Overexpression of Methoprene-tolerant, a Drosophila melanogaster gene that is critical for juvenile hormone action and insecticide resistance

Abstract

The Methoprene-tolerant (Met) gene of Drosophila melanogaster is involved in both juvenile hormone (JH) action and resistance to JH insecticides, such as methoprene. Although the consequences of Met mutations on development and methoprene resistance are known, no studies have examined Met+ overexpression. Met+ was overexpressed in transgenic lines with various promoters that drive overexpression to different levels. Flies expressing either genomic or cDNA Met+ transgenes showed higher susceptibility to both the morphogenetic and toxic effects of methoprene, consistent with the hormone-binding property of MET. Both the sensitive period and lethal period were the same as seen for non-overexpressing Met+ flies. However, continual exposure of high-overexpressing Met+ larvae to borderline-toxic or higher methoprene doses advanced the sensitive period from prepupae to first instar and the lethal period from pharate adults to larvae and early pupae. When expression of transgenic UAS-Met+ was driven to high levels by either an actin-GAL4 or tubulin-GAL4 promoter, larvae showed high mortality in the absence of methoprene, indicating that high MET titer is lethal, perhaps resulting from expression in an inappropriate tissue. Adults overexpressing Met+ did not show enhanced oogenesis, ruling out MET as a limiting factor for this hormone-driven physiology.

Figure 1

RT-PCR determinations of Met transcripts from late third-instar larvae and adults of the Met⁺ overexpressing strains. The strain labeled w; Met⁺/actin expresses the UAS-Met⁺ transgene driven by GAL4-actin, and the strain labeled w; Met⁺/TM3 is a sibling TM3-balancer control.

Figure 2

Stage-specific death of w; p[V2] individuals raised on one of three lethal doses of methoprene compared with w raised on 26.8 μg/vial. Values for larval death were determined by subtracting the number of dead pupae from the total individuals in each of 3 separate cultures of 30. The predominant pharate adult stage of death of w is typical of the effect of methoprene on non-overexpressing Met⁺ strains. Error bars represent SEM values.