A possible novel link between organophosphorus and DDT insecticide resistance genes in Anopheles: Supporting evidence from fenitrothion metabolism studies: Supporting evidence from fenitrothion metabolism studies

Fenitrothion metabolism studies on a multiple organophosphorus resistant strain of Anopheles subpictus from Sri Lanka indicated that oxidases and glutathione S-transferases play a major role in the detoxication of this insecticide. The oxidases, as well as producing detoxication products, also produce two highly active acetylcholinesterase inhibitors, fenitrooxon and 3-hydroxymethyl fenitrooxon. Further metabolism studies on fenitrooxon indicated that the glutathione S-transferases in A. subpictus were able to detoxify these activation products. DDT resistance in the field population of A. subpictus has increased since 1983, although there has been no direct DDT selection pressure during this period. DDT resistance in this species is due to an increase in DDT dehydrochlorination which is correlated with an increase in glutathione S-transferase activity. It is possible that the same enzyme is responsible for both DDT and secondary organophosphate (fenitrothion) metabolism. If so, organophosphorus insecticide selection pressure will actively maintain, or select for, DDT resistance in this population. Linkage disequilibrium between the oxidase and glutathione S-transferase-based resistance genes would be expected if the same glutathione S-transferase fulfils both functions, and this is now being investigated in the field population in Sri Lanka.