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Many plant secondary compounds induce detoxification activity in herbivorous insects. Although inducibility may be advantageous as a means of reducing costs associated with maintenance of metabolism, another benefit of inducibility is that it may allow insects to tailor their detoxification profiles to multiple substrate toxins in their diets. The parsnip webworm, Depressaria pastinacella, must contend with many types of furanocoumarins, toxins present in abundance in all of its host plants. Previous studies have documented that cytochrome P-450s are responsible for metabolism of furanocoumarins in this species and that this overall activity is inducible. In this study, we examined the effects of ingestion of single furanocoumarins on metabolism of multiple furanocoumarins and the ability of webworms to adjust their metabolism profiles to match artificial diets with furanocoumarin content differing qualitatively and quantitatively from the average content found in their principal host. That detoxification rates of newly molted sixth instars prior to feeding did not differ from those of actively feeding fifth or sixth instars suggests that constitutive activities of furanocoumarin-metabolizing enzymes are maintained in the absence of substrates. All of the induction assays in this study were performed with ultimate instars. Each of the furanocoumarins assayed was found to induce metabolism of five different furanocoumarin substrates; however, the induction profile was independent of the inducing agent. Consistent with this finding, webworms were incapable of matching their detoxification profiles to diets with different furanocoumarin compositions. Thus, the profile of detoxification within individuals of this species appears to be genetically fixed, although there is considerable variation in profiles among individuals. |
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