Clonal diversity and turnover in an overwinteringDaphnia pulexpopulation, and the effect of fish predation

Abstract

SummaryDaphniaare key species in lake ecosystems and are a model system in ecology. Their ecological success is very much due to their dual reproductive mode: females reproduce parthenogenetically when environmental conditions are favourable but switch to sexual reproduction and resting egg production in unfavourable conditions such as winter. Phases of clonal reproduction lower the overall genotypic diversity ofDaphniapopulations due to the random loss of genotypes. During phases of sexual reproduction, new genotypes enhance the genotypic variability of the population and compensate for the loss of clonal lineages.We hypothesised that clonal diversity should be low during the winter months due to high mortality rates (clonal erosion) but will be restored in the spring most likely from the resting egg bank. To test this hypothesis, we investigated the clonal composition of aDaphnia pulexpopulation in a small mesotrophic lake in Norway.Daphniawere sampled every month from September 2007 to September 2010, andc. 60 individuals per month were genotyped using 11 microsatellite markers. Marker resolution was high, and 582 different genotypes could be detected in the 936 individuals analysed.An unexpected complication for the study was the illegal introduction of planktivorous fish into the lake between autumn 2006 and spring 2007. This led to significant predation pressure on theD. pulexpopulation and shifted the cladoceran community to smaller species. However, while theD. pulexpopulation density declined severely over the study years, the overall genotypic variability increased. We observed very high turnover rates, with many clones being present for only a single month. Nevertheless, and as expected, genotypic diversity was higher during summer and lower during winter, and some parthenogenetic individuals seemed to be able to survive the winter. The large majority of new genotypes came from the resting egg bank.We conclude that, due to the new predation pressure, the resting egg bank has become much more important in restocking genotypic diversity as well as forD. pulexabundance. However, despite the buffering capacities of the resting egg bank, the predation pressure will most likely lead to the extinction ofD. pulexin Lake Myravatn.