Mating Behavior and Sexual Isolation in the Drosophila Virilis Species Group

Abstract

The mating behavior of fourteen strains of the D. virilis group, representing two or more strains each of D. virilis, D. americana americana, D. americana texana, and D. novamexicana, when studied by direct observation, appears to be identical in all qualitative aspects although quantitative differences can be detected. The sexual isolation that occurs between the various strains is due to the interaction of two elements of the mating behavior, i.e., courtship discrimination and sexual drive. Both sexes exhibit these two elements in their mating behavior. Within the group of strains studied, courtship discrimination is mainly but not exclusively the function of the males, while both the males and females display high sexual drive. In the interstrain crosses, the degree of sexual isolation appears to be randomly distributed. Males of D. novamexicana display increased courtship discrimination toward those females farthest derived (geographically) from them. D. americana texana males show no consistent courtship discrimination toward females from any area. D. virilis males discriminate against all females except their own. In general the females of all species accept most frequently the overtures of those males that court most often and persistently. In the intra-strain crosses a definite balance exists between the two components of the sexual behavior of the males and females, and is of such a nature that it appears to be of adaptive value in ensuring copulations. Sexual isolation, on the basis of the data from this study, appears to have originated as either genetic drift within small populations, or responses to selection for local ecotypes, or both.