Biochemical Factors Contributing to Response Variation among Resistant and Sensitive Clones of Daphnia magna Straus Exposed to Ethyl parathion

Biochemical factors contributing to response variation in five clones of Daphnia magna exposed to ethyl parathion were studied. Differences in sensitivities to ethyl parathion between sensitive and resistant clones varied between four- and ninefold. Acute toxicity and in vivo acetylcholinesterase (AChE) inhibition responses to ethyl parathion were similar, whereas in vitro AChE sensitivities to paraoxon were higher. Thus indicating that, in vivo, not all ethyl parathion present is converted by Daphnia juveniles to active metabolite, or if so, only a proportion of the paraoxon produced is in direct contact with its target. Comparison of AChE activities obtained during in vitro and in vivo exposures with acute responses revealed clonal concordance only between in vivo AChE sensitivities and acute toxicity. This latter finding indicates that clonal differences in ethyl parathion metabolism rather than differences in AChE sensitivities are the most likely explanations of the observed genetic differences in tolerance.