MALE MATING DISCRIMINATION IN DROSOPHILA MELANOGASTER, D. SIMULANS AND THEIR HYBRIDS.

Little is known about the relative importance of the sexes in maintaining reproductive isolation between closely related species in Drosophila. A great deal of evidence demonstrating the importance of females in sexual selection in Drosophila has been amassed, but this work concerns only intraspecific courtship and copulation (Petit and Ehrman, 1969; Spiess, 1970; Ehrman, 1972). The picture regarding interspecific courtship is not clear. It may be that the female determines whether or not copulation will occur between closely related species to the same extent as she does within her own species (Merrell, 1954). However, it may be that males can discriminate well, and do not court interspecifically as readily or persistently as they do intraspecifically. If this is the case, males would exert primary control over the maintenance of ethological isolating barriers, regardless of the female's discriminatory abilities, since the male initiates courtship and copulation. Spieth (1974) has argued that in Drosophila it is indeed the male that is primarily responsible for sexual isolation in nature. One of the best ways to determine whether or not a male has an important role in maintaining ethological isolating barriers is actually to observe and measure interspecific courtship. Direct observation experiments using the Drosophila melanogaster group of species has added support to Spieth's position. Von Schilcher and Dow (1977) found that male Drosophila melanogaster and Drosophila simulans do not court females of their sibling species as persistently or intensely as their own and that it is the males who most often terminate interspecific courtship, while it is the females who usually cause the termination of intrasDecific courtshiD. Manning (1959), working with D. melanogaster and D. simulans, also found a reluctance on the part of males to court interspecifically, but in addition, he found that this reluctance was stronger in D. simulans. Von Schilcher and Dow (1977) did not find such a difference between the species. Both Manning (1959) and von Schilcher and Dow (1977) looked only at pair matings. To date, no one has reported if males in the D. melanogaster species group discriminate to the same extent when both conspecific females and females from a closely related species are present, a situation that is presumably the more natural and meaningful one. Also, mating discrimination in the male hybrids of D. melanogaster and D. simulans has not been studied extensively. Von Schilcher and Manning (1975) measured mating speed and certain parameters of the wing vibration of hybrids, but they did not analyze their discriminatory abilities. Possibly an analysis of the discrimination shown by hybrids will give some clue to the genetic basis of male discrimination in the parent species. The purpose of this study, then, was to examine the abilities of male D. melanogaster, D. simulans and their hybrids to discriminate between conspecific and nonconspecific females in "no choice" pair matings and "male choice" situations to evaluate both the probable importance of the male sex in maintaining ethological isolating barriers and the genetic basis of these abilities.