The sex of a fetus in humans and other mammals is determined by a male sex-determining gene located on the Y chromosome.
However, the human Y chromosome is declining and could disappear in a few million years, leading to the extinction of humanity unless we evolve a new sex-determining gene.
The good news is that there are currently two branches of rodents that have lost their Y chromosome but are still thriving. The National Academy of Sciences in the U.S. has recently pointed out evidence showing how the spiny mouse has developed a new male sex-determining gene.
In humans, as well as in other mammals, females have two X chromosomes and males have one X and a small chromosome called Y. The names are unrelated to their shapes; X stands for “unknown.”
(Illustrative image: Nature).
The X chromosome contains about 900 genes that perform various functions unrelated to sex. In contrast, the Y chromosome contains few genes (about 55) and a lot of non-coding DNA – simple repeats that seemingly have no function.
Yet, the Y chromosome is critical because it contains a vital gene that initiates male development in the fetus.
About 12 weeks after conception, this master gene activates other genes that regulate the development of the testes. The fetal testes produce male hormones (testosterone and its derivatives), ensuring the fetus develops into a boy.
This main sex-determining gene is identified as SRY (sex-determining region Y) and was discovered in 1990. It works by activating a genetic pathway that begins with a gene called SOX9, which is key to determining male sex in all vertebrates, although it is not located on the sex chromosomes.
Most mammals have X and Y chromosomes that are similar to ours. An X has many genes and a Y has SRY along with a few other genes. The issue with this system is that the dosage of the X gene is not equal in males and females.
So how did such a strange system develop? Surprisingly, the Australian platypus has a completely different sex chromosome system, resembling that of birds more closely.
In the platypus, the XY pair functions as a regular chromosome pair, with equal contributions from both. This suggests that in mammals, X and Y were once a normal chromosome pair not long ago.
In return, this means that the Y chromosome has declined over 166 million years during which humans and platypuses have evolved separately: about 5 genes per million years. At this rate, the 55 genes will eventually disappear in another 11 million years.
Notably, some rodents have no Y chromosome. Eastern European moles and spiny mice in Japan are typical examples where the Y chromosome and SRY have completely vanished. The X chromosome remains, with equivalent or doubled amounts in both sexes.
While it is still unclear how moles determine sex without the SRY gene, a research group led by biologist Asato Kuroiwa from Hokkaido University has had more success with spiny mice on the islands of Japan. All of them are at risk of extinction.
Kuroiwa’s team discovered that most Y chromosome genes in spiny mice have moved to other chromosomes. They also found slight variations near the important sex gene SOX9, located on chromosome 3 of spiny mice.
This small duplicative DNA segment contains a “switch” that activates SOX9 in response to SRY. When they introduced this copy into mice, they found it enhanced the activity of SOX9, suggesting this change might allow SOX9 to function without SRY.
The future disappearance of the Y chromosome in humans has sparked speculations about our future.
Some lizards and snakes are species that have only females and can produce eggs from their own genes through parthenogenesis. However, this cannot occur in humans or other mammals because we have at least 30 crucial genes that only function if inherited from the father through sperm.
For reproduction, we need sperm, and we need men, meaning the end of the Y chromosome could herald the extinction of humanity.
However, humans might evolve a new sex-determining gene. This evolution could come with risks. What if more than one new system develops in different parts of the world?
The “battle” of sex genes could lead to the divergence of new species, which is exactly what has happened with moles and spiny mice.
Thus, on Earth in another 11 million years, there are two scenarios. There may be no trace of humanity left, or a few different human species may be separated by different sex-determining systems.
