If the Y chromosome disappears, will humans become solitary reproduction?

The argument that the ratio of men to women is imbalanced once caused a lot of male anxiety. Some statistics show that there are nearly 30 million more men than women. Of course, taking into account the different marriageable ages of the two sexes and various complex factors, this data does not represent the final number of bachelors, but what is more frightening than being single is that the Y chromosome, which is closely related to the male compatriots, seems to be slowly degenerating and disappearing.

We know that there are 23 pairs of chromosomes in human somatic cells, 22 of which are autosomes, and one pair of sex chromosomes, the X and Y chromosomes, which determine our gender.

Autosomes are the same in shape, size, and similarity, so they can pair well with each other, but X and Y chromosomes are very different in form and function.

23 pairs of chromosomes in males

In general, females have two completely normal X chromosomes, while males have one normal X chromosome and one atrophied Y chromosome. We can simply assume that the Y chromosome determines whether a person becomes a man or not.

Unlike the topsy-turvy male figure, the human Y chromosome is very short and compact. And some scientists have found that the Y chromosome is already degenerating rapidly, and if it continues at this rate, the Y chromosome will disappear completely in 4.6 million years.

This may sound like a long time, but life on Earth has existed for 3.5 billion years. So we have to think about the harsh fact that if the Y chromosome finally disappears, there will be no men left in the world. It all starts with the function and origin of the Y chromosome.

Sex determination and sex chromosomes were only linked after the first discovery of sex chromosomes in Orthoptera by McClung in 1902. However, it was not until 1990 that Sinclair et al. discovered the sex-determining region of the Y chromosome (SRY).

This is a segment of a gene found at the end of the short arm of the human Y chromosome and is the primary gene that determines testicular development in males.

The protein product of SRY affects the expression of another gene called Sox9, which in turn causes individuals to develop as males. 2018 researchers located and knocked out the enhancer of Sox9 in embryos of mice that were about to sexually differentiate so that Sox9 was not expressed, and showed that XY mice never became male, but instead remained female.

This shows that XY chromosome carriers can differentiate into males solely by relying on the Sox9 gene protein to guide the testicular development of XY chromosome carriers, and that SPY can be seen as the master switch that determines sex inheritance.

XY chromosome carriers may be female, and in turn, XX chromosome carriers may become male. During meiosis, the Y chromosome sometimes interchanges with the X chromosome, which could lead to this situation if SRY happens to be in the middle of it.

The discovery of SRY has shed light on how X and Y evolved, and it is now generally accepted that sex chromosomes evolved from the same autosomes. the Sox3 gene was originally located on a pair of autosomes in an ancient mammalian ancestor, and the promoter sequence drove Sox3 expression in sperm precursors and the central nervous system.

Sox3 on one chromosome was later replaced by a sequence that drives the expression of undifferentiated gonads (a tissue that can develop into ovaries or testes), forming SRY to direct testicular development. Over time, genes on this chromosome that were not required for male development were degraded, thus giving rise to the Y chromosome, and the one that possessed the original SOX3 now became the X chromosome.

The sex chromosomes are still relatively young compared to the evolution of life as a whole, and the X and Y chromosomes were once thought to have evolved in different directions for 300 million years. However, studies sequencing the platypus genome suggest that the XY sex-determination system may not have emerged until about 166 million years ago, beginning when monotremes separated from other mammals.

The sex of most current mammals, including placentals and marsupials, is determined by the XY sex-determination system, but most metazoan vertebrates do not have sex chromosomes in their bodies.

We must have been surprised when we were told as children that the sex of crocodiles was determined by temperature, when in fact their sex was determined by the external environment rather than by individual genotype. Of course, there are some taxa that have diverged sex chromosomes, such as the ZW sex-determination system in snakes, which is similar to sex determination in birds.

As the lowest mammal, monotremes, which look like both reptiles and mammals and birds, are naturally ideal for determining how the XY chromosome evolved.

Platypus

Unlike most mammals, the platypus has 10 sex chromosomes, with males having five X and five Y chromosomes, and females having 10 X chromosomes, which are linked together by nine autosome-like regions during cell division.

Little is known about the genetic content of the platypus' 10 sex chromosomes, but the few data available are striking. Studies have shown that platypus chromosome X1 is similar to that of mammalian X; while chromosome X5 contains the avian sex-determining gene DMRT1.

This means that the mammalian X and Y chromosomes evolved from autosomal pairs after the divergence of monotremes 166 million years ago, so they are 145 million years younger than previously thought. This may not be a good thing, because the shorter time of existence means that the Y chromosomes have degenerated more rapidly than originally estimated.

The earliest Y chromosomes were the same size as the X chromosome and contained all the same genes, but unlike autosomes, the Y chromosome could only be passed from father to son as a single copy, and, more embarrassingly, the genes on the Y chromosome could not be genetically recombined.

X chromosome (left) and Y chromosome (right)

The genetic recombination that occurs in each generation may help eliminate harmful genetic mutations, but recombination between the X and Y chromosomes is harmful to living organisms by causing males to lose essential genes on the Y chromosome and females to have more non-essential or even harmful genes that would otherwise only appear on the Y chromosome.

The inability to benefit from recombination causes the genes on the Y chromosome to degenerate over time and eventually disappear from the genome. Studies have shown that over hundreds of millions of years of evolution, the X chromosome has retained hundreds of genes, while the Y chromosome appears to be in "evolutionary free fall," with only 70 or 80 genes remaining.

This may not be sensational, but examples of the complete disappearance of the Y chromosome have been found in several rodent species, such as the mole vole, where the genotype of both males and females is XO, and the genotype of all Tanzanian mole voles is XX.

Mole voles

To date, no placental conserved sex determinant SRY has been found in their respective genomes, but rather the CBX2 gene on chromosome 2 plays a role in determining male sex.

Of course, on the question of whether the human Y chromosome will disappear, there are also "reservations" that the Y chromosome's defense mechanism can save itself, and some studies have shown that the Y chromosome's gene loss has been stalled for 25 million years.

In 2010, MIT researchers completed a detailed comparison of chimpanzee and human Y chromosomes and found that the genes on the Y chromosomes in chimpanzees and humans were changing rapidly.

The Y chromosome appears to be "the most evolutionarily dynamic part of our genome," said David Page, co-author of the study and director of MIT's Whitehead Institute. Their research shows that even a small number of genes can undergo a great deal of evolution, "as if the Y chromosome is a house that is constantly being modified.

This remodeling has been going on at an alarming rate for about 6 million years since chimpanzees and humans evolved from a common ancestor.

It is not clear, however, exactly what these genetic changes have accomplished, but in chimpanzees, there is evidence that such changes have increased sperm production.

Now to return to the question we posed at the beginning of the Y chromosome disappears, does the male disappear with it? The answer is probably no.

The Y chromosome is indeed important and contains genes necessary to make normal sperm in addition to SRY. However, the degeneration of the Y chromosome may cause other chromosomes to "take over" its genes, and eventually, the Y chromosome will disappear completely and a new sex determination system will be born.