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Drosophila melanogaster; DDT (Insecticide); Chromosome inversions


Inversion polymorphism may be maintained by some form of balancing selection, A laboratory population maintained for many years at small size should contain little genetic variation due to the effects of genetic drift. This study analyzed a Drosophila population which exhibits a persistent inversion polymorphism. The population has been subjected to selection for DDT resistance since 1959. The purpose of this study was to determine if: a) balancing selection was involved and b) if the selection was DDT-related. To study these questions, three pairs of populations were set up at different inversion frequencies: 95%, 55% (equilibrium value) and 5%. One population of each pair was exposed to DDT. Changes in inversion frequencies were determined over a period of about four months. All populations returned to near equilibrium inversion frequencies indicating balancing selection. Unexpectedly, the DDT-exposed populations seemed to return to equilibrium at a slower rate. It was hypothesized that DDT decreased larval density and hence the intensity of selection. Two additional projects were initiated. First, larval population density in the DDT -exposed and non-exposed populations was measured; non-exposed populations were about twice as densely populated. Second, the effects of artificially lowering larval density on balancing selection were studied. Selective intensities were apparently relaxed in the low density populations.

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