The genetics of pre-mating isolation at the incipient stage of speciation in Drosophila melanogaster

A central question of Evolutionary Biology is what sorts of genetic changes underlie the process of speciation. To address this question, I examined characters that contribute to mate choice in a model system. The Zimbabwe (Z/M) system of D. melanogaster is an apparent case of incipient speciation (HOLLOCHER et al. 1997b; Wu et al. 1995). Zimbabwe (Z) lines exhibit asymmetric premating isolation with lines from the rest of the world (M). The pre-mating barrier between Z females and M males is nearly absolute. Gene flow between Z and M races is reduced but not eliminated.;In this thesis, I characterize Z and M mating behavior in detail, map the chromosomal regions contributing to behavioral differences, and demonstrate a correlation between mate choice and specific behaviors. The differences all map to the right arm of the third chromosome, a region that is highly differentiated between Z and M and includes the candidate gene fruitless . I have also shown that both Z and M females exhibit mate choice.;I demonstrated that Z and M races differ in time of day of peak mating behavior. This difference may be an indication of allochronic speciation, or divergence that occurs due to different timing of reproductive events.;I showed that desat2 contributes to make choice in the M genetic background. desat2 encodes a fatty acid desaturase which determines the cuticular hydrocarbon (pheromone) profile of D. melanogaster females. Z strains carry a functional allele whereas M strains carry a non-functional allele. Our gene-replacement experiments implicate desat2 in ecological adaptation: it confers starvation tolerance but hinders cold tolerance. Using double-choice mating experiments under low-light conditions (depriving the flies of visual cues), I have shown that the gene-replacement lines display assortative mating. Thus, desat2 changes mating behavior. Together, these experiments suggest that, as D. melanogaster spread out of southern Africa, it was exposed to the selective pressure of changing ecological conditions. Reproductive isolation may have evolved as a byproduct of ecological adaptation.