Things that donors know: are men an endangered species?

Shmuel is worried about our future, the men "I heard that the male Y chromosome is getting depleted over time, is it possible that every generation male mammals are losing more and more of their characteristics? Is it possible for them to disappear?"

Indeed, there are those who estimate that the man's days are coming to an end. In 2002, an article in the prestigious journal "Nature" put our life expectancy at 10 million years. It's scary to think that we only have 9,999,981 years left before we go extinct (and depressing to think that we might not be that far). To understand why some biologists eulogize the males, some facts concerning the genetics of the redundant species should be mentioned.

While all the other chromosomes appear in pairs, one is found in the mother's egg and the other from the father's sperm, one pair of chromosomes is unusual: men have an X chromosome from the mother and a strange, small and almost gene-free chromosome, called the Y. When sex cells are formed in the ovaries or testes, an important process takes place that is the basis of sexual reproduction: recombination. Each pair of chromosomes (and there are 22 of these in addition to the sex chromosomes) adheres to each other and the chromosomes, one from the father and the other from the mother, exchange DNA segments. This is how the offspring gets mixed chromosomes: each one originates from combining pieces from a grandparent. This process enables the operation of natural selection. Thanks to the replacement, each mutation is tested in the cruel test of the war of existence within new combinations of genes and not necessarily that of the original chromosome in which it occurred. Thus, a defective gene can be separated from a normal environment, and a positive mutation can spread even if the original chromosome has less successful genes. The Y chromosome is the only one that passes from father to son without passing, that is, as one block of hereditary information without a way for the mutation to express itself separately from the genes surrounding it. In fact, the Y chromosome is a kind of asexual heredity unit within a sexual being. The Y is also unusual in other ways: it is the smallest of the chromosomes - less than 50 active genes compared to about 1,000 on the X chromosome. While each chromosome contains a random variety of genes, some are active in the ongoing maintenance of each cell and some are unique to certain tissues, after all, a chromosome The Y is the only selector and almost all its genes work in one tissue: the testes.

XY is a common but not the only mechanism

The XY mechanism in which the female has two X chromosomes and the male has XY exists in mammals, flies, some species of turtles and lizards and even in plants. In another variety of creatures: birds, amphibians, butterflies and snakes, the female actually has a small and degenerate unique chromosome, while the male receives two identical sex chromosomes. In the bee series (Hymenoptera), males have one set of chromosomes (haploid) and females two. There are also creatures in which the sex chromosome appears without a partner, meaning that the male or female has an odd number of chromosomes. There is no evolutionary "tree" that divides the animal world in this way and this is the result of convergent evolution in which creatures that choose to divide into two distinct species, male and female, create a unique chromosome for one of the species and then degenerate it until it disappears or close to it. In the hermaphrodite creatures - a group that includes, for example, the molluscs, worms and the monoecious plants, each individual carries both male and female reproductive organs and activates all the genes necessary for that purpose, and accordingly there are no sex-specific chromosomes.

In many creatures, the species is not genetically determined but is determined by environmental conditions. Crocodiles and various fish species let the temperature of the environment determine the sex of the embryo and some fish (the most famous of which is the clownfish from "Finding Nemo") change their sex depending on the presence of other males and females in the environment. The origin of mammals is apparently reptiles whose species was determined by temperature, but such a mechanism does not work in warm-blooded animals, meaning an internal environment where every embryo will experience the same heat.

A dominant mechanism in mammals

When the first mammal species appeared about 300 million years ago, the mechanism was also created in which one dominant gene activates the switch and causes a gland with double potential to become a testicle and not an ovary. Sex is a complex trait and the difference between males and females is expressed in several genes. A gene that is beneficial to the female will be harmful to the male and vice versa and because of this there is a pressure of natural selection to concentrate several male genes close to each other on the same chromosome that carries the male gene and female genes on the other chromosome and what is very important for the continuation: to prevent the exchange of genes between these chromosomes so that the offspring will be male or female But not intermediate forms. A way to suppress crossing over is to invert a section of the chromosome so that in that section the chromosomes will not stick to each other and crossing over will not take place. Indeed, the Y chromosome is rich in segments that underwent such an inversion during evolution. The first stage in which an immune segment was formed from a replacement on the ancestor of the Y chromosome happened about 240 million years ago and included the region where the SRY sex-determining gene is located and the last one in which the Y became a completely separate unit 30 million years ago. This evolutionary mechanism of gradual separation of the sex chromosomes has occurred repeatedly in the evolution of animal and plant species that are not hermaphroditic, i.e. those in which there is a genetic difference between males and females. But the animal world is diverse and evolution has different ways to reach the same goal: the chromosome corresponding to "our" Y in butterflies (where it actually determines femaleness) was apparently created in a different way - in the splitting of the gene that determines the species from a normal chromosome.

The second chromosome that appears in two copies in female mammals (or males in birds) also undergoes a unique evolution: since female mammals have double the number of copies of the X chromosome genes, one of these chromosomes is silenced in each cell. That is, the female is a "hybrid creature" (chimera) in which half of the cells express the X from the mother and half from the father. In the male, the X chromosome from the mother is expressed without balance from the chromosome from the father. While a mutation in any other chromosome will usually be hidden in the shadow of the neighboring chromosome, any change in the X chromosome will be expressed in half the cells of the girls and in all the boys. The X chromosome is the place where a positive mutation, one that improves the fitness of the individual, "should" occur. as mentioned,

In humans, the X is the sexiest and most intelligent of the chromosomes. It has a relatively high concentration of genes that act on the reproductive system and the brain, and the rapid development of human intelligence over the past millions of years is explained in no small part by mutations on the X chromosome. Accordingly, hereditary diseases resulting from damage to the X are often manifested simultaneously in mental retardation and sexual development Hurt. The Y chromosome, on the other hand, suffers from all the harms that befall those who cannot refresh themselves and exchange information with friends - it is very difficult for evolution to correct defects in the embryo's chromosome as a single unit without ever isolating good genes separately and defective genes separately. Negative mutations hitch a ride on the Y more quickly and positive mutations can disappear if they occur on an "Abu Al Banat" that has no male offspring. In addition, the Y chromosome undergoes far more mutations than any other chromosome - 5 times more mutations than any other part of the genome. This is because the Y chromosome goes through all its cell division cycles in the hostile environment of the testicle: mutations occur during division and a sperm cell goes through many more divisions than an egg. The sperm is more exposed to oxidative damage and lacks DNA repair enzymes that work in the egg, and as I recall Y does not have a partner to find in the protected shelter of the ovary. The more intense the sperm production, the greater the chance of mutations in the Y chromosome.

Since we separated from the ancestors of the chimpanzees, about 5 million years have passed, from every anatomical and behavioral point of view we have changed more, while the chimpanzee has remained similar to the same ancestor. The Y chromosome is unusual: precisely in the chimpanzees where the competition between males for fertilization is aimed at more vigorous activity of the testicles, the Y chromosome has undergone more dramatic changes and in that tiny corner of the genome we have remained relatively similar to the ancestor.

Evolution's way of dealing with a wayward chromosome is to silence it. If the erosion rate in the Y chromosome is constant, then the math is simple: in 300 million years from the first mammals, the Y chromosome was depleted by 95%, so it will take about 15 million years to completely empty it. The disappearance of the Y will be the end of the male species and therefore also of Homo sapiens in general because there are genes that only work when they originate in the sperm cell. But the animal world provides comfort. Some rodents have already completed the process and their male is satisfied with the X chromosome that does not have a partner. In such cases, the male-determining gene moves to another chromosome, as has already happened in marsupials, and there is evidence of the re-formation of a young sex chromosome around this gene after the previous one has been lost.

The end of the Y will indeed be the end of the human race because reproduction will not be possible between those with the XY system and those with the mutation that will remove it, but biology shows that creatures continue to reproduce even when genes move. Another reassuring fact is that the erosion is not a linear process but fades away and there are models that predict its stopping when the Y chromosome contains a small but stable number of genes. Reinforcement for this model is obtained from a comparison between sex chromosomes of several species of Drosophila flies in which sex chromosomes were formed in the distant or recent past. Humans and rhesus monkeys: two species that split about 30 million years ago have a similar set of genes on the Y chromosome, which reinforces the belief that our Y chromosome has already reached its final size and no further shortening is expected.

 
Did an interesting, intriguing, strange, delusional or funny question occur to you? Send to: ysorek@gmail.com

More of the topic in Hayadan:

• The women's chromosomes have the X factor
• The chromosome stays in the family
• A DNA chain has been discovered that increases fertility and prolongs the lives of Europeans
• Modern man has lost genes that were on the Y chromosome of the Neanderthals
• The Y chromosome was first formed in the ancestors of mammals 180 million years ago