Degenerating male genes push humans to the brink of extinction

The sex of humans and mammals is determined by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years.

A recent scientific report in the Proceedings of the National Academy of Sciences suggests that the loss of the Y chromosome will lead to extinction unless we evolve a new sex gene.

How does the Y chromosome determine human sex?

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

The X chromosome contains about 900 genes that do all sorts of jobs that have nothing to do with sex. But the Y contains few genes (about 55) and a lot of noncoding DNA—simple repetitive DNA that doesn’t seem to do anything, or junk code.

But the Y chromosome contains a critically important gene that helps initiate male development in the embryo.

Around 12 weeks after conception, this master gene switches to other genes that regulate testicular development. The testicles in the embryo produce male hormones (testosterone and its derivatives), ensuring that the baby develops as a boy.

This sex gene was identified as SRY (sex region on the Y) in 1990. It works by activating a genetic pathway that begins with a gene called SOX9, which is key to male sex determination in all vertebrates.

The Y chromosome is shrinking.

Most mammals have an X and Y chromosome pair similar to ours; an X with many genes and a Y with SRY and a few other genes. This system is problematic because the X gene dosage in males and females is not equal.

The surprising discovery was that the Australian platypus has a completely different sex chromosome, more like that of birds.

In the platypus, XY is just a pair of autosomes, with the same number of genes. This shows that in mammals, X and Y are a normal pair of chromosomes.

Put another way, this means that the Y chromosome has lost 900–55 active genes in the 166 million years that humans and platypuses have been evolving separately. That’s a loss of about 5 genes every million years. At this rate, the last 55 genes on our Y chromosome will be gone in 11 million years.

Rodents have no Y chromosome.

The good news is that we know of two lines of rodents that have lost their Y chromosomes – and survived.

The mole rat of Eastern Europe and the spiny rat of Japan both boast several species in which the Y and SRY chromosomes have completely disappeared, leaving only the X chromosome.

While it's still unclear how mole rats determine sex without the SRY gene, a team led by biologist Asato Kuroiwa of Hokkaido University had better luck with spiny rats—a group of three species on different islands of Japan, all of which are endangered.

Kuroiwa's team found that most of the genes on the spiny mouse's Y had moved to other chromosomes. But the team found no trace of SRY, nor its replacement gene.

In 2022, they published a successful identification in PNAS. The team found sequences that were in the male genome but not the female, then refined them and tested the sequences on individual mice.

What they found was a tiny difference near the sex-critical gene SOX9, on spiny mouse chromosome 3. The tiny duplication (just 17,000 out of more than 3 billion base pairs) was present in all males and none in females.

They propose that this small cloned piece of DNA contains a “switch” that activates SOX9 in response to SRY. When they introduced this clone into mice, they found that it enhanced SOX9 activity, so this change might allow SOX9 to function without SRY.

What does this mean for the future of humanity?

The threat of the human Y chromosome gradually disappearing has prompted speculation about our future.

Some lizards and snakes are female only and can produce eggs from their own genes through parthenogenesis. But this cannot happen in humans or other mammals because we have at least 30 important genes that are “imprinted” and only turn on if they come from the sperm of a male who acts as a father.

To reproduce, we need sperm, or rather we need men, which means the disappearance of the Y chromosome could spell the end of the human species.

The new findings support an alternative possibility — that humans may have evolved a new sex-determining gene.

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

A sexual "gene war" can lead to the separation of new species, which is exactly what happened with mole rats and spiny mice.

So if someone were to visit Earth in 11 million years, they might find no humans at all – or find several different human species, separated by different sex-determining genetic systems.

(According to 1thegioi)

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