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

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Abstract
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 modes of action and known sublethal effects—cadmium, copper, and fluoranthene—were selected to test the previously mentioned hypothesis. Specifically, we compare how genetic variability in feeding responses among the studied Daphnia clones changed across increasing nonlethal exposure levels (expressed as increased exposure levels of feeding inhibition, EC) and how these changes converge relative to acute lethal responses (expressed in terms of the 48‐h LC50). Our data show that cadmium fully supports the convergence hypothesis and that data with fluoranthene may also support it. Nevertheless, the most important findings of the present study are to have provided experimental evidence that (1) lethal responses in acute exposures and sublethal responses such as feeding inhibition are not related, (2) genetic variability for sublethal responses tend to be lower than acute responses, and (3) trade‐offs between low and high sublethal exposure levels rather than genetic heterogeneity governed sublethal responses. The first two findings have important implications for risk assessment if further supported with more data on other toxicants and clones since they suggest that concerns voiced about the need to include more genetic variability in chronic toxicity tests may be groundless. The third finding may indicate that selection under low and variable environmental levels of toxicants with sublethal effects of feeding may select genotypes with high phenotypic plasticity in response to toxic stress rather than genotypes that perform well within particular toxicant exposure levels.