The good news is that two branches of rodents have already lost their Y chromosome and have lived to tell about it.
A new paper in the Proceedings of the National Academy of Science shows how the spiny rat evolved a new male-defining gene.
How does the Y chromosome determine a person’s gender?
In humans, as in other mammals, females have two X chromosomes, and males have one X chromosome and a small chromosome called Y. The names have nothing to do with their shape; X meant “unknown”. X contains about 900 genes that perform all sorts of non-sex related functions. But Y contains a few genes (about 55) and a lot of non-coding DNA – simple, repetitive DNA that doesn’t seem to do anything.
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But the Y chromosome has the advantage because it contains a crucial gene that triggers male development in the embryo. About 12 weeks after conception, this master gene turns on others that regulate testicular development. Embryonic testicles produce male hormones (testosterone and its derivatives), which ensures the development of the child as a boy.
This major sex gene was identified as SRY (sex area on Y) in 1990. It works by triggering a genetic pathway starting with the SOX9 gene, which is key to male sex in all vertebrates, although it is not on the sex chromosomes.
Most mammals have an X and Y chromosome similar to ours; X with more genes and Y with SRY plus a few others. This system is fraught with problems due to the unequal number of X genes in men and women.
How did such a strange system come about? The surprising discovery is that the Australian platypus has completely different sex chromosomes, more similar to those of birds.
In the platypus, the XY pair is a regular chromosome with two equal members. This suggests that mammals X and Y were not so long ago a normal pair of chromosomes.
In turn, this should mean that the Y chromosome lost 900-55 active genes over the 166 million years that humans and platypuses evolved separately. That’s about five genes lost in a million years. At this rate, the last 55 genes will be gone in 11 million years.
Our announcement of the inevitable death of the human Y chromosome caused a sensation, and to this day there are claims and counterclaims about the expected lifespan of our Y chromosome – estimates from infinity to several thousand years.
Rodents without a Y chromosome
The good news is that we know of two lines of rodents that have already lost their Y chromosome and are still surviving.
Both the mole voles of Eastern Europe and the spiny rats of Japan can boast of some species in which the Y chromosome and SRY have completely disappeared. The X chromosome remains in single or double dose in both sexes.
Although it is not yet clear how mole voles determine sex without the SRY gene, a team led by Hokkaido University biologist Asato Kuroiwa had better luck with spiny rats, a group of three endangered species on various Japanese islands.
Kuroiwa’s team found that most of the genes on Y spiny rats have been moved to other chromosomes. But she did not find any signs of SRY, nor the gene that replaces it.
Now, finally, they have published a successful identification to PNAS. The team found sequences that were in the genomes of males but were not in the genomes of females, then refined them and verified the sequence in each individual rat.
They found a tiny difference near the key sex gene SOX9 on chromosome 3 of the spiny rat. A small duplication (only 17,000 base pairs out of over 3 billion) was present in all men and none of the women.
They suggest that this small piece of duplicated DNA contains a switch that normally turns on SOX9 in response to SRY. When they injected this duplication into mice, they found that it increased SOX9 activity, so this change could allow SOX9 to work without SRY.
What does this mean for the future of men
The evolutionarily inevitable disappearance of the human Y chromosome has raised speculation about our future.
Some lizards and snakes are only females and can produce eggs from their own genes through what is known as parthenogenesis. But this can’t happen to humans or other mammals because we have at least 30 important “imprinted” genes that only work if they’re passed down from the father via sperm.
We need sperm and men to reproduce, which means the end of the Y chromosome could herald the extinction of the human race.
The new discovery supports the alternative possibility that humans may develop a new sex-determining gene. Ugh!
However, the evolution of a new sex-determining gene comes with risks. What if more than one new system emerges in different parts of the world?
The “war” of the sex genes can lead to the isolation of new species, which happened with the mole voles and prickly rats.
So if someone visits Earth 11 million years from now, they might not find humans — or several different human species separated by their different systems of sex determination.
(This is a PTI story syndicated through The Conversation)