A locus for female discrimination behavior causing sexual isolation in Drosophila

The genetic basis of sexual isolation that contributes to speciation is one of the unsolved questions in evolutionary biology. Drosophila ananassae an d Drosophila pallidosa are closely related, and postmating isolation has no t developed between them. However, females of both species discriminate the ir mating partners, and this discrimination contributes to strong sexual is olation between them. By using surgical treatments, we demonstrate that mal e courtship songs play a dominant role in female mate discrimination. The a bsence of the song of D. pallidosa dramatically increased interspecies mati ng with D. ananassae females but reduced intraspecies mating with D. pallid osa females. Furthermore. genetic analysis and chromosomal introgression by repeated backcrosses to D. pallidosa males identified possible loci that c ontrol female discrimination in each species. These loci were mapped on dis tinct positions near the Delta locus on the middle of the left arm of the s econd chromosome. Because the mate discrimination we studied is well develo ped and is the only known mechanism that prevents gene flow between them, t hese loci may have played crucial roles in the evolution of reproductive is olation, and therefore, in the speciation process between these two species .