The Y chromosome may be a symbol of masculinity, but according to modern scientists, it is not the most stable or even the most essential collection of genes in mammals.
Gender Determinant
Although the Y chromosome carries the “master sex determinant” or SRY gene, which determines whether an embryo will develop male sexual characteristics or not, other than the SRY gene, there are no other vital genes on the Y chromosome that are not present on the X chromosome. Accordingly, the Y chromosome is the only chromosome that is not necessary for life. Women, after all, survive just fine with two X chromosomes.
Rate of degeneration
In addition, the Y chromosome quickly weakens, as if fading over time. Because of this, women have two completely normal, healthy X chromosomes, while men have one full-fledged X chromosome and a Y chromosome that has “dried up” during evolution.
If this rate of degeneration continues at its current rate, the Y chromosome has only four and a half million years left. After this time, scientists predict the possible degeneration of this chromosome.
This period may seem very long, but it is not entirely true, especially when you consider that life on Earth existed for three and a half billion years.
Genetic recombination
The Y chromosome was not always a degenerate and unnecessary part of the DNA code. If you look at things 166 million years ago, at the time of the evolution of the very first mammals, the position of the “male” chromosome was completely different.
The early "proto-y chromosome" was originally the same size as the x chromosome and contained a set of the same genes. However, the Y chromosome has one fundamental flaw. Unlike all other chromosomes, of which we have two copies in each of our cells, Y chromosomes are present there in a single copy and are passed on from fathers to sons.
This means that the genes contained on the Y chromosome are not subject to genetic recombination, a kind of “shuffling” of genes that occurs in each generation and helps eliminate destructive gene mutations.
Deprived of the benefits of "recombination", the genes on the Y chromosome deteriorate over time and are eventually eliminated from the genome.
Defense Mechanisms
Despite this, recent research has shown that genes on the Y chromosome have developed effective defense mechanisms aimed at slowing down genetic degradation.
For example, a recent Danish study published in PLoS Genetics focused on studying in detail genetic code Y chromosomes have 62 different members. Scientists have concluded that the Y chromosome regularly undergoes large-scale structural rearrangements aimed at "gene amplification" - numerous copies of healthy genes responsible for the formation of sperm. This "amplification" mitigates gene loss on the Y chromosome.
Genetic palindromes
The study also showed that the Y chromosome had evolved unusual genetic structures, called palindromes (DNA sequences that read the same on both ends, like the word "stomp", for example). Genetic palindromes protect the Y chromosome from further degradation. In fact, palindromic DNA sequences are able to “convert” genes, that is, restore damaged genes, using an undamaged backup copy as a template.
By looking at other varieties of Y chromosomes, such as in other mammals and some other species, scientists have concluded that amplification of Y chromosome genes is general principle for representatives of various species.
Scientific debate
On the question of whether the Y chromosome will disappear over time, or will be able to develop sufficient protective mechanisms, science community is divided into two camps. One group insists that defense mechanisms do an excellent job of protecting the chromosome, while the other argues that these processes can only briefly delay the inevitable - the complete disappearance of the Y chromosome from the genetic code of living organisms. The debate on this issue continues and shows no sign of abating.
Disappearance
A leading proponent of the Y chromosome extinction argument, Jenny Graves of La Trobe University in Australia, argues that long term The Y chromosomes are doomed, even if they manage to survive a little longer than expected.
In a 2016 paper, she points out that Japanese spiny rats and voles have completely lost their Y chromosomes. She argues that gene loss processes on the Y chromosome inevitably lead to fertilization problems, which in turn can stimulate the formation of entirely new species.
What awaits men?
According to scientists, even if the Y chromosome disappears in people, this does not mean that men will disappear with it. Even in animal species that do not have a Y chromosome, there is still a division into males and females and natural fertilization and reproduction occur.
In these cases, the SRY gene, which determines membership male, moves to another chromosome, meaning that over time, men may completely lose the need for the Y chromosome. However, the new sex-determining chromosome - the one to which the SRY gene has passed - will have to undergo the same slow process of degeneration due to the same lack of recombination that doomed the Y chromosome to degradation.
Artificial methods of insemination
While the Y chromosome is essential for normal human reproduction, it no longer contains any genes that are useful or necessary for existence. It turns out that if you use modern artificial fertilization methods, then the Y chromosome is completely unnecessary.
It means that Genetic Engineering may soon replace the function of the Y chromosome gene, allowing female same-sex couples or infertile men to produce offspring. However, even if it were possible for everyone to become pregnant this way, it is very unlikely that most healthy people They will simply stop giving birth to children in the traditional way, switching to artificial insemination.
Although the fate of the Y chromosome is an interesting and hotly debated area genetic research, no need to worry just yet. We don't even know if the Y chromosome will disappear altogether. It is quite possible that her genes will be able to find a way to protect themselves from degeneration and everything will remain as it was.
The male chromosome, the notorious Y, is different from the other 45 included in the gene set normal person. She doesn't have a mate. It is she who is more characterized by various mutations. As some researchers say, in the near future civilization will face the complete disappearance of this element. On the other side, latest research showed that the reproductive process can easily proceed without the participation of this chromosome.
What do scientists say?
According to researchers, male chromosomes will disappear in the next ten million years. Of course, there can be no certainty about this, but the forecasts are confirmed by fairly reliable calculations. This will happen due to the loss of functionality by an element of the DNA structure.
Today it is reliably known that male chromosomes are significantly different from others, including X, since they cannot exchange genetic information during the reproductive process. This led to the loss of hereditary material and the accumulation of various mutations transmitted between generations. However, scientists pay attention: the presence of this particular chromosome, or rather its absence, will not become an obstacle to having offspring.
Latest research
Often this is followed by rather implausible information about space aliens, but not in our case. Scientists have actually found out exactly when chromosomes were formed as a tool for determining the sex of the fetus. Previously, there was an opinion that this happened for the first time three million centuries ago. Conducted in the recent past research papers showed: 166 million years before our time, both male and female chromosomes were absent from the gene pool of our genus.
Many adhere to the theory that the sex (male, female) chromosomes have the same gene set as their source. In ancient times, the evolution of mammals led to the appearance of a gene, the allele of which became the basis for male type body. Allele in modern science called Y, the second began to be designated X. That is, in fact, at the beginning there were almost identical chromosomes, the difference was in one gene. Over time, Y became the carrier of genetic information that was more useful for the male half of the family, but not important or harmful for the female half.
Some features of the human body
Researchers, figuring out the specific characteristics of male and female chromosomes, discovered that Y is not able to recombine with X during gametogenesis, that is, at the moment when germ cells mature. Hence, possible changes are caused exclusively by mutations. The genetic information formed during such a process cannot be assessed by natural mechanisms as a defect, and there is no dilution by genetic variations. Consequently, the father passes on the complete set to his son - and so on time after time, generation after generation. Gradually, the number of modifications accumulates.
The process of maturation of germ cells is associated with division, characteristic of sperm. Each such division is another opportunity for additional mutations accumulating in the male sex chromosome. The acidity of the environment in which this process occurs also plays a role - this factor additionally provokes unplanned mutations. Scientists have found that statistically, Y is the most frequently damaged chromosome of the entire gene set.
It was, it became, it will be
Currently, the number of genes on the Y chromosome, as scientists say, is no less than 45, but no more than 90. Specific estimates differ somewhat, depending on the methods used by researchers. But the female sex chromosome contains almost one and a half thousand genes. This difference is due evolutionary processes, leading to the loss of genetic information.
In earlier times, scientists studying the dynamics of the Y chromosome estimated that, on average, about 4.6 genes are lost per million years. If this progression continues in the future, completely genetic information through this object will cease to be transferred within the next ten million years.
Alternative Approach
Of course, X and Y are chromosomes, the study of which, in principle, became available to humanity quite recently, so scientists mostly have only theoretical calculations, without data confirmed by practical observations, which is always associated with a small probability of error and discrepancies. Already, some are convinced that the opinion voiced above is incorrect.
Specialized research was carried out at the Whitehead Institute. Scientists, examining the male set of chromosomes, came to the conclusion that genetic decay had completely stopped. It was just evolutionary stage associated with features human body, and a stable state has now been achieved, which will remain so for at least ten million years.
How it happened
The alternative study mentioned, focusing on the X and Y chromosomes, involved sequencing 11,000,000 base pairs of the male chromosome. Genetic data from rhesus macaques were used as experimental samples. The sequence obtained during the work was compared with the corresponding section of the male chimpanzee chromosome, and a sample of human genetic information was taken as a control. Based on the data obtained, it was possible to confirm the assumption that the genetic content of male chromosomes has been constant for 25 million years.
One of the authors of this study is Jennifer Hughes, who explained that the Y (the symbol for the male chromosome) lost just one gene, which is strikingly different from the experimental samples obtained from macaques. This indicates that in the near future (however, time intervals measured in millions of years can only be called so arbitrarily) time, humanity is not in danger of any loss of chromosomes.
Is this scary?
Currently, scientists know exactly which chromosome is responsible for the sex of the unborn child: it depends precisely on this very 23rd pair, which in the male body is not even represented by the same pair, because for women XX is characteristic, and for men - XY . Therefore, theories about possible disappearance Many people are concerned: will humanity go extinct then? Will we become same-sex?
Scientists assure: there is no cause for concern. Recently, research conducted in scientific institute in Hawaii, clearly showed that healthy offspring are quite possible with the presence of two genes on the male chromosome - and this is in relation to mice. This means that in the future it will be possible to completely bypass this chromosome, successfully reproducing without it. This also applies to humans. Scientists note: such research results are important not only for those who fear for the fate of humanity in the distant future. It is quite possible that they will help find the answer to questions about eliminating male infertility.
How the experiment was carried out
The researchers' workflow involved interacting with the reproductive cells of male mice. They worked on them, leaving only two genes from the male chromosome. One of them is responsible for the formation of the male structure of the body, including hormonal development, spermatogenesis, and the second is responsible for the proliferation factor.
During the research, it became clear that the gene that determines the proliferation of spermogonia is the only one in which reproductive system mice really need to form offspring.
What happened next?
To test the results of their theoretical conclusions, in laboratory conditions, scientists fertilized mouse eggs using improved male chromosomes. For this purpose, a highly precise intracytoplasmic injection method was used. The embryos that developed were implanted into the female body - into the uterus.
Statistics showed: 9% of all pregnancies were successful, and the offspring were born completely healthy. But if the reproductive process occurs with the participation of a male mouse whose chromosome has not been changed, the percentage of successful pregnancies without deviations in the development of the offspring is only 26%. This clearly demonstrates that men's sex chromosome in the future, perhaps, it will become only a relic of past millennia. It will probably be possible to find on other chromosomes elements responsible for genetic information that have a correspondence with the male chromosome. If you activate their functionality, the object in question will become completely redundant.
Oncology and genetics
Some time ago, studies were published that showed a relationship between the likelihood of developing malignant tumors and the loss of the male chromosome. This sometimes occurs in old age. Leukocytes are primarily affected. Scientists have also found that this is one of the reasons for early mortality: men with gene changes usually die earlier, but women live longer.
First said phenomenon were described about half a century ago, but the consequences, as well as the causes, to this day remain a sealed secret to the public. As part of the study in Sweden, blood samples were taken from 1,153 people aged 70-84 years. Only blood samples from men were studied, and the sample was based on people who were regularly observed in clinics across the country from at least forty years of age. The collected information clearly showed that the loss of the male chromosome is characteristic of those whose life expectancy is approximately 5.5 years shorter compared to men who have not experienced such a change. If the number of leukocytes with altered gene information increased, the likelihood increased fatal outcome, provoked by malignant processes.
Stereotypes and reliable information
It is generally accepted that Y is the chromosome that determines gender child, and with this its functions are exhausted. In fact, the genetic information it stores is important for many functions. Scientists hope that it is by studying the features of this chromosome that it will be possible to invent effective medicine against tumors. Doctors suggest that the loss of chromosomes with age leads to weakening immune system. This, in turn, creates conditions for the growth of malignant cells.
The Y chromosome is the shortest of the chromosomes. 22 out of 23 pairs of human chromosomes have approximately the same volume genetic information, and only the last, 23rd pair, which determines gender, violates this dimension. The Y chromosome, which contains genes encoding the development of male sexual characteristics, is much smaller in volume than the X chromosome, which is paired with it (the male sex corresponds to the combination of XY chromosomes, and the XX pair is responsible for the female). Today male Y chromosome has only 19 of the approximately 600 genes that 200-300 million years ago it should have shared with X. The small volume of the Y chromosome and its gradual loss of genes have prompted some scientists to say that in the future it will disappear altogether, particularly in humans (even today in individual mammals the combination XX is responsible for both female and male sex). However, a study by MIT biologists published in Nature showed that the male chromosome is immune from extinction, and nature appears to be preserving it forever.
Over the past 26 million years, the genetic content of the Y chromosome has remained unchanged. This is due to the fact that many of her genes play key role in survival, and its role is not limited solely to determining sex. This chromosome, in particular, contains genes that are involved in protein synthesis, regulate the activity of other genes and play important role in the fusion of RNA molecules together. Its role is manifested in the cells of the heart, blood, lungs and other tissues and organs of the body. As David Page, a biologist at the Institute for Biomedical Research at the University of Massachusetts, figuratively put it, Institute of Technology"Y-chromosome genes are powerful players in the body's central command room." Page led a team of researchers who, in a paper in Nature, showed that the concept of a degraded Y chromosome should be said goodbye.
However, not everyone was convinced by Page and his team's findings. In particular, genetics from the Australian national university Jennifer Graves, who says 26 million years isn't that long long period in a long-term trend of Y-chromosome degradation. In addition, there are mammals that already manage without it.
In 2002, Graves, in her article, which, by the way, also appeared in Nature, showed that the Y chromosome gradually decreased in size starting with early mammals and predicted that in 10 million years it would disappear altogether. This, in turn, raises a logical question: what then will happen to the male sex and the sexual differences necessary for the continuation of life? Graves and other biologists who support the hypothesis of further degradation of the Y chromosome argue that other chromosomes will take over its functions, and the mechanisms of sexual differentiation will continue.
David Page and his colleagues set out to study in detail the evolutionary history of the male chromosome. Scientists compared and analyzed the complete DNA sequences of eight species of mammals, ranging from the most ancient ones, such as opossums, rats and mice, to the more recent primates, including the youngest ones - rhesus macaques, chimpanzees and humans.
The study showed that the detrimental loss of the Y chromosome of its genes has been going on for hundreds of millions of years, but 26 million years ago, when chimpanzees separated from the rest of the monkeys, and especially 7 million years ago, when the first representatives of the genus Homo (people) appeared, the process The “wear and tear” of the male chromosome has stopped. As Page put it, “It was very surprising how stable this chromosome has been over the last 26 million years.”
This stability comes from vital important core male chromosome, which includes 12 genes that have nothing to do with sex determination or the development of the male reproductive system. But the expression of these genes occurs in other tissues, such as heart and blood cells. They are responsible for key cellular functions, such as protein synthesis and regulation of transcription of other genes. This means that the Y chromosome is important for the survival of the entire organism, so its future survival is guaranteed by evolution.
Graves responded to this conclusion from Page’s team that the degradation of the Y chromosome is not a linear process, and its final stages are highly likely to have a tendency to fluctuate, and therefore stability may be temporary. Graves argues that two species of Japanese spiny rats (Echimyidae) have completely lost the male Y chromosome and transferred its genes to other chromosomes, and two species of hamsters (Cricetidae) have completely lost some genes on the Y chromosome, and their functions have apparently been taken over by genes on other chromosomes. “Although it seems that nature with new forms genetic systems decided to experiment first on rodents, we should not think that this will not threaten us, humans, in the future,” summarizes Graves.
In addition to the discussion about further evolution male chromosome, Page's research forced doctors and biologists to think seriously: male and female cells may be biochemically different. Because Page's team has shown that the Y chromosome has functions far beyond sex determination, men's Y-related genes lead to slightly different cells than women. When biologists experiment with cell lines, they usually do not take into account their male or female origin. Therefore, the importance of many previous studies may be questioned because experiments with an XY cell line may lead to different results than experiments with an XX cell line.
First of all, this concerns genetic origin individual diseases. It is known, for example, that autoimmune diseases amaze more women, while autism-related disorders are more common in men. Trying to get to the bottom of the reason for this, biologists, as a rule, did not take into account the subtle biochemical and genetic features on cellular level. It's time to get rid of these illusions, explains David Page.
Male Y chromosome
Brief information (video, English): ,
Women and men each have 23 pairs of chromosomes. Of each pair, one was received from the father and one from the mother. Unlike the autosomal chromosomes, which are named in order from “1” to “22,” the two “sex” chromosomes have letter designations. XX for women and XY for men. From the mother - always the X chromosome. From the father, the child will inherit either the X chromosome (girl) or the Y chromosome (boy). The X chromosome from the father turns into the XX combination - and this is the female sex. The Y chromosome from the father turns into an XY combination, and determines the male gender. Almost all chromosomes undergo mixing (recombination), a process where each pair of chromosomes exchanges different fragments with each other. Since each man has only one Y chromosome, it, unlike the X chromosomes, does not recombine. For these reasons, genealogical analysis on X chromosomes becomes much more complicated. We also inherit mitochondrial DNA (mtDNA) from our mother, but none from our father.
The main tools of DNA genealogy are analyzes of mutations, their number and location in mtDNA and Y chromosomes. The Y chromosome, due to the very low frequency of mutations and the absence of mixing (recombination), unlike mitochondrial DNA, is transmitted almost unchanged from generation to generation. Based on mutation variations, chromosomes are divided into haplotypes, which are combined into haplogroups and subclades (subgroups). The letter designations of haplogroups are alphabetical and indicate the time of appearance of the next mutation. That is, haplogroup A (the Y chromosome of the so-called Adam, appeared about 75,000 years ago, is localized today mainly in South Africa) older in age (about 30,000 years ago), etc. alphabetically.
Estimated distribution of Y-DNA haplogroups 2000 BC. e.
Distribution of Y-DNA haplogroups
Distribution of Y-DNA haplogroups in Europe
The male Y chromosome is not a dead end of evolution, but is changing very actively. Such conclusions were made by geneticists when comparing the set of genes in the chromosome of humans and chimpanzees, which survived 6 million years of separate evolution. Unexpected genetic diversity explained by the peculiarities of the functioning of genes involved in the formation of germ cells.
In most mammals, sex is determined by them: the male body is the carrier of the X- and Y-chromosomes, and women “make do” with two X-chromosomes. Once this division did not exist, but as a result of evolution about 300 million years ago, chromosomes differentiated. There are variations whereby some men's cells contain two X chromosomes and one Y chromosome, or one X chromosome and two Y chromosomes; Some women's cells 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. For example, the phenomenon of hemophilia is associated with this feature. The defective gene that impairs blood clotting is linked to the X chromosome and is recessive. For this reason, women only endure 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.
One way or another, the Y chromosome has traditionally been considered weak point male organisms, reducing genetic diversity and hindering evolution. However latest research showed that fears about the extinction of the male race 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 a specific part of the human Y chromosome and one of its immediate family– chimpanzees – very different. Over the 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 actually quite constant.
Human evolution. Source "Eternal Youth"
Scientists' assumptions about the conservatism 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 options - two from the mother and one from the father, all of them can exchange genes), it cannot gain genetic diversity from the outside, changing only due to the loss of genes. According to this theory, in 125 thousand years the Y chromosome will finally die out, which could be the end of all humanity.
However, for 6 million years of separate evolution of humans and chimpanzees, the Y chromosome has been successfully changing and progressing. IN new job, conducted at the Massachusetts Institute of Technology, talks about the Y chromosome of chimpanzees. The human Y chromosome was deciphered 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 on the two chromosomes would be very similar. For comparison: in total mass The DNA of humans and chimpanzees is different in only 2% of genes, and the Y chromosome differs by more than 30%!
Professor Page compared the process of evolution of the male chromosome to a change in the appearance of a house, the owners of which remain the same. “Despite the fact that the same people live in the house, almost constantly one of the rooms is completely updated and renovated. As a result, after a certain period of time, as a result of “room-by-room” renovation, the entire house changes. However, this trend is not normal for the entire genome,” he noted.
The reason for this unexpected instability of the Y chromosome is not yet precisely clear. Scientists suggest that genetic diversity in it is ensured by instability to mutations. The usual mechanism for “repairing” genes fails on the Y chromosome, opening the way for new mutations. Statistically, a larger number of them become fixed and change the genome.
In addition, these mutations are subject to significantly greater selection pressure. This is determined by their function - the production of germ cells. Any beneficial mutations will be fixed with to a greater extent probabilities, since they act directly - increasing the ability of an individual to reproduce. At the same time, ordinary mutations have an indirect effect - increasing the body's resistance to disease or harsh conditions environment, For example. Thus, the benefit of a mutation in a nonspecific DNA section will only be revealed if the organism falls into the corresponding unfavourable conditions. In other cases, mutant and non-mutant organisms will perform similarly. Fertility appears very quickly - already in the second generation. An individual either reproduces as a result of mutation more successfully and leaves numerous offspring, or reproduces noticeably worse and cannot increase the share of its genes in the general population. This mechanism functions more efficiently in chimpanzees, whose females constantly mate with big amount males. As a result, the germ cells enter into direct competition, and “selection” occurs as efficiently as possible. In humans, due to more conservative models of reproduction, the Y chromosome has not evolved so rapidly, geneticists say. This hypothesis is supported by the fact that the parts of the chromosome involved in sperm production are most different between humans and chimpanzees.