Population genetics and local adaptation in a cyclically parthenogenetic aquatic invertebrate: An examination of the population response of Daphnia pulex to contamination by de-icing road salt

The population genetics of Daphnia pulex from First and Second Sister Lakes was qualified. The lakes differed greatly in the amount of de-icing salt contamination, as was measured by conductivity. First Sister Lake had measurements that were consistently over twice as high as those of Second Sister Lake. These lakes are contiguous for part of the year via a marshy area. Allozyme electrophoresis was used to track the frequencies of genotypes during the summer months for two years. As expected, the greatest genetic diversity in both populations occurred at the time of ephippial (sexually produced) egg hatching in the spring. Generally, there was a decline in diversity over the summer, as expected with clonal growth and selection. The intrinsic rate of growth was estimated with data from chronic bioassays using dilutions of road salt and selected clones from both lakes. Standard quantitative genetics was used to determine the components of variance for each clone using the growth rate estimates. The environmental component of variance dominated the chronic response. There was a highly significant component of interaction (genotype x environment) indicating open evolutionary options for the populations with respect to road salt contamination. Genotype components of variance were insignificant for most clones, leading to the conclusion that most of the genotypes tested were "ecological equivalents" with respect to tolerance to road salt contamination. One exception was a clone present in large numbers in First Sister Lake at the end of summer (as a result of clonal selection). This clone was narrowly adapted to high concentrations of road salt. This is a case of the generation and maintenance of genetic variance in the wild, since this clone is unlikely to persist in lakes not experiencing road salt contamination. Population genetic divergence between the two lakes was significantly greater than expected. It is concluded that genotype-environment interactions play a key role in determining the chronic responses and adaptation of Daphnia pulex to de-icing road salt contamination in these two lakes.