Enzyme polymorphism and cyclic parthenogenesis in Daphnia magna. I. Selection and clonal diversity.

Genotype frequencies and fecundities were recorded over a period of two years for three polymorphic enzyme loci (Est, Mdh and Got) in a parthenogenetic natural population of Daphnia magna Straus (Crustacea: Cladocera). There was a large excess of heterozygotes at each locus, and some nonrandom association between loci, although 29 different three-locus genotypes were detected. There were small but significant changes in genotype frequencies that did not follow any clear seasonal cycles or overall trends, and the genotypes often differed significantly in fecundity, although the direction of the difference was not constant. These fitness differences were probably not attributable to the specific loci studied.--Models of balancing selection are of two types: segregation-balanced (e.g., heterosis) and competition-balanced (e.g., frequency dependence). Only the latter type can stabilize diversity in a clonal population. The observed selection was not heterotic, but it is not certain that it was stabilizing either. Clonal competition did not lead to victory by a single, fittest clone; genotypic diversity remained high.