HQ Team
December 7, 2022: The Y chromosome that determines the sex of human and other mammal babies is degenerating and might disappear in a few million years. This means most mammals might become extinct unless a new gene evolves.
But a new paper published in Nature suggests the degradation of the chromosome has stabilised. The research found that two branches of rodents have evolved a new male-determining gene.
In humans, the female has two X chromosomes and males have a single X and a chromosome called Y. The chromosomes carry 900 genes that determine certain functions. The Y chromosome contains 55 genes and a gene that kick-starts male development in the embryo at about 12 weeks of conception.
This master sex gene was identified as SRY in 1990.
This X and Y combination is a little imbalanced because of the unequal dosage of X genes in males and females.
The Y chromosome
The platypus, a mammal, has X and Y chromosomes with two equal gene numbers. This monotreme mammal shared a common ancestor with humans 166 million years ago.
This suggests that X and Y were just an ordinary pair of chromosomes not that long ago.
In turn, this must mean the Y chromosome has lost 900–55 active genes over the 166 million years that humans and platypi have been evolving separately. That’s a loss of about five genes per million years. The last 55 genes will be gone in 11 million years at this rate.
Claims of the ultimate demise of the Y chromosome have led to a lot of contention and debate. A recent study found that the sperm count is falling.
Researchers have found two rodent lineages that have already lost their Y chromosome and are still surviving.
The mole voles of eastern Europe and the spiny rats of Japan each boast some species in which the Y chromosome, and SRY, have completely disappeared. The X chromosome remains in a single or double dose in both sexes. But the scientists have not been able to determine the reason for it.
Recently, a team led by Hokkaido University biologist Asato Kuroiwa 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.
The team published a successful identification in PNAS. The team found sequences that were in the genomes of males but not females, then refined these and tested for the sequence on every individual rat.
They discovered 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 more than 3 billion) was present in all males and no females.
This was supposed to be the crucial DNA that switches on SOX9 in response to SRY. When they introduced this duplication into mice, they found that it boosts SOX9 activity, so the change could allow SOX9 to work without SRY.
So how will the future be for us without the Y chromosome? There are some species such as reptiles, where females self-reproduce via what’s known as parthenogenesis. But this can’t happen in humans or other mammals because we have at least 30 crucial “imprinted” genes that work only if they come from the father.
To reproduce, we need sperm and we need men, meaning that the end of the Y chromosome could herald the extinction of the human race.
The new finding supports an alternative possibility – that humans can evolve a new sex-determining gene. But how this new evolution will play out in the coming millions of years is still up for speculation.
