Complex epistasis and the genetic basis of hybrid sterility in the Drosophila pseudoobscura Bogota-USA hybridization

We analyzed the genetic basis of postzygotic isolation between the Bogota a nd USA subspecies of Drosophila pseudoobscura. These subspecies diverged ve ry recently (perhaps as recently as 155,000 to 230,000 years ago) and are p artially reproductively isolated: Bogota and USA show very little prezygoti c isolation but form sterile F-1 males in one direction of the hybridizatio n. We dissected the basis of this hybrid sterility and reached four main co nclusions. First, postzygotic isolation appears to involve a modest number of genes: we found large chromosome regions that have no effect on hybrid f ertility. Second, although apparently few in number, the factors causing hy brid sterility show a remarkably complex pattern of epistatic interaction. Hybrids suffer no hybrid sterility until they carry the "right" allele (Bog ota vs. USA) at at least four loci. We describe the complete pattern of int eractions between all chromosome regions known to affect hybrid fertility. Third, hybrid sterility is caused mainly by X-autosomal incompatibilities. Fourth, hybrid sterility does not involve a maternal effect, despite earlie r claims to the contrary. In general, our results suggest that fewer genes are required for the appearance of hybrid sterility than implied by previou s studies of older pairs of Drosophila species. Indeed, a maximum likelihoo d analysis suggests that roughly 15 hybrid male steriles separate the Bogot a and USA subspecies. Only a subset of these would act in F-1 hybrids.