Do genotype responses always converge from lethal to nonlethal toxicant exposure levels? Hypothesis tested using clones of Daphnia magna straus

This study examines the hypothesis that genotypic differences in tolerance will converge with decreasing levels of toxicity: from lethal to sublethal responses and from high to low sublethal exposure levels of toxic chemicals . Four laboratory Daphnia magna clones and three chemicals with different m odes of action and known sublethal effects-cadmium, copper, and fluoranthen e-were selected to test the previously mentioned hypothesis. Specifically, we compare how genetic variability in feeding responses among the studied D aphnia clones changed across increasing nonlethal exposure levels (expresse d as increased exposure levels of feeding inhibition, EC) and how these cha nges converge relative to acute lethal responses (expressed in terms of the 48-h LC50). Our data show that cadmium fully supports the convergence hypo thesis and that data with fluoranthene may also support it. Nevertheless, t he most important findings of the present study are to have provided experi mental evidence that(I) lethal responses in acute exposures and sublethal r esponses such as feeding inhibition are not related, (2) genetic variabilit y for sublethal responses tend to be lower than acute responses, and (3) tr ade-offs between low and high sublethal exposure levels rather than genetic heterogeneity governed sublethal responses. The first two findings have im portant implications for risk assessment if further supported with more dat a on other toxicants and clones since they suggest that concerns voiced abo ut the need to include more genetic variability in chronic toxicity tests m ay be groundless. The third finding may indicate that selection under low a nd variable environmental levels of toxicants with sublethal effects of fee ding may select genotypes with high phenotypic plasticity in response to to xic stress rather than genotypes that perform well within particular toxica nt exposure levels.