Present and Future aspect of Y-chromosome
The sex of human and other mammal babies is decided by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years, leading to our extinction unless we evolve a new sex gene.
The good news is two branches of rodents have already lost their Y chromosome and have lived to tell the tale.A new paper in Proceedings of the National Academy of Science shows how the spiny rat has evolved a new male-determining gene.
How the Y chromosome determines human sex
In humans, as in other mammals, females have two X chromosomes and males have a single X and a puny little chromosome called Y. The names have nothing to do with their shape; the X stood for “unknown”.
The X contains about 900 genes that do all sorts of jobs unrelated to sex. But the Y contains few genes (about 55) and a lot of non-coding DNA — simple repetitive DNA that doesn’t seem to do anything.
But the Y chromosome packs a punch because it contains an all-important gene that kick-starts male development in the embryo. At about 12 weeks after conception, this master gene switches on others that regulate the development of a testis. The embryonic testis makes male hormones (testosterone and its derivatives), which ensures the baby develops as a boy.
This master sex gene was identified as SRY (sex region on the Y) in 1990. It works by triggering a genetic pathway starting with a gene called SOX9 which is key for male determination in all vertebrates, although it does not lie on sex chromosomes.
The disappearing Y
Most mammals have an X and Y chromosome similar to ours; an X with lots of genes, and a Y with SRY plus a few others. This system comes with problems because of the unequal dosage of X genes in males and females.
How did such a weird system evolve? The surprising finding is that Australia’s platypus has completely different sex chromosomes, more like those of birds.
In platypus, the XY pair is just an ordinary chromosome, with two equal members. This suggests the mammal X and Y were an ordinary pair of chromosomes not that long ago.
In turn, this must mean the Y chromosome has lost 900–55 active genes over the 166 million years that humans and platypus have been evolving separately. That’s a loss of about five genes per million years. At this rate, the last 55 genes will be gone in 11 million years.
Our claim of the imminent demise of the human Y created a furore, and to this day there are claims and counterclaims about the expected lifetime of our Y chromosome – estimates between infinity and a few thousand years
Rodents with no Y chromosome
The good news is we know of two rodent lineages that have already lost their Y chromosome – and are still surviving.
The mole voles of eastern Europe and the spiny rats of Japan each boast some species in which the Y chromosome, and SRY, have completely disappeared. The X chromosome remains, in a single or double dose in both sexes.
Although it’s not yet clear how the mole voles determine sex without the SRY gene, a team led by Hokkaido University biologist Asato Kuroiwa has had more luck with the spiny rat — a group of three species on different Japanese islands, all endangered.
Kuroiwa’s team discovered most of the genes on the Y of spiny rats had been relocated to other chromosomes. But she found no sign of SRY, nor the gene that substitutes for it.
Now at last they have published a successful identification in PNAS. The team found sequences that were in the genomes of males but not females, then refined these and tested for the sequence on every individual rat.
What they discovered was a tiny difference near the key sex gene SOX9, on chromosome 3 of the spiny rat. A small duplication (only 17,000 base pairs out of more than 3 billion) was present in all males and no females.
They suggest this small bit of duplicated DNA contains the switch that normally turns on SOX9 in response to SRY. When they introduced this duplication into mice, they found that it boosts SOX9 activity, so the change could allow SOX9 to work without SRY.
What this means for the future of men
The imminent — evolutionarily speaking — disappearance of the human Y chromosome has elicited speculation about our future.
Some lizards and snakes are female-only species and can make eggs out of their own genes via what’s known as parthenogenesis. But this can’t happen in humans or other mammals because we have at least 30 crucial “imprinted” genes that work only if they come from the father via sperm.
To reproduce, we need sperm and we need men, meaning that the end of the Y chromosome could herald the extinction of the human race.
The new finding supports an alternative possibility — that humans can evolve a new sex determining gene. Phew!
However, evolution of a new sex determining gene comes with risks. What if more than one new system evolves in different parts of the world?
A “war” of the sex genes could lead to the separation of new species, which is exactly what has happened with mole voles and spiny rats.
So, if someone visited Earth in 11 million years, they might find no humans – or several different human species, kept apart by their different sex determination systems.
The Y-chromosome is one of a pair of chromosomes that determine the genetic sex of individuals in mammals. When the chromosomes were discovered then both the X & Y chromosomes had same size and contain all the same genes. But during evolution the size of Y chromosome becomes decreased. Y chromosomes have a fundamental flaw. Y chromosomes are present as a single copy, inherited from fathers to his sons, unlike all other chromosomes, which contain two copies. Because there cannot be genetic recombination between genes on the Y chromosome, there can be no reassortment, which prevents harmful mutations from being separately chosen against. The master-switch gene for sex determination, also known as the sexdetermining region Y, is located on the Y chromosome (SRY gene). If a fertilized egg cell, called a zygote, has the SRY gene, the zygote develops into an embryo that has male sex traits. If the zygote lacks the SRY gene, the zygote develops into an embryo that has female sex traits. The Ychromosome lost most of its ability to recombine after the evolution of the SRY gene, and a large inversion occurred on the Y-chromosome. There is a tendency for non-paired chromosomes or nonrecombining chromosomes to decay to the point where they no longer function, only genes essential for male fertility have retained their function on the Y-chromosome, while most of the other genes are decayed versions of genes located on the Xchromosome. It contains very few other genes and is the only chromosome not necessary for life. Women, after all, manage just fine without one. What is happening with the Y chromosome? Some studies shown that, it has been degenerating rapidly and just 4.6 million years are left before it disappears completely. Unlike others, the Y chromosome has no pair, so it might fall weak before the evolutionary forces. You might have pondered why a boy with a bald father always inherits this quality. It is impossible to quiet or find a substitute for the defective Y chromosomal genes. Compared to women, who have two X chromosomes, defective genes on one chromosome cannot be silenced by genes on the opposite chromosome. As a result, genetic recombination, or the “shuffling” of genes that takes place every generation and helps in the elimination of harmful gene mutations, cannot occur on the Y chromosome. According to certain research, Y chromosomes have evolved through time into selfpreserving “Palindroms,” which protect it from further degeneration. The chromosome has considerably shrunk over the span of 200 million years of evolution, according to scientific observation. When we hear that the Y chromosome is disappearing, our thoughts will start to wander and we’ll start to worry about what will happen to males. Are we making space for a new species or will they eventually disappear? Researchers at Hokkaido University in Japan have made sure that men won’t disappear any relatively soon. In some ways, the way the Y chromosome disappeared from a particular male species of rat has provided us with a roadmap for how the same thing may occur with human males. What this means for the future of men? Future scenarios have been discussed in the context of the Y chromosome’s approaching extinction, at least in evolutionary terms. A process known as parthenogenesis allows some lizard and snake species, which are only found in females, to produce eggs from their own DNA. However, neither humans nor other mammals can experience this as we possess at least 30 essential genes that are “imprinted” and only function when passed on from the father by sperm. Because males and sperm are required for reproduction, the loss of the Y chromosome may indicate the end of the human race. The latest discovery gives support to a different hypothesis, namely that humans may develop a new sex-specific gene. However, there are dangers involved with the introduction of a new sex gene. The split of new species as a result of a “struggle” of the sex genes has occurred in mole voles and spiny rats. Thus, if someone travelled to Earth in 11 million years, they may not find any people there or find a variety of different human species, each with its own method of determining sex. This gene heteromorphism is caused by the majority of the Y chromosome’s genes no longer functioning. Due to the Y’s unidirectional transmission from father to son and absence of sexual recombination, it has undergone degeneration. Natural selection is less effective in non-recombining genomes because selection at one site interacts with the actions of selection at related sites. On a nonrecombining Y chromosome, two types of natural selection can prevent the elimination of harmful mutations. According to models that only include negative selection, the actual number of Y chromosomes decreases as harmful mutations are continuously eliminated by purifying selection. Thus increase the effects of genetic drift, decreasing the overall effectiveness of purifying selection. Due to background selection and Muller’s ratchet models, harmful mutations can only accumulate on the Y as a result of other deleterious mutations. An alternate hypothesis is that the Y chromosome’s degradation is significantly influenced by positive selection. According to this theory, associated harmful mutations are carried along by repeated fixations of highly desirable mutations (selective sweeps). Type of gene located on the Y chromosome that is working like the growth of testes, the production of sperm and determining the sex of the child. Y chromosomes and other older Y chromosomes hardly show any signs of the degenerative processes that lead to them. Analyzing patterns of diversity on young Y chromosomes that are still experiencing degeneration offers the best chance of differentiating between positive and negative selection as reasons of degeneration. The selective sweep model predicts a marked excess of low-frequency mutations relative to neutral expectations, whereas negative selection models produce a less severe distortion in the frequency spectrum of mutations, even though both positive and negative selection models might account for reduced variability on an evolving Y chromosome.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
