Abstract
The asymmetric sexual isolation between Zimbabwe (Z-type) and Cosmopolitan (M-type) populations in Drosophila melanogaster provides an opportunity to investigate the incipient speciation process. Under multiple choice conditions, Z females mate only with males of their own type while M females accept both types of males. This reproductive isolation system raises several interesting issues such as the evolutionary forces maintaining the Z/M polymorphism, the candidate discrimination cues of male characters, and the female recognition mechanism. In this study, I investigated the differences of courtship behavior between two races which probably play an important role in preventing gene flow at premating stage. The courtship song and behavioral sequential module were inferred as possible discrimination cues. Furthermore, through observing the interaction between two sexes in interracial crosses, the previous hidden M female preference was revealed, which could contribute on the Z/M polymorphism maintenance. Cuticular hydrocarbons were further identified to be discrimination cues of M females via perfuming experiments and single choice behavior assays. The third chromosome has the predominate role on male pheromones recognized by M females. By surgically cutting the olfactory organs, I demonstrate that both olfaction and gustation involved in M female discrimination but the discrimination in M/Z (via olfactory system) and that in polymorphic Z races (at least via gustatory system) are different. Through different experimental designs, we can further understand the details between recognition traits and preference in this premating isolation system.