Microevolution in predominantly clonal populations of pelagic Daphnia (Crustacea: Phyllopoda): Selection, exchange, and sex

Abstract

The study addresses the problem of gene pools and units of microevolution in mainly apomictic species. The genetic variation of Daphnia cucullata at the phosphoglucose‐isomerase locus was studied in 6 lakes for 3–6 consecutive years. The two most distant lakes were 12 km apart. All lakes were sampled once a year; in one of the lakes (Klostersee), the main basin (WT) and the largest bay (WB, volume 1/7 of WT) were sampled every month for 2½ years. Three alleles and all six expected genotypes were found. Spatial differentiation was substantial, indicating lake‐specific selection. There was a geographic gradient of allele frequencies without an apparent environmental gradient, suggesting exchange processes. Genotype frequencies in WB and WT showed marked fluctuations. They differed consistently between the two parts of the lake yet were highly correlated. The correlation was best if the WB frequencies were allowed to lag about one month behind the WT frequencies. This is to be expected if bay and basin experience different selection pressures but exchange parts of their populations, which requires some time. No parallel time lags were observed in environmental or demographic parameters. There was a significant correlation between fecundity (eggs/female) and genotype frequency, suggesting clone‐specific differences in fitness. Biparental reproduction was very rare and superimposed on a continuum of parthenogenesis. Deviations from Hardy‐Weinberg proportions were common and significant unless one allele was predominant. The direction of deviation of a given genotype frequency was usually the same in all lakes, suggesting overall differences in fitness between genotypes apart from lake‐specific conditions of selection. Heterozygote excess dominated. Biparental reproduction appears to be unimportant for the observed changes in genotype frequencies. The populations behave as if they are composed of independent clones. The main function of biparental reproduction may be the occasional replenishment of genotypic diversity and dispersal via resting eggs. The smallest local entities with distinct selection characteristics seem to be sections of lakes, but microevolution may involve several lakes because of exchange processes. Copyright © 1990, Wiley Blackwell. All rights reserved