Neutral evolution of the sex-determining gene transformer in Drosophila

The amino acid sequence of the transformer (tra) gene exhibits an extremely rapid rate of evolution among Drosophila species, although the gene perfor ms a critical step in sex determination. These changes in amino acid sequen ce are the result of either natural selection or neutral evolution. To diff erentiate between selective and neutral causes of this evolutionary. change , analyses of both intraspecific and interspecific patterns of molecular ev olution of tra gene sequences are presented. Sequences of 31 tra alleles we re obtained from Drosophila americana. Many replacement and silent nucleoti de variants are present among the alleles; however, the distribution of thi s sequence variation is consistent with neutral evolution. Sequence evoluti on was also examined among six species representative of the genus Drosophi la. For most lineages and most regions of the gene, bath silent and replace ment substitutions have accumulated in a constant, clock-like manner. In ex on 3 of D. virilis and D. americana we find evidence for an elevated rate o f nonsynonymous substitution, but no statistical support for a greater rate of nonsynonymous relative to synonymous substitutions. Both levels of anal ysis of the tra sequence suggest that, although the gene is evolving at a r apid pace, these changes are neutral in function.