Genes on the mammalian X chromosome just can't wait to get off.

J. J. Emerson and colleagues' analysis of the human and mouse genomes shows that the X chromosome has a clear excess of genes that have functional duplicates on other chromosomes. Of the 94 genes that the authors identified to have been functionally retroposed between chromosomes in the human genome, 15 were derived from X-chromosome genes: far more than the 3 or 4 expected on the basis of the size of this chromosome. Similarly, the 17 out of 105 functional retropositions in the mouse genome were of X-chromosome genes, although only 4 or 5 were expected.

So why do genes want out of the X chromosome? One possibility is that genes that benefit males at a cost to females are moving because, compared with the X chromosome, an autosome spends on average less time in females and so would be more difficult to select against. Alternatively, the inactivation of X-linked genes during meiosis might favour the export of genes to the autosomes, where they are more likely to be expressed to the benefit of the male during meiosis. Either of these mechanisms could cause functional retrogenes that are exported from the X to be selectively favoured over genes that are retroposed from other chromosomes.

Despite the X chromosome being a popular place for genes to leave, paradoxically, it also seems that it is a favourite destination. The authors show that there are relatively many more functional retrogenes recruited to the X than any other chromosome in both human and mouse genomes. However, they also show that human pseudoretrogenes, which are less likely to be subject to selection, are also more common than expected on the X chromosome. So, although selection once again has a key role in causing this bias, in this case there is likely to be a purely mechanistic component to the bias.

So, it seems that selection primarily powers the genic traffic that travels to and from the mammalian X chromosome. However, is the cement still wet on this busy genomic highway or is it a well-worn track? Emerson and colleagues answer this question with a comparative analysis of the mouse and human genomes, which showed that most retrogenes that have escaped the X (12/15) or have moved to it (10/13) did so before the mouse–human divergence. Clearly, the turnover of genes on the X chromosome is an ancient but ongoing process.

The fascinating picture of dynamic X-chromsome evolution that Emerson and colleagues have revealed invites a bit of genomic crystal-ball gazing. Is it just a matter of time before the X chromosome becomes the exclusive preserve of genes that are advantageous to males when hemizygous and that are silenced in female tissues, whereas all genes that are favourable to males when homozygous will be shifted to the autosomes?