The mysteries of the Y chromosome
A fragile creature that will soon disappear
The Y chromosome is not like the other 45 chromosomes of the human genome. She does not have a pair, she "collects" all possible mutations in herself, and many researchers are sure that soon the male sex chromosome will disappear altogether. Moreover, as it turned out recently, it is not really needed for reproduction.
Scientists predict that the human Y chromosome could potentially completely lose its function and disappear from the genome within the next ten million years. The "male" sex chromosome differs significantly from other chromosomes, and, in particular, from the X chromosome, in that during reproduction an individual is not able to exchange genetic sites. As a result, her hereditary material has become impoverished and the chromosome has accumulated mutations that are transmitted from generation to generation. But do not panic: as recent studies have shown, in the future people will be able to have children without the participation of the Y chromosome.
Male featureUntil recently, it was believed that the X- and Y-chromosomes appeared about 300 million years ago, but more recently, scientists have found out that chromosomal sex determination was absent 166 million years ago.
According to the most common theory, the X- and Y-chromosomes originated from a pair of identical chromosomes, when a gene appeared in ancient mammals, one of the alleles of which directed the development of the organism according to the male type. The chromosomes carrying this allele became Y chromosomes, and the second chromosome in this pair became the X chromosome. Thus, the X- and Y-chromosomes initially differed by only one gene. Over time, genes that are beneficial to males and harmful or irrelevant to the body of females began to develop on the Y chromosome.
An image of the Y chromosome under an electron microscope.
Photo from the website visualphotos.com .
The Y chromosome does not recombine with the X chromosome during the maturation of germ cells (gametogenesis), so it can only change as a result of mutations. The resulting genetic information is not rejected and is not "diluted" with new gene variations, therefore, it is transmitted from father to son for many generations with virtually no changes. Over time, the number of harmful mutations inevitably increases.
In the process of gametogenesis, spermatozoa undergo multiple cell divisions, and each of them provides an opportunity for the accumulation of mutations. In addition, spermatozoa are located in the highly oxidizing environment of the testicles, which contributes to the appearance of new mutations. That is why the Y chromosome "breaks" much more often than other chromosomes.
Stop the decay of the "male" chromosomeIn the course of evolution, the human Y chromosome has lost most of the genes originally present in it, and now, according to various estimates, it contains from 45 to 90 genes compared to about 1400 genes on the X chromosome.
Previously, scientists made a forecast according to which, with an estimated rate of loss of 4.6 genes per million years, the human Y chromosome could potentially completely lose its function within the next 10 million years.
But there is another opinion: the authors of a study conducted at the Whitehead Institute for Biomedical Research believe that the rapid loss of genes – the genetic "decay" that characterized the early evolution of the male sex chromosome has come to naught, and the Y chromosome will remain relatively stable in the next tens of millions of years.
The researchers sequenced 11 million pairs of nucleotide bases of the Y chromosome of rhesus monkeys. Comparing this sequence with a similar site on the male sex chromosome, as well as on the Y chromosome of chimpanzees, scientists came to the conclusion that the genetic composition of the male sex chromosome has hardly changed over the past 25 million years.
According to one of the authors of the study, Jennifer Hughes, in view of the fact that "in humans, only one gene was lost by the Y chromosome in comparison with rhesus monkeys, we can be sure that in the next millions of years the male chromosome will not disappear."
Conception without the Y chromosomeHawaiian researchers have demonstrated that only two genes from the Y chromosome are enough for male mice to conceive healthy offspring.
The authors of the article believe that in the future there may be a technique that allows you to do without the Y chromosome at all during human reproduction. In addition, the result obtained is potentially of great importance for the fight against male infertility.
The scientists used germ cells obtained from male mice, in which only two genes were left from the Y chromosome - SRY (Sex–determining Region of Y) - the most significant gene on the Y chromosome, which is responsible for the development of the male–type organism, the production of male hormones and spermatogenesis, and the spermatogony proliferation factor Eif2s3y. As the researchers found, Eif2s3y is the only Y–chromosome gene required for normal sperm formation.
The resulting male germ cells were then fertilized in vitro eggs using the method of intracytoplasmic injection (ROSI). The developed embryos were implanted into the uterus of the females. As a result of this procedure, 9 percent of pregnancies ended with the birth of healthy offspring, and in males with a full Y chromosome, this figure was equal to 26 percent. In the future, according to scientists, it is possible to do without the Y chromosome altogether in case of its defect. If genes interacting with Y-chromosome genes are found on other chromosomes, then activation of such partner genes can theoretically completely replace their functions.
Protection from cancer?Recently, the journal Nature published data that found that the loss of the Y chromosome in blood cells (leukocytes), often observed in older men, is associated with an increased risk of cancer and earlier mortality compared to women.
This phenomenon was first described about 50 years ago and until now its causes and consequences have remained largely unexplained. Now Swedish scientists have studied blood samples of 1,153 elderly men aged 70 to 84 years, who were observed in clinics from the age of 40. As it turned out, men, in most of whose blood samples the loss of the Y chromosome was detected, lived on average 5.5 years less than those who did not have such a phenomenon. In addition, an increase in the number of such blood cells significantly increased the risks of men dying from cancer.
"Many believe that the Y chromosome contains only genes involved in sex determination and sperm production, but in fact its genes are also involved in performing other important functions, for example, they can potentially play a role in preventing the development of tumors," the authors noted in their article. "Our hypothesis is that the age–related loss of the Y chromosome disrupts the immune "vigilance" of blood cells, which allows tumor cells to grow uncontrollably and transform into cancer."
The results suggest that a blood test for the presence of leukocytes that have lost the Y chromosome may become a new approach to identifying an increased risk of cancer in men. At the same time, the researchers stressed that the presence of such cells in a small number is not very dangerous, but their predominance may indicate a high risk of cancer.
The Y chromosome, the most important male sexual trait, is extremely susceptible to the influence of external factors. Due to the fact that the chromosome is unpaired, it does not participate in recombination and accumulates all mutations, both harmful and useful. Scientists have repeatedly predicted the imminent end of this strange gene cluster, but it still holds – as befits a real male chromosome.
Portal "Eternal youth" http://vechnayamolodost.ru05.11.2014