Disruptive Selection for Sternopleural Chaeta Number in Various Strains of Drosophila melanogaster

Abstract

Disruptive selection for sternopleural chaeta number was carried out on various laboratory and wild strains of Drosophila melanogaster. The methods employed were those described by Thoday and Gibson (1962). Experimentation was carried out for 10 generations in some strains and for 42 generations in others. Sterility was a common problem in the selected low lines. Reproductive isolation was not obtained in any of the strains, and preferential mating was not observed during the course of the experimentation. However, differences in response to disruptive selection by different strains were clearly shown. One strain (South Africa) showed significant divergence between high and low chaeta numbers only after 10 generations. Another (Ithaca) showed significant divergence between high and low chaeta numbers after five generations of selection. Still other strains (Bermuda and Monroe County) showed a response only after 27 generations of selection. It is concluded that disruptive selection for sternopleural chaeta number leads to reproductive isolation in the laboratory very rarely. An attempt was made to distinguish between divergence for chaeta number with eventual reproductive isolation as observed by Thoday and Gibson (1962) and divergence for chaeta number with no subsequent mating preference or reproductive isolation as observed in the experiments reported here.