13 January 2014

For the particularly gifted, I repeat: men will not die out!

The disappearance of the Y chromosome was canceled

Kirill Stasevich, CompulentaA person has two sex chromosomes in the genome, X and Y, and each of them should have a set of genes that are identical in function.

This was once the case, but over time, 90% of the genes shared between the X and Y chromosomes disappeared from the male chromosome. When biologists discovered this, a hypothesis appeared that the Y chromosome was destined to disappear altogether: not tomorrow and not the day after tomorrow, of course, but in 5 million years. By evolutionary standards, the term is quite short.

This hypothesis invariably enjoys increased attention, even from those who are little interested in biology. Well, really, how is it, there was a Y chromosome – and suddenly it's not there! After all, one of the symbols of masculinity, or, if you like, masculinity. Moreover, this hypothesis is often interpreted very loosely, abusing headings like "The future without men", etc. But it does not take into account that some rodents do not have a Y chromosome at all, and they nevertheless retained their separate sex. In addition, it turned out not so long ago that only two genes from the Y chromosome are enough for male fertility in general. That is, even if the male chromosome disappears, it will not necessarily drag down the male sex itself.

Over millions of years of evolution, the Y chromosome has kept only the best for itself

However, Rasmus Nielsen from the University of California at Berkeley (USA) and his colleagues went even further: they stated that there is no disappearance of the Y chromosome at all. Scientists compared the male sex chromosomes of eight Europeans and eight Africans, compared the variability of the Y chromosome genes among themselves, with other chromosomes and with mitochondrial DNA - and proposed their own version of the development of sex chromosomes.

About 200 million years ago, when animals were still new to Earth, they had XY chromosomes in the genome of their ancestors, which were not much different from other chromosome pairs. In each generation, X and Y exchanged genes, and so it went on until "male" genes began to accumulate in the Y chromosome, which turned out to be tightly registered here, so they also attracted others. These genes responsible for the formation of testes, spermatozoa, etc., behaved badly in the female body, so that eventually X and Y stopped exchanging them.

And this meant that when a defect appeared in the male chromosome, it could no longer be corrected due to a similar gene in the X chromosome (this possibility remained with the X chromosomes themselves). The damaged pieces eventually just fell off the Y chromosome. That is, only universal healthy gene variants remained in the degraded male chromosome. Unlike others, Y chromosomes in different people are much more similar to each other, even if we take populations of different ages, such as Africans and Europeans. According to the authors of the work, the male chromosomes turned out to be more similar to each other than one would expect.

However, there are two possible reasons for this uniformity of the male chromosome: either very few men passed it on to the next generation, or everyone participated in reproduction, but natural selection "cut" the chromosome itself. According to the first variant, it turned out that only one out of four men had to reproduce throughout the history of mankind, and this contradicts the data on variations in all other chromosomes.

That is, the Y chromosome became what it is, as a result of strict natural selection, which cleaned out all mutations along with their owners. No mutation here could evade the eye of evolution: the male sex chromosome was deprived of a partner whose genes could perform the work of non-functional Y-genes.

In other words, the authors of the work do not deny that once the degradation of the male chromosome was in full swing. But then she stopped–simply because she had reached her limit. And it also follows that the Y chromosome is not at all as useless as it is sometimes said: being useless, it would not be so uniform in different individuals. The question is why such work has not been carried out before, but the scientists themselves say that inter-population studies on the Y chromosome are just beginning. In addition, it would be good to back up such studies with information on other mammalian species, however, although the genomes of 36 species have been sequenced today, and only three have fully read the Y chromosome.

And here's another thing. The year before last, we wrote that the rate of disappearance of the male chromosome is greatly exaggerated: in 25 million years, it has lost only one gene. Perhaps this indirectly indicates in favor of the fact that its degradation has simply stopped or is stopping for a long time.

The results of the study are published in PLoS Genetics: Natural Selection Reduced Diversity on Human Y Chromosomes.

Prepared based on the materials of the University of California at Berkeley: Study dispels theories of Y chromosome's demise.

Portal "Eternal youth" http://vechnayamolodost.ru13.01.2014

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