowledgements. -I am grateful to Dr. A. Saura for comments on the manuscript; to Mr. K. Tankersley and Ms. K. Vainio for valuable aid during the work; and to Dr. H. Gyurkovics, Ms. J. Kaukinen and Mr. M. Vogeli for help in collecting some of the Daphnia samples. This study has been supported by grants from the Emil Aaltonen Foundation and the Jenny and Antti Wihuri Foundation.

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