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

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

The sex of babies of humans and other mammals is determined by a male determinant gene on the Y chromosome. But man Y chromosome degenerates and could disappear in a few million years, leading to our extinction unless we develop a new sex gene.

The good news is that two branches of rodents have already lost their Y chromosome and have been able to tell the tale.

A new paper inside Proceedings of the National Academy of Science shows how the spiny rat evolved 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 tiny little 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 kinds of tasks that have nothing to do with sex. But the Y contains few genes (about 55) and lots of non-coding DNA – simple repetitive DNA that doesn’t seem to do anything.

But the Y chromosome works out because it contains an extremely important gene that triggers male development in the embryo.

About 12 weeks after fertilization, this master gene turns on other genes that regulate the development of a testis. The embryonic testis produces male hormones (testosterone and its derivatives), which help the baby develop into a boy.

This main sex gene was identified as SRY (sex region on the Y) in 1990. It works by activating a genetic pathway that starts with a gene called SOX9, which is key for male determination in all vertebrates, even though it is not located on sex chromosomes.

The disappearing Y

Most mammals have an X and Y chromosome similar to ours; an X with many genes, and a Y with SRY plus a few others. This system poses problems because of the unequal dosing of X genes in males and females.

How did such a weird system evolve? The surprising finding is that Australia’s platypus has completely different sex chromosomes, more like those of birds.

In platypus, the XY pair is just a regular chromosome, with two equal members. This suggests that the mammalian X and Y were just a normal pair of chromosomes not so long ago.

This in turn must mean that the Y chromosome has lost 900-55 active genes in the 166 million years that humans and platypus evolved separately. That is a loss of about five genes per million years. At this rate, the last 55 genes will go in 11 million years.

Our claim of the imminent demise of human Y made a splashand to this day there are claims and counter claims about the expected lifespan of our Y chromosome – estimates between infinity and a few thousand years.

Rodents without a Y chromosome

The good news is that we know of two lineages of rodents that have already lost their Y chromosome — and are still surviving.

The mole voles of Eastern Europe and the spiny rats of Japan each have a number of species in which the Y chromosome and SRY have completely disappeared. The X chromosome remains, in a single or double dose in both sexes.

Although it is not yet clear how the moles determine the sex without the SRY genea team led by Hokkaido University biologist Asato Kuroiwa has had better luck with the spiny rat — a group of three species on several Japanese islands, all of which are threatened with extinction.

Kuroiwa’s team found that most of the genes on the Y of spiny rats had been moved to other chromosomes. But she found no trace of SRY or the gene that replaces it.

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

What they discovered 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 SOX9 normally turns on in response to SRY. When they introduced this duplication into mice, they found that it increases SOX9 activity, so the change would allow SOX9 to work without SRY.

What this means for the future of men

The imminent – ​​evolutionary speaking – disappearance of the human Y chromosome has led to speculation about our future.

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

To reproduce we need sperm and 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. Relief!

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

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

So if someone were to visit Earth 11 million years from now, they wouldn’t be able to find humans — or different human species, kept separate by their different sexing systems.The conversation

Jenny GravesDistinguished Professor of Genetics and Vice Chancellor’s Fellow, La Trobe University

This article has been republished from The conversation under a Creative Commons license. Read the original article.

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