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Men are slowly losing their Y chromosome

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The sex of human and other mammalian babies is determined by the male-determining gene on the Y chromosome. However, the human Y chromosome is degenerating, and unless we evolve new sex genes, it may disappear in a few million years and we may become extinct.

However, in the mouse family, two species that have already lost the Y chromosome are alive.

A new paper in the Proceedings of the National Academy of Science shows that spiny rats have evolved a new male sex-determining gene.

How the Y chromosome determines sex in humans

In humans, like other mammals, females have two X chromosomes and males have one tiny X and Y chromosome. The name had nothing to do with its shape, the X meaning “unknown”.

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X has about 900 genes and performs a variety of non-sexual jobs. However, Y contains few genes (about 55) and a lot of noncoding DNA, simple repetitive DNA that seems to do nothing.

But the Y chromosome packs a punch because it contains a key gene that initiates male development in the embryo. About 12 weeks after conception, this master gene turns on other genes that control testicular development. The embryo’s testicles make male hormones (testosterone and its derivatives) to ensure that the baby grows up as a boy.

This master gene was identified in 1990 as SRY (sex region on the Y). SRY functions in a genetic pathway that originates in a gene called SOX9. SOX9 is absent on the sex chromosome, but plays an important role in male sex determination in all vertebrates.

Disappearing Y

Most mammals have X and Y chromosomes, just like us. X has many genes, Y has SRY and a few others. A problem with this system is that the amount of the X gene is unequal in males and females.

How did such a bizarre system evolve? The Australian platypus has made the surprising discovery that it has sex chromosomes that are quite different from those of birds.

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

So during the 166 million years that humans and platypus evolved separately, the Y chromosome lost between 900 and 55 active genes. This translates to about 5 genes being lost every million years. At this rate, the last 55 genes will be gone in 11 million years.

The lifespan of the human Y chromosome is estimated to be between infinity and thousands of years, and is still argued and argued to this day.

The Amami Spiny Mouse is an animal endemic to Japan that lives on Amami Oshima. (Credit: Asato Kuroiwa)

 

It is still unknown how sex is determined in mole voles that lack the SRY gene, but a research team led by Mari Kuroiwa, Professor of the Department of Biology, Hokkaido University, found three species of spiny rats living in the Japanese archipelago. endangered species) had better results.

Kuroiwa’s team found that most of the genes on the spiny mouse Y chromosome had moved to other chromosomes. However, no gene for SRY or its substitute was found.

This time, we finally succeeded in identifying it and published it in “PNAS”. The researchers found a sequence in the male genome but not in the female genome, purified it, and tested the sequence in all individual rats.

They found subtle differences near the key sex gene, SOX9, on chromosome 3 in spiny mice. This small duplication (only 17,000 base pairs out of more than 3 billion base pairs) was present in all males and absent in females.

The team suggested that this slightly duplicated DNA contained a switch that normally turns on SOX9 in response to SRY. Introduction of this duplicated DNA into mice enhanced the activity of SOX9, suggesting that this change may have enabled SOX9 to function in the absence of SRY.

What it means for the future of men

The imminent demise of the human Y chromosome has sparked a lot of speculation about our future.

There are all-female species of lizards and snakes that can produce eggs from their own genes in a process called parthenogenesis. But this is not possible because humans and other mammals have at least 30 important ‘imprinted’ genes that must be passed on from the father through sperm to function.

Sperm are needed for reproduction, and a man is needed. In other words, the demise of the Y chromosome could portend the end of humanity.

This new finding supports another possibility that humans could evolve new sex-determining genes.

But the evolution of new sex-determining genes carries risks. What if multiple new systems evolved in different parts of the world?

A “war” of sex genes could lead to the segregation of new species, just like mole voles and spiny rats.

So if someone were to visit Earth 11 million years from now, humans might not be there. Alternatively, we might find several different human species, segregated by different sex-determining systems.

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