Variation in toxicity of a current-use insecticide among resurrected Daphnia pulicaria genotypes

This study examined how genotypes of Daphnia pulicaria from a single population, separated by thousands of generations of evolution in the wild, differ in their sensitivity to a novel anthropogenic stressor. These genotypes were resurrected from preserved resting eggs isolated from sediments belonging to three time periods: 2002–2008, 1967–1977, and 1301–1646 A.D. Toxicity of the organophosphate insecticide chlorpyrifos was determined through a series of acute toxicity tests. There was a significant dose–response effect in all genotypes studied. Moreover, significant variation in toxicity among genotypes within each time period was detected. Importantly, a significant effect of time period on sensitivity to chlorpyrifos was found. Analysis of the median effect concentrations (EC50s) for genotypes within each time period indicated that the 1301–1646 genotypes were 2.7 times more sensitive than the 1967–1977 genotypes. This trend may be partially explained by microevolutionary shifts in response to cultural eutrophication.