Differential response to environmental alcohol among second-chromosome arrangements in experimental populations of Drosophila buzzatii.

Drosophila buzzatii feeds and breeds on the decaying cladodes and fruits of several species of Opuntia (prickly pear) which contain significant levels of ethanol and isopropanol. The potential influence of these two alcohols on the inversion polymorphism of the second and fourth chromosomes was investigated in ten experimental populations with different amounts of alcohol (either ethanol or isopropanol) added to the culture medium. All populations were started with the offspring of 29 wild females collected at Adeje (Tenerife, Canary Islands) and their genetic composition was monitored for about two years (more than 30 generations). Consistent changes in the frequency of most second- and fourth-chromosome arrangements occurred in all populations including those without alcohol (control). The comparison of inversion frequency through the various treatments revealed a significant influence of the alcohol on the frequency changes of the four second-chromosome arrangements. Moreover, this influence was of a different type for every one of them, sometimes with opposite effects between alcohols and/or concentrations. These results indicate genetic differentiation among second-chromosome arrangements with regard to alcohol and suggest that the alcohol heterogeneity found in the species' trophic niche may play an important role in the maintenance of this polymorphism and also in the recent historical changes in the frequency of some arrangements associated with colonization.