Recently, I found old interesting news from internet. Evolutionary biologists have long believed that the mammalian Y chromosome is essentially stagnant, having lost most of its genes hundreds of millions of years ago. But new research from MIT's Whitehead Institute, published in this week's issue of Nature, overturns that theory. The research team, led by Whitehead Institute director and MIT biology professor David Page, showed that the Y chromosome is actually evolving rapidly and continuously remaking itself. The results overturned the expectation that the chimp and human Y chromosomes would be highly similar. Instead, they differ remarkably in their structure and gene content. The chimp Y, for example, has lost one third to one half of the human Y chromosome genes--a significant change in a relatively short period of time. Page points out that this is not all about gene decay or loss. He likens the Y chromosome changes to a home undergoing continual renovation.
I agree with Page on his result that Y chromosomes indeed evolved faster than expected, but I don’t agree the explanation they gave. The followings are my questions.
1, "The region of the Y that is evolving the fastest is the part that plays a role in sperm production," say Jennifer Hughes, first author on the Nature paper and a postdoctoral researcher in Whitehead Institute Director David Page's lab.
I don’t agree with Jennifer on this point. Frankly speaking, I don’t think the Y chromosome plays any role directly in sperm production, because in the end of MeiosisⅠ, the X chromosome and the Y chromosome have already been divided into different cells, and in simple words, the MeiosisⅡ is just a normal Mitosis. At the end of MeiosisⅡ, male produce four spermatids, two have X chromosome, other two have Y chromosome, and formation of sperm is just a deformation based on meiosis finished.
If Jennifer was right that some region of the Y plays a role in sperm production, the sperm which contains X never exist, because they don’t Y chromosome at all, but the fact is that the spermatids, whatever it contains X or Y, always can turn into sperm cells.
About Azoospermia, in general, it can be classified into two forms:
Obstructive Azoospermia: In this situation, sperm are produced but not ejaculated. The main cause is a physical obstruction (obstructive azoospermia) of the posttesticular genital tracts. The most common reason is a vasectomy done to induce contraceptive sterility.
Non-obstructive Azoospermia: it can be divided into two situations.
Pretesticular azoospermia: it is characterized by inadequate stimulation of otherwise normal testicles and genital tract. Typically, follicle-stimulating hormone (FSH) levels are low (hypogonadotropic) commensurate with inadequate stimulation of the testes to produce sperm. In simple words, the reason why a little boy don't have sperm is that there is not enough sex hormones to make him produce a sperm.
Testicular azoospermia: In this situation the testes are abnormal, atrophic, or absent, and sperm production severely disturbed to absent. In simple words, This may be due to chromosomal abnormalities, such as Klinefelter syndrome (karyotype 47XXY), or microdeletions of the Y chromosome, which is the loss of genetic material involved in spermatogenesis.
About Klinefelter syndrome (karyotype 47XXY) or XYY, XXXYY, XXXY, XXXXY gene types, due to the odd chromosomes, spermatocytes have disorder in Synapsis MeiosisⅠ, so they can’t produce a sperm.
About Y chromosome infertility, some research illustrates that it is usually caused by deletions of genetic material in regions of the Y chromosome called azoospermia factor (AZF) A, B, or C. Genes in these regions are believed to provide instructions for making proteins involved in sperm cell development, although the specific functions of these proteins are not well understood. Deletions in the AZF regions may affect several genes. The missing genetic material likely prevents production of a number of proteins needed for normal sperm cell development, resulting in Y chromosome infertility. I agree with this explanation very much, as everyone knows, genes not only control the synthesis of protein, but also steroid substances and enzymes. The Y chromosome is still there, but lack of some genes result lack of some special protein or steroid or enzymes, then result azoospermia.
In summary, the mechanism of sperm deformation is not completely understood so far, but I really don’t think Y directly plays a role in sperm production. I think Jennifer Hughes overestimated the Y chromosome too much, maybe she fell into the trap of "penis envy” of Freud.
If conditions permit, we can change the positions between spermatocyte and oocyte, and check whether spermatocyte can still produce into sperm and oocyte can still produce into egg cell and polar bodies or not. I guess it is impossible, and Thai Shemale is the biggest evidence for that. They can’t produce sperm, because they were injected estrogen since very little boys, but Y chromosomes are still in their genes. This can illustrate that very well chromosome is not directly involved in sperm formation.
2, The researchers suspect several factors are at play in the divergent evolution of human and chimp Y chromosomes, including differences in mating behaviors. Because a female chimpanzee may mate with many male chimpanzees around the same time, any genes on the Y chromosome that lead to enhanced sperm production offer a distinct competitive advantage. If a Y chromosome with genes for enhanced sperm production also carries mutations that alter or eliminate a gene not related to sperm production, those less advantageous mutations also get passed on, resulting in a Y chromosome with far fewer genes than the human Y.
The reason why I don’t agree with them on this point is that I don’t deny that positive selection is driving rapid evolution, but apparently, human evolution is much more far than chimps, in other words many many years ago, human also have the group sex as same as chimps that a female human may mate with many male human during her estrus, but with the evolution of human beings, we get rid of the chimps mode. There must be something which drives human to evolve out of chimps. Please notice that: male chimps only have sex with female chimps during her estrus, but male human don't have sex only during female human estrus, and apparently the purpose of male chimps having sex is only for reproduction, but it doesn’t apply to male human. I think a rational thought is that we should focus on some behaviors which human have but chimps don’t have, not on some behaviors which chimps have but human don't’ have. If we go to the opposite direction, we are not going to find the truth at all. Now we don’t have to consider that why the chimp Y has lost one third to one half of the human Y chromosome genes, we have to reconsider why human has increased one half to one of the chimp Y chromosome genes. Maybe the extra genes in male human are the key why the purpose for male human having sex is not only for reproduction.
In general, I agree that Y chromosomes indeed evolve fast than other chromosomes, whatever in horizontal comparison or vertical comparison. In horizontal dimension, as a female, I have to admit that men in general are more outstanding, smart or rational than women in all areas, wherever politics, science or sports. Why is that? Because women don’t have Y. Does Y really have so magic power? Yes, it does, men have really jumped out of the trap of reproduction, but women still as tool as reproduction. In vertical dimension, male human indeed evolved much faster than male chimps, male chimps are still stuck in the trap of reproduction.