The Y chromosome slowly disappears.  A new sex gene could be the future of men

The Y chromosome slowly disappears. A new sex gene could be the future of men

The sex of human babies and other mammals is determined by a male-determining gene on the Y chromosome. But the human Y chromosome degenerates and could die out in a few million years, leading to our extinction unless we develop a new sex gene. .

The good news is that two rodent branches have already lost their Y chromosome and lived to tell the tale.

A new paper in Proceedings of the National Academy of Sciences shows how the spiny rat developed a new male-determining gene.

How the Y Chromosome Determines Human Sex

In humans, as in other mammals, females have two X chromosomes and males have a single X and a small chromosome called Y. The names have nothing to do with their shape; the X stood for ‘unknown’.

The X contains about 900 genes that perform all sorts of tasks unrelated to sex. But the Y contains few genes (about 55) and a lot of non-coding DNA – just repetitive DNA that doesn’t seem to do anything.

But the Y chromosome packs a punch because it contains a very important gene that triggers male development in the embryo.

About 12 weeks after conception, this master gene activates others that regulate the development of a testicle. The embryonic testicle produces male hormones (testosterone and its derivatives), which ensure the development of the baby as a boy.

This master sex gene was identified as SRY (sex region on the Y) in 1990. It functions by triggering a genetic pathway beginning with a gene called SOX9 which is essential for male determination in all vertebrates, although it does not is not on the sex chromosomes.

The disappearing Y

Most mammals have an X and Y chromosome similar to ours; an X with lots of genes, and a Y with SRY plus a few more. This system poses problems because of the unequal dosage of X genes in men and women.

How did such a strange system evolve? The surprising discovery is that the Australian platypus has completely different sex chromosomes, more like those of birds.

In the platypus, the XY pair is just an ordinary chromosome, with two equal members. This suggests that mammals X and Y were an ordinary pair of chromosomes not so long ago.

In turn, this must mean that the Y chromosome has lost 900 to 55 active genes over the 166 million years that humans and the platypus have evolved separately. That’s a loss of about five genes per million years. At this rate, the last 55 genes will be gone in 11 million years.

Our claim of the impending demise of the human Y caused furor, and to this day there are claims and counter-claims about the expected lifespan of our Y chromosome – estimates ranging from infinity to a few thousand. years.

Rodents without 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.

Eastern European mole voles and Japanese spiny rats each have species in which the Y chromosome and SRY have completely disappeared. The X chromosome remains, in single or double doses in both sexes.

Although it’s 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 the spiny rat – a group of three species on different Japanese islands, all endangered.

Kuroiwa’s team found that most of the genes on the spiny rat Y had been moved to other chromosomes. But she found no signs of SRY, nor the gene that replaces it.

Now, finally, they have published a successful identification in PNAS. The team found sequences that were in the genomes of males but not females, then refined and tested them for the sequence on each individual rat.

What they found was a small 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 males and no females.

They suggest that this small piece of duplicated DNA contains the switch that normally activates SOX9 in response to SRY. When they introduced this duplication in mice, they found that it stimulated SOX9 activity, so the change could allow SOX9 to function without SRY.

What this means for the future of men

The impending disappearance – evolutionarily speaking – of the human Y chromosome has sparked speculation about our future.

Some lizards and snakes are female-only species and can make eggs from their own genes through what is called parthenogenesis. But that can’t happen in humans or other mammals because we have at least 30 crucial “printed” genes that only work if they come from the father via sperm.

To reproduce, we need sperm and we need men, which means that the end of the Y chromosome could herald the extinction of the human race.

The new finding supports an alternative possibility – that humans could develop a new sex-determining gene. Phew!

However, the evolution of a new sex-determining gene carries risks. What if more than one new system evolved in different parts of the world?

A “war” of sex genes could lead to the separation of new species, which is exactly what happened with mole voles and spiny rats.

So if someone visited Earth 11 million years from now, they might find no humans – or several different human species, separated by their different sex determination systems.The conversation

Jenny Graves, Emeritus Professor of Genetics and Member of the Vice-Chancellor, La Trobe University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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