Land use, genetic diversity and toxicant tolerance in natural populations of Daphnia magna.

Provided that gene flow is not too high, selection by local environmental conditions in heterogeneous landscapes can lead to genetic adaptation of natural populations to their local habitat. Pollution with anthropogenic toxicants may create pronounced environmental gradients that impose strong local selection pressures. Toxic contaminants may also directly impact genetic structure in natural populations by exhibiting genotoxicity or by causing population declines resulting in genetic bottlenecks. Using populations of Daphnia magna established from the dormant egg banks of ponds located in a landscape dominated by anthropogenic impact, we aimed at detecting evidence for local adaptation to environmental contamination. We explored the relationship between land use around the 10 investigated ponds, population genetic diversity as measured by neutral genetic markers (polymorphic allozymes) and the tolerance of the populations originating from these ponds to acute lethal effects of two model toxicants, the pesticide carbaryl and the metal potassium dichromate. Genetic diversity of the populations as observed by neutral markers tended to be negatively impacted by agricultural land use intensity (Spearman rank correlation, R=-0.614, P=0.059), indicating that genetic bottlenecks may have resulted from anthropogenic impact. We experimentally observed differences in susceptibility to both carbaryl and potassium dichromate among the studied pond populations of D. magna (analysis of deviance, P<0.001). Because the experimental design excluded the possibility of physiological adaptation of the test animals to the toxicants, we conclude that the differences in susceptibility must have a genetic basis. Moreover, carbaryl tolerance levels of the populations tended to increase with increasing agricultural land use intensity as described by ranked percentage of land coverage with cereal and corn crop in the proximity of the ponds (Spearman rank correlation, R=0.602, P=0.066). Together, these two findings provide evidence for local adaptation of D. magna populations to pesticide contamination. Overall, the results demonstrate the potential selection pressure imposed by anthropogenic pollution and provide evidence that genetic erosion in natural Daphnia populations is related to anthropogenic impact.