Men! Y u so SRY? There was a time when our gender was determined by environmental cues such as temperature, rather than our genes. Then, about 200-300 million years ago, the SRY gene evolved from the related SOX gene, and thus was born the Y chromosome. While all the other chromosome pairs swapped genetic material (thus repairing damaged genes), the Y chromosome began to lose its genes at an alarming rate, eventually only swapping with the X at its very tips.
• Nearly Gone: Over time, the Y lost 1,393 of its 1,438 original genes:that’s a rate of 4.6 genes per million years! Today, only 19 genes are shared between X and Y.
• Bombshell: In a 2002 Nature paper, Australian scientists dropped a bombshell, predicting that the Y may be lost altogether in about 10 million years! After all, spiny rats and male voles have already lost their Y, resorting to sex determining genes on other chromosomes.
• Enter monkey business: A new report shows that the rhesus macaque’s Y has 20 genes shared with its X, of which 19 are shared with the human Y. This suggests that the human Y chromosome has lost only one gene since humans and macaques last shared a common ancestor 25million years ago. So the linear extrapolation model is flawed . It is likely that the genes remaining have an essential function in fertility and are not going to be shed any time soon.
• Sneaky Backups: David Page of MIT found that the Y chromosome has a sneaky way of making backups: the most important genes are stored in the DNA as mirror images, or palindromes — which read the same way forwards and backwards. (It’s the Y’s way of saying “Madam, I’m Adam”.) This means that Y genes can repair themselves when they get damaged. But Jennifer Graves, the Australian scientist who published the 2002 story considers these duplications to be the chromosome’s dying gasps. “The Y could disappear tomorrow if another sex-determining gene were to arise on an autosome,” she says.
Serious stuff: Hughes, J. F. et al. Nature http://dx.doi.org/10.1038/nature10843 (2012).