Sex drives evolution

Following species divergence, differences arise in the expression levels of various genes, as a result of natural selection. Differences in gene expression levels also occur within a species but between the sexes, with most of these differences being attributable to sexual dimorphism, and sex-specific behavior and reproduction. In the 13 June issue ofScience, Jose Ranz and colleagues at Harvard University, USA, report the use of microarray technology coupled with statistical analysis to examine the evolutionary pattern of gene expression differences in D. melanogaster and D. simulans, which diverged about 2.5 million years ago, across the whole genome.

Ranz and coworkers analyzed both transcription products and genomic DNA in multiple hybridizations against microarrays containing 4,776 Drosophila coding sequences. Of these, 2,493 genes had not changed in relative expression levels since species divergence, and 57.5% of these showed sex-biased expression. The remaining half of the sequences on the microarrays, 2283 genes, showed expression levels that had altered between the species. Of these, only 16.6% showed the same alterations in expression levels between the sexes in both species; the remainder showed an evolutionary pattern that was sex-specific. Half of these retained the same gender bias, while the remainder showed gain or loss, and in 20 there was reversal of gender-specific bias. Most of the interspecific differences were found in male-biased genes, in agreement with the observation that male-specific morphological features evolve more rapidly than those of females. The distribution of the sex-biased genes was non-random, in contrast to the situation observed in mammals.

"The observations that 83% of the interspecific changes in gene expression are sex-dependent and that divergence in expression levels is greater in males suggest that sex-dependent selection is a major force driving the recent evolution of the Drosophilaexpression profile," the authors conclude.