Molecular evolution of sex-biased genes in Drosophila

Studies of morphology, interspecific hybridization, protein/DNA sequences, and levels of gene expression have suggested that sex-related characters (particularly those involved in male reproduction) evolve rapidly relative to non-sex-related characters. Here we report a general comparison of evolutionary rates of sex-biased genes using data from cDNA microarray experiments and comparative genomic studies of Drosophila. Comparisons of nonsynonymous/synonymous substitution rates (d(N)/d(S)) between species of the D. melanogaster subgroup revealed that genes with male-biased expression had significantly faster rates of evolution than genes with female-biased or unbiased expression. The difference was caused primarily by a higher d(N) in the male-biased genes. The same pattern was observed for comparisons among more distantly related species. In comparisons between D. melanogaster and D. pseudoobscura, genes with highly biased male expression were significantly more divergent than genes with highly biased female expression. In many cases, orthologs of D. melanogaster male-biased genes could not be identified in D. pseudoobscura through a Blast search. In contrast to the male-biased genes, there was no clear evidence for accelerated rates of evolution in female-biased genes, and most comparisons indicated a reduced rate of evolution in female-biased genes relative to unbiased genes. Male-biased genes did not show an increased ratio of nonsynonymous/synonymous polymorphism within D. melanogaster, and comparisons of polymorphism/divergence ratios suggest that the rapid evolution of male-biased genes is caused by positive selection.