Patterns of evolution of genes disrupted in expression in Drosophila species hybrids.

Divergence between species in regulatory pathways may contribute to hybrid incompatibilities such as sterility. Consistent with this idea, genes involved in male fertility often evolve faster than most other genes both in amino acid sequence and in expression. Previously, we identified a panel of male-specific genes under-expressed in sterile male hybrids of Drosophila simulans and D. mauritiana relative to pure species, and we showed that this under-expression is associated with infertility. In a preliminary effort to assess the generalities in the patterns of evolution of these genes, I examined patterns of mRNA expression in three of these genes in sterile F 1 hybrid males of D. pseudoobscura and D. persimilis . F 1 hybrid males bearing D. persimilis X chromosomes under-expressed all these genes relative to the parental species, while hybrids bearing D. pseudoobscura X chromosomes under-expressed two of these three genes. Interestingly, the third gene, CG5762 , has undergone extensive amino acid evolution within the D. pseudoobscura species group, possibly driven by positive natural selection. We conclude that some of the same genes exhibit disruptions in expression within each of the two species groups, which could suggest commonalities in the regulatory architecture of sterility in these groups. Alternative explanations are also considered.