The Y chromosome is the engine of evolution
Genetically changeable menAlexandra Borisova, "Newspaper.
Ru»The male Y chromosome is not a dead end of evolution, but is changing very actively.
Such conclusions were made by geneticists when comparing a set of genes in the chromosome of a human and a chimpanzee who survived 6 million years of separate evolution. The unexpected genetic diversity is explained by the peculiarities of the functioning of genes involved in the formation of germ cells.
As stated in 2006 by the now Prime Minister (then President) Russia's Vladimir Putin, "if grandma had certain sexual characteristics, she would be a grandfather." It was about the possibility of Russia adopting sanctions against Iran, but the comparison is not entirely true. Thanks to the achievements of genetics, we know that grandmother differs from grandfather not only in appearance, but also in a set of sex chromosomes.
In most mammals, gender is determined by them: the male body is the carrier of the X and Y chromosomes, and women "get by" with two X chromosomes. Once this division did not exist, but as a result of evolution about 300 million years ago, the chromosomes differentiated. There are deviations, as a result of which the cells of some men contain two X chromosomes and one Y chromosome or one X chromosome and two Y chromosomes; the cells of some women contain three or one X chromosome. Occasionally, female XY organisms or male XX organisms are observed, but the vast majority of people still have a standard configuration of sex chromosomes. This feature, for example, is associated with the phenomenon of hemophilia. A defective gene that impairs blood clotting is linked to the X chromosome and is recessive. For this reason, women only carry the disease without suffering from it themselves due to the presence of a duplicate gene due to the second X chromosome, but men in a similar situation carry only a defective gene and get sick.
Anyway, the Y chromosome has traditionally been considered a weak point of male organisms, reducing genetic diversity and hindering evolution. However, recent studies have shown that fears about the extinction of the male gender are greatly exaggerated: the Y chromosome does not even think of stagnating. On the contrary, its evolution is very active, it changes much faster than other parts of the human genetic code.
A study published in Nature (Jennifer F. Hughes et al., Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content) showed that the specific part of the human Y chromosome and one of its closest relatives – chimpanzees – differs very much. For 6 million years of separate evolution of monkeys and humans, the fragment of the chromosome responsible for the production of germ cells has changed by a third or even half. The rest of the chromosome is really quite constant.
Scientists' assumptions about the conservativeness of the Y chromosome were based on objective factors: being transmitted from father to son without changes (for the X chromosome there are as many as three variants - two from the mother and one from the father, all of them can exchange genes), it cannot draw genetic diversity from the outside, changing only due to the loss of genes. According to this theory, after 125 thousand years, the Y chromosome will finally fade away, which could be the end of all mankind.
However, for 6 million years of separate evolution of humans and chimpanzees, the Y chromosome has been successfully changing and progressing. A new paper conducted at the Massachusetts Institute of Technology talks about the Y chromosome of a chimpanzee. The human Y chromosome was decoded in 2003 by the same group led by Professor David Page.
The results of the new study surprised geneticists: they expected that the sequence of genes in the two chromosomes would be very similar. For comparison: in the total mass of human and chimpanzee DNA, only 2% of the genes are different, and the Y chromosome differs by more than 30%!
Professor Page compared the process of evolution of the male chromosome with a change in the appearance of a house whose owners remain the same. "Despite the fact that the same people live in the house, almost constantly one of the rooms is completely renovated and renovated. As a result, after a certain period of time, as a result of "room-by-room" repairs, the whole house changes. However, this trend is not normal for the whole genome," he said.
The reason for this unexpected instability of the Y chromosome is not yet exactly clear. Scientists suggest that genetic diversity in it is provided by instability to mutations. The usual mechanism of "fixing" genes fails on the Y chromosome, opening the way for new mutations. Statistically, more of them are fixed and change the genome.
In addition, these mutations are subject to much stronger selection pressure. This is determined by their function – the production of germ cells. Any beneficial mutations will be fixed with a higher degree of probability, since they act directly – increasing the ability to reproduce an individual. At the same time, ordinary mutations have an indirect effect – increasing the body's resistance to disease or to harsh environmental conditions, for example. Thus, the benefits of mutation in a non-specific DNA site will be revealed only if the organism falls into the appropriate unfavorable conditions. In other cases, mutant and non-mutant organisms will work similarly. Fertility manifests itself very quickly – already in the second generation. An individual either reproduces more successfully as a result of mutation and leaves numerous offspring, or reproduces noticeably worse and cannot increase the share of its genes in the general population. This mechanism functions more effectively in chimpanzees, whose females constantly mate with a large number of males. As a result, the germ cells enter into direct competition, and the "selection" is as efficient as possible. In humans, due to more conservative reproduction models, the Y chromosome has not evolved so rapidly, geneticists believe. This hypothesis is supported by the fact that the parts of the chromosome involved in sperm production are most different in humans and chimpanzees.
Professor Page's group, in collaboration with the University of Washington Genome Center, continues to work on decoding the Y chromosome of other mammals. They hope to shed light on the evolution of sex chromosomes and its relationship to behavioral patterns in the population.
Portal "Eternal youth" http://vechnayamolodost.ru14.01.2010