Men May Lose Their Y Chromosome but a Gene Study in Spiny Rats Gives Hope for Mankind

The Y chromosome in men is slowly disappearing but research in rodents shows there might be hope for human kind.

Highlights:

Y chromosome which has genes that are involved in male sex determination and development is present in men and is slowly going extinct
Although this might be alarming, studies in spiny rats show that a new male-determining gene has evolved
While this is promising, this supports a different possibility -that this can lead to different species

The gender of mammals including humans is decided by a male-determining gene on the Y chromosome. However, the human Y chromosome is degenerating and may go extinct in a few million years unless there is evolution of a new sex gene.

The good news is that two rodent species have already lost their Y chromosome and survived.

A recent study shows how the spiny rat has evolved a new male-determining gene. Most of the genes on the Y -chromosome of spiny rats have been shifted to other chromosomes ().

Genetics of Male Infertility - Y-Chromosome Microdeletions
Encyclopedia section of medindia briefs you about the Y-chromosome Microdeletions of Male Infertility

‘The Y chromosome present in men is slowly degenerating but research in spiny rats shows that there might be hope for humanity; not without risk of different species.’

How Human Sex is Determined by the Y- Chromosome

In humans, as in other mammals, females have two X chromosomes and males have a single X and a chromosome called Y. The X contains about 900 genes having various functions which are not even related to sex. However, the Y contains few genes (about 55) and a lot of non-coding DNA – simple repetitive DNA that does not seem to have a function.

The Y chromosome is integral because it contains an important gene that starts male development in the embryo. At about 12 weeks after conception, this gene switches on others that regulate the development of a testis. The embryonic testis makes male hormones (testosterone and its derivatives), which ensures the baby develops as a boy.

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

Is There a Link Between Heart Failure and Y Chromosome in Men?
Yes. There is a link between the mosaic loss of Y chromosome and heart failure of aging men as revealed by a study and it has been successfully done in mice.

The Disappearance of Y Chromosome

Most mammals have an X and Y chromosome; an X with lots of genes, and a Y with SRY plus a few others. This system comes with problems because of the disproportionate dosage of X genes in males and females.

How did such a weird system evolve? The astonishing finding is that Australia’s 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 the mammal X and Y were an ordinary pair of chromosomes not that long ago.

This implies that the Y chromosome has lost 900–55 active genes over the 166 million years that humans and platypus have been evolving 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 imminent demise of the human Y created a craze, and to this day there are claims and counterclaims about the expected lifetime of our Y chromosome – estimates between infinity and a few thousand years

Genes on the Y Chromosome Prove Essential for Survival of Men
The human Y chromosome has over the course of millions of years of evolution managed to preserve a small set of genes that has ensured not only its own survival but also the survival of men, despite a well-documented history of dramatic genetic decay.

Rodents Without Y chromosome

The good news is we know of two rodent lineages that have already lost their Y chromosome – and continue to survive.

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.

Although it’s not yet clear how the mole voles determine sex without the SRY gene, researchers have had more luck with the spiny rat - a group of three species on different Japanese islands, all endangered.

They discovered most of the genes on the Y of spiny rats had been relocated to other chromosomes. But there is no sign of SRY, nor the gene that substitutes for it.

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.

What they discovered was 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.

They suggest this small bit of duplicated DNA contains the switch that normally turns 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.

Alzheimer’s Risk Increases With Loss of Y Chromosome
About one in five men over age 80 lose the Y chromosome from their blood cells and the condition is known a loss of Y, or LOY.

What Does This Mean for the Future of Men

The disappearance of the human Y chromosome has elicited speculation about our future.

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

The new finding supports an alternative possibility – that humans can evolve a new sex determining gene. However, evolution of a new sex determining gene comes with risks.

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

Which means if someone visited Earth in 11 million years, they might find no humans – or several different human species, kept apart by their different sex determination systems.

Reference:

Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9 - (https://www.pnas.org/doi/10.1073/pnas.2211574119)

Source-Medindia