The Y chromosome is decaying. The male could disappear
The sex of human and other mammalian infants is decided by a male sex-determining gene on the Y chromosome. But the human Y chromosome is degenerating and could disappear within a few million years, leading to our extinction, unless it evolves a new sex gene.
The good news is that two offshoots of rodents have already lost their Y chromosome and lived to tell the tale. They have not gone extinct
A recent paper published in the Proceedings of the National Academy of Science shows how the spiny rat has evolved a new gene that determines male sex.
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 a Y. Names have nothing to do with their shape; the X stands for “unknown”.
The X contains about 900 genes that perform all sorts of non-sex-related activities. The Y chromosome, on the other hand, contains a few genes (about 55) and a lot of non-coding DNA – simple repetitive DNA that doesn't seem to do anything, but the Y chromosome is very important because it contains a key gene that initiates male development in the embryo .
At about 12 weeks after conception, this main gene activates others that regulate testicular development. The embryonic testicle produces male hormones (testosterone and its derivatives), which ensure that the child develops like a boy.
This master sex gene was identified as SRY (sex region of the Y) in 1990. It works by triggering a genetic pathway that starts with a gene called SOX9, which is crucial for male sex determination in all vertebrates, even though it is not found on sex chromosomes.
The disappearance of the Y
Most mammals have an X and Y chromosome similar to ours: an X with many genes and a Y with SRY and a few others. This system poses problems due to the unequal dosage of X genes in males and females.
How did such a strange system evolve? The surprising finding is that the Australian platypus has completely different sex chromosomes, more similar to those of birds.
In the platypus, the XY pair is just a normal chromosome, with two equal members. This suggests that the mammalian X and Y were a normal pair of chromosomes not too long ago.
In turn, this means that the Y chromosome lost 900-55 (845) active genes in the 166 million years that humans and platypuses evolved separately. This is 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 disappearance of the human Y chromosome created a stir, and even today there are claims and counterclaims about the expected lifespan of our Y chromosome – estimates ranging from infinity to a few thousand years.
Rodents without a Y chromosome
The good news is that we know of two rodent lineages that have already lost their Y chromosome and are still surviving.
Eastern European mole voles and Japanese spiny rats boast some species in which the Y chromosome and SRY have completely disappeared. The X chromosome remains, in single or double dose in both sexes.
While it's still unclear 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 several Japanese islands, all within risk of extinction.
Kuroiwa's team found that most of the genes present in the Y of spiny rats were transferred to other chromosomes. But he found no trace of SRY, nor of the gene that replaces it.
In 2022, they published a successful identification in PNAS. The team found sequences that were found in the genomes of males but not females, then refined and analyzed them on each individual rat.
They discovered 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.
The researchers suggest that this small, duplicated piece of DNA contains the switch that normally activates SOX9 in response to SRY. When they introduced this duplication to mice, they found that it increases SOX9 activity, so the modification could allow SOX9 to function without SRY.
What does this mean for the future of men
The impending evolutionary demise of the human Y chromosome has sparked speculation about our future.
Some lizards and snakes are exclusively female species and can produce eggs with their own genes through so-called parthenogenesis. But this can't happen in humans or other mammals because we have at least 30 crucial genes 'imprinted' 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 evolve a new gene that determines sex. Thus Y may disappear, but the distinction between males and females will not disappear.
However, the evolution of a new gene that determines sex carries risks. What if more than one new system evolved in different parts of the world? We would have species that would be different and no longer crossbreedable.
However, the times are quite long and for now there is nothing to worry about. The man exists and has a Y gene.
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The article The Y chromosome is decaying. The male could disappear comes from Economic Scenarios .
This is a machine translation of a post published on Scenari Economici at the URL https://scenarieconomici.it/il-cromosoma-y-sta-decadendo-potrebbe-sparire-il-maschio/ on Sat, 03 Jun 2023 21:20:38 +0000.