Incipient speciation by sexual isolation in Drosophila: Concurrent evolution at multiple loci

Drosophila melanogaster from Zimbabwe and nearby regions shows strong but a symmetric: sexual isolation from its cosmopolitan counterparts. By creating stable chromosome-substitution lines, earlier studies were able to show th at the two major autosomes have very large effects on both male mating succ ess and female mating preference, In this study, we genetically dissect thi s sexual isolation by recombination analysis between a whole-chromosome sub stitution line (which carries a Zimbabwe-derived third chromosome) and a st rain with seven visible markers on that chromosome. Four loci are responsib le for male mating success and three others are found to control female mat ing preference. Because male and female traits are not closely linked, thei r strong association among isofemale lines is most likely a reflection of s exual selection in nature. The results suggest that a large number of behav ioral loci may evolve concurrently in the incipient stage of speciation bef ore other aspects of reproductive isolation (such as hybrid sterility) have become evident. The results shed light on the population genetic processes underlying the formation of nascent species, as well as modes of speciatio n.