Interestingly, genes with sex-biased expression patterns tend to evolve rapidly at the coding region level26. This observation is consistent with the notion that many differences in gene expression between the sexes are the result of sexual selection (Box 1). The evolution of sex-biased genes was recently reviewed by Ellegren and Parsch26 and will not be discussed in detail here. However, it is relevant to note that although sex-biased genes often evolve rapidly at the protein coding level, differences in gene regulation between the sexes are often conserved in evolution. For example, Zhang and colleagues showed that sexually dimorphic expression patterns of a large number of genes are conserved across seven Drosophila species32. Similarly, Reinius et al. found a signature of evolutionary conserved sexually dimorphic gene expression in the brain of three primate species, including humans23. Specifically, they compared gene expression profiles in the occipital cortex of male and female humans and cynomolgus macaques (Macaca fascicularis), and identified hundreds of genes with sex-biased expression patterns in both species.
