Opposing Modes of Selection on the Alcohol Dehydrogenase Locus in Drosophila Melanogaster

Abstract

Three experiments have been carried out which show that exogenous environments of ethanol impose selection on the alcohol dehydrogenase (Adh) locus of D. melanogaster. This locus is widely polymorphic for two alleles, AdhF and Adhs, and AdhF generally produces about twice as much alcohol dehydrogenase activity as Adhs. In the first experiment, AdhF IAdhF and AdhF/Adhs flies survived equally often and Adhs/Adhs flies less frequently after exposure for 7 days to medium impregnated with ethanol. The same pattern of survival differences was found in the second experiment in which flies were exposed for 1 day to an aqueous solution of ethanol and sucrose. In contrast, in the third experiment survival was scored after exposure for 45-min to ethanol fumes, and Adhs/ Adhs flies survived more often than AdhF/Adhs, both these genotypes surviving more frequently than Adh F / Adh F. We doubt whether anyone of the three experiments by itself adequately represents the ecology of natural populations of D. melanogaster exposed to ethanol. It is likely that mixtures of the three experimental conditions approximate more closely the natural environments and therefore we suggest that, overall, selection might favour intermediate levels of alcohol dehydrogenase activity, producing a net advantage for heterozygotes at the Adh locus.