Short-range genetic structure of Drosophila melanogaster populations in an Afrotropical urban area and its significance.

Alcohol dehydrogenase (Adh) (alcohol:NAD+ oxidoreductase, EC 1.1.1.1) gene frequencies and ethanol tolerance in Drosophila melanogaster are known to exhibit long-range latitudinal variations on different continents; this has led to the argument that the clines are adaptive. Accordingly, tropical populations are characterized both by a low frequency of Adh-F and by a low ethanol tolerance. In the urban area of Brazzaville (Congo) under an equatorial African climate, an original genetic structure of local populations has been found: Adh-F frequency varies from 3% to 90% when countryside and brewery populations are compared. This variation is accompanied by an increase of ethanol tolerance (from 6% to 13% alcohol). Such differences, which have remained stable for the past 3 years, were observed between collection sites less than 1 km apart. Two other enzyme loci exhibited a correlated variation with Adh-F--i.e., an increase of the S allele of glycerol-3-phosphate dehydrogenase (NAD+) (sn-glycerol-3-phosphate:NAD+ 2-oxidoreductase, EC 1.1.1.8) and of the F allele of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1-oxidoreductase, EC 1.1.1.49). Such observations suggest very strong selective pressures exerted by environmental ethanol that oppose the gene flow due to adult dispersal between contiguous habitats. A functional relationship between the polymorphisms of the three enzyme loci seems likely, and a metabolic interaction involving NAD and NADP cofactors is proposed.