Rapidly evolving genes of Drosophila: differing levels of selective pressure in testis, ovary, and head tissues between sibling species.

Investigations of rapidly evolving sex- and reproduction-related genes are expected to reveal important information about the process of speciation and species divergence. We screened testis, ovary, and head tissues to identify and characterize rapidly evolving genes (REGs) between closely related species. The results show differential patterns of evolution of genes expressed in reproductive and nonreproductive tissues. (1) There is a differential distribution of REGs in the Drosophila genome, with most REGs localized in the testis, followed by ovary, and then head. (2) Sequence analysis indicates that differential selective pressures are driving the rapid evolution of genes expressed in sex and nonsex tissues. Testis REGs from our data, on average, yielded higher rates of nonsynonymous substitutions relative to transcripts in ovary and head, indicating stronger selective pressures on the male reproductive system. (3) We identified REGs in the testis, ovary, as well as in head tissue that show evidence of evolving under positive selection. Identification of rapidly evolving sex genes is important for detailed investigations of cryptic female choice, sexual conflict, and faster male evolution and is pertinent to our understanding of the process of species divergence and speciation.