Biochemical factors contributing to response variation among resistant andsensitive clones of Daphnia magna straus exposed to ethyl parathion

Biochemical factors contributing to response variation in five clones of Da phnia magna exposed to ethyl parathion were studied. Differences in sensiti vities to ethyl parathion between sensitive and resistant clones varied bet ween four- and ninefold. Acute toxicity and in vivo acetylcholinesterase (A ChE) 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 activ e metabolite, or if so, only a proportion of the paraoxon produced is in di rect contact with its target. Comparison of AChE activities obtained during in vitro and in I,hto exposures with acute responses revealed clonal conco rdance only between in vivo AChE sensitivities and acute toxicity. This lat ter finding indicates that clonal differences in ethyl parathion metabolism rather than differences in AChE sensitivities are the most likely explanat ions of the observed genetic differences in tolerance. (C) 2001 Academic Pr ess.