Morton_1985_Pestic.Biochem.Physiol_24_19

The basis of malathion resistance was examined by comparisons of susceptible and resistant strains of Drosophila melanogaster. Increased levels of mixed-function oxidase (MFO) activity were highly correlated (r = +0.95, P < 0.001, N = 7) with resistance, as was glucose-6-phosphate dehydrogenase activity (r = +0.85, P < 0.01, N = 7). Cross-resistance to other insecticides and the synergistic effect of MFO inhibitors suggested that malathion was oxidatively degraded in these Drosophila strains. Fly extracts and -Ymicrosomal fractions catalyzed the NADPH-dependent oxidation of [14C]malathion to malaoxon and of [14C]malaoxon to a major water-soluble product tentatively identified as the malaoxon beta-monoacid. Both activities were greater in resistant than in susceptible strains. Differences in carboxylesterase and glutathione-S-transferase activities in fly extracts were not associated wth resistance. The malathion resistance of the selected strains may also have partly depended upon the presence of an altered acetylcholinesterase (AChE) which reacted more slowly with malaoxon, although both the altered AChE and high MFO activity were required for the full expression of resistance.