SEXUAL ISOLATION STUDIES IN THE SPECIES COMPLEX DROSOPHILA VIRILIS

Abstract

The Pasadena strain of Drosophila virilis virilis was tested for sexual iso- lation with strains of D, v. americana from four localities in Ohio. In experiments in which males of one subspecies are placed with equal numbers of females of the other, virilis shows sexual isolation in matings with all americana strains. The matings in which virilis enters as the male show sig- nificantly greater isolation than the reciprocal ones. The americana strains vary in regard to the sexual isolation they show when mated to virilis. Similar “no choice” experiments show no sexual isolation among any of the four americana strains. In experiments in which males are placed with equal numbers of conspecific and alien females, matings between the two subspecies show that the males exercise choice and inseminate more of the females of the con- specific than the alien strain. Similar matings involving a choice of females demonstrate sexual isolation among all americana strains. This isolation is much less complete than that found between americana and virilis. Strains of americana of the same geographical origin may behave differently in crosses to virilis. The degree of isolation existing among different americana strains in intraspecific matings does not appear to be correlated with the distance separating the localities from which they were collected.Observations on pair-matings between and within the two subspecies show that the sexual isolation mechanisms in interspecific crosses depend on two types of behavioristic peculiarities. One type is normally found in intraspecific matings; the other type, found only in interspecific matings, is associated with lack of coordination between the male and female.