Factors Affecting Sexual Isolation between Drosophila melanogaster and Drosophila simulans

Abstract

An analysis was made of the effects of reciprocal crosses, male and female age, number of flies per mating bottle, and sex ratio on the degree of sexual isolation. When the isolation was measured as the percentage of fertile mating bottles it was higher (as expected) than when measured as the percentage of females fertilized. This resolves the apparent discrepancies between the results of earlier investigators (using mass matings, generally with an excess of males, and measuring percentage of fertile mating bottles), and those of Barker (1962, using pair matings, and measuring percentage of fertilized females). The frequency of interspecific hybridization increased with increase in the proportion of males (1:5, 1:1, 5:1), and was higher for the reputedly less successful D. melanogaster [female]~ x D. simulans ~[male] than for the reciprocal cross. Further, the effects of male and female age were not as expected from previous studies, particularly for the cross D. simulans ~[female] x D. melanogaster [male]~. Results are discussed in terms of the effect of age on inter-species mating discrimination, and the possibility of variation between strains of each species in such discrimination. The variety of factors affecting sexual isolation is such that a single measure of the degree of isolation between two species is meaningless, except for the specific experimental conditions used.