Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits.

Two genetically distinct habitat races of Drosophila melanogaster coexist in Brazzaville (Congo). One is the typical field type of Afrotropical populations, the other mainly breeds in beer residues in breweries. These two populations differ in their ethanol tolerance, in their allelic frequencies at several enzyme and microsatellite loci and in the composition of their cuticular hydrocarbons. The brewery population is quite similar to European temperate populations with regard to all these traits. Previous investigations of two morphological traits (ovariole number and sternopleural bristle number) failed to detect any difference between the two habitat races. Here we investigated other morphological traits (wing and thorax length, thorax pigmentation and female abdomen pigmentation). The reaction norms of these traits according to growth temperature were compared in the two Afrotropical habitat races and in a French temperate population. As expected, the French population was very different from the field African population: as a general rule, the brewery population (Kronenbourg) was intermediate in several aspects between the other two. We conclude that the strong selective forces that maintain the genetic divergence between the two habitat races also act on morphometrical traits, and the possible selective mechanisms are discussed.