Endangered rat species lacks Y chromosome

A new genetic mechanism for determining sex has been discovered in spiny rats, shedding light on the evolution and degeneration of the Y chromosome.

Researchers at Hokkaido University, Japan, led by Professor Asato Kuroiwa, have shown that sex determination in the endangered Amami spiny rat, where males do not carry a Y chromosome, occurs through directly increasing expression of the SOX9 gene on chromosome 3 in male rats.

'To demonstrate this beyond doubt, the team would have to delete the duplication in spiny rats to show that no males develop', Professor Robin Lovell-Badge from the Francis Crick Institute in London, and trustee at the Progress Educational Trust, who was not involved in the research, told the New Scientist. As the spiny rat is an endangered species, this experiment would not be allowed. 'However, the evidence they have is all quite convincing,' Professor Lovell-Badge continued.

For the vast majority of mammals, male sex is determined by the Y chromosome, a small chromosome containing around 55 genes, compared to around 1000 on the X chromosome. One of these genes, SRY, causes male determination by activating SOX9.

However, the Y chromosome has been gradually degenerating and has lost many of the genes it previously carried, leading some to speculate about how much longer the Y chromosome might exist.

Publishing their research in the Proceedings of the National Academy of Sciences, the team has demonstrated how SOX9 is activated in the Amami spiny rats without the need for SRY. Showing how male determination can still occur without the Y chromosome. This allows for further questions to be asked about the evolution of sex determination.

'[Professor] Kuroiwa's team discovered most of the genes on the Y of spiny rats had been relocated to other chromosomes. But she found no sign of SRY, nor the gene that substitutes for it', wrote evolutionary geneticist Emeritus Professor Jenny Graves from Australian National University in the Conversation. Professor Graves, who was not involved in the study, originated the suggestion that the human Y chromosome is in demise.

'What they discovered was a tiny difference near the key sex gene SOX9, on chromosome 3 of the spiny rat', continued Professor Graves. 'They suggest that this small bit of duplicated DNA contains the switch that normally turns on SOX9 in response to SRY'.

The authors note that this mechanism of male determination has not yet been extended to other rodent species lacking Y chromosomes, indicating that it is not yet confirmed if this is the only alternative method of sex determination in mammals, or even in a single species. Future work will investigate whether these findings can be extended to the other three mammals known to lack a Y chromosome.

'A "war" of the sex genes could lead to the separation of new species, which is exactly what has happened with mole voles and spiny rats,' said Professor Graves. 'So if someone visited Earth in 11 million years, they might find no humans – or several different human species, kept apart by their different sex determination systems.'