ABSTRACT
In the glacial lakes of the Palaearctic three species of Cladocera commonly coexist: Daphnia hyalina, D. galeata, and D. cucullata. Frequently these populations contain not only animals which are morphologically typical for the species but also individuals of an intermediate phenotype. Electrophoretic investigations of allozyme-patterns in morphologically typical individuals reveal that each species is fixed for a different allele at the GOT locus. Morphologically intermediate animals are heterozygous for the alleles of the two species which they resemble. The allelic pattern at other loci is also consistent with the assumption that morphological intermediates are formed via interspecific hybridization. Very few backcrosses between galeata-hyalina hybrids and their parent species are found, and there is no indication of gene flow between D. cucullata and the other species.
ABSTRACT
Cyclical parthenogenesis allows study of the genetic and evolutionary characteristics of groups exhibiting both asexual and sexual reproduction. The cladoceran genus Daphnia contains species which vary with respect to the relative incidence of sexual reproduction; pond species tend to undergo sexual reproduction more regularly than species found in large lakes. Previous genetic studies have focused on pond populations, generating expectations about large-lake populations that have not been fully met by recent studies. The present study of the Palearctic species Daphnia galeata further examines the genetic structure of large-lake populations. Nine local populations, from lakes in northern Germany, are examined for genetic variation at seven enzyme loci. Populations exhibit similar allelic arrays and often similar allele frequencies at the five polymorphic loci; values of Nei's genetic distance (D) ranged from 0.002 to 0.239, with a mean of 0.084. FST values range from 0.012 to 0.257, and spatial autocorrelation coefficients range from -0.533 to 0.551, for the eight alleles analyzed. With few exceptions, within-population genotypic frequencies were in Hardy-Weinberg equilibrium. There was, however, significant heterogeneity in genotypic frequencies among populations. The number of coexisting clonal groups, as determined by three locus genotypes, is high within populations. Clonal groups are widely distributed among localities. The amount of genetic divergence observed among these large-lake populations is smaller than that previously observed among pond populations and suggests that different processes may be important in determining the genetic structure and subsequent phenotypic divergence of lake versus pond populations.
