Effects of a low dose of ethanol on mating success ofDrosophila melanogastermales: implications for the evolution of ethanol resistance?

Ethanol occurs naturally in the decaying fruit in which many species of Drosophila (Diptera: Drosophilidae) breed, potentially generating selection for resistance to its toxic and sedating effects. Studies measuring mortality of flies exposed to a range of ethanol concentrations have shown that within Drosophila melanogaster Meigen, populations from temperate regions are more ethanol resistant than ancestral tropical African populations. The high ethanol resistance of temperate D. melanogaster presents a puzzle, however, because breeding and feeding sites in the wild seldom contain enough ethanol to kill even more ethanol-sensitive Afrotropical genotypes. We hypothesize that the ethanol concentrations encountered by temperate populations, though usually sub-lethal, are nonetheless high enough to reduce fitness in other ways, potentially generating indirect selection for genotypes that can survive exposure to unnaturally high ethanol concentrations. As a first step in testing this hypothesis, we compared the effects of a sub-lethal dose of ethanol, comparable to that obtainable from fermenting fruit, on the mating success of males from one European and one Afrotropical population. Ethanol significantly reduced mating success of males from the Afrotropical population, but had no effect on that of males from the European population. We also show that when flies are placed on medium with a realistic concentration of ethanol, considerably more ethanol is absorbed through vapor than through feeding, suggesting that courting males may be unable to avoid being exposed to ethanol. We hypothesize that the higher resistance of temperate populations to being killed by high, unnatural ethanol concentrations may have evolved in part as a correlated response to selection for behavioral insensitivity to natural concentrations.