The Y chromosome may be a symbol of masculinity, but it becomes increasingly clear that this chromosome is not strong and durable. Despite carrying the master switch, SRY, which determines whether an embryo will develop as a male (XY) or female (XX), this chromosome contains only very few other genes and this is the only chromosome that is not absolutely necessary for life. What is clear, women proved to be fine without this chromosome.
Moreover, the Y chromosome degenerates aka deterioration rapidly, so women have two completely normal X chromosomes, while men have one X chromosome and one shrinking Y chromosome. If the same degeneration rate continues, the Y chromosome has only 4.6 million years left before it disappears altogether. It seems like a very long time, but not really if you contemplate that life has been on Earth for 3.5 billion years.
Not always Y chromosome like that. If we turn back at 166 million years to the earliest mammals, the story is completely different. The early "proto-Y" chromosomes were initially the same size as the X chromosome and contained all of the same genes. Only, the Y chromosome has a fundamental flaw. Unlike all the other chromosomes, the two copies we have in each of our cells, the Y chromosome only appears as a single copy, passed from father to son.
This means the genes on the Y chromosome are not capable of undergoing genetic recombination. Genetic recombination is the "shuffling" of genes that occur in each generation and helps to eliminate harmful gene mutations. Not benefiting from recombination, the Y chromosomal gene degenerates over time and eventually disappears from the genome.
Nevertheless, recent research shows that the Y chromosome develops some very convincing mechanisms to "stop", slowing the rate of gene loss to a halt.
For example, a recent Danish study published in PLoS Genetics, sequencing Y chromosome portions from 62 different males and found that these chromosomes readily accept large-scale structural reorganization that allows for "gene amplification" - the acquisition of multiple copies of genes that improve sperm function healthy and reduce the loss of genes.
The study also shows that the Y chromosome develops unusual structures called "palindroms" (same readable DNA sequence from front and back - like the word "kayak"), which protects it from further degradation. They record the rate of "gene conversion events" in the sequence of palindromes on the Y chromosome - this is essentially a "copy-paste" process that enables the repair of genes using undamaged backup copies as an archetype.
Looking at other species (Y chromosomes present in mammals and some other species), more and more evidence suggests that the amplification of the Y chromosome gene is a general principle that applies to all. The amplified genes play a very important role in sperm production and (at least in rodents) in regulating the sex ratio of offspring. Writing in Molecular Biology and Evolution recently, the researchers provide evidence that the increasing number of copies of genes in mice is the result of natural selection.
Regarding the question of whether the Y chromosome will actually disappear, the scientific community, like the British today, is split into "leavers" and "remainers" groups. The latter group argues that its defense mechanisms do a remarkable job and save Y chromosomes. But the leavers say that the mechanism is only allowing the Y chromosome to hang on, before it finally falls into the abyss. And the debate continued.
The leading proponent of the disappearing argument, Jenny Graves of La Trobe University in Australia, states that, if you use a long-term perspective, the Y chromosome must be destroyed - although it sometimes lasts longer than expected. In a 2016 paper, he points out that Japanese spiked mice and mole voles have completely lost their Y chromosomes - and that the process of missing or creating genes in the Y chromosome must have caused fertility problems. This could then lead to the formation of entirely new species.
Male extinction?
As we pointed out in a chapter on the new e-book, even if the Y chromosome in humans did disappear, it does not necessarily mean that men are in the process of becoming extinct. Even in species that completely lose the Y chromosome completely, males and females are still equally necessary for reproduction.
In such cases, the SRY master switch genes that determine the genetic trait of males have migrated to different chromosomes. That is, the species produce males without the need for a Y chromosome. However, the new sex-determining chromosome - which SRY displaces - must then begin to degenerate again in the absence of the same recombination that destroy their previous Y chromosome.
Interestingly, however, with respect to humans is that although a Y chromosome is necessary for normal human reproduction, many of the genes it carries are not necessary if you use reproductive techniques with help. This means that genetic engineering can soon replace the function of the Y chromosome gene, allowing couples of infertile women or men to conceive. However, if everyone may be pregnant in this way, it seems highly unlikely that fertile humans will stop reproducing naturally.
While this is a fascinating and contentious genetic research area, there is not much to worry about. We do not even know whether the Y chromosome will disappear altogether. And, as we have pointed out, if it does, it is likely that we will continue to need men to keep normal reproduction going.
In fact, the prospect of a "farm animal" type system where a handful of "lucky" men are chosen to be fathers for the majority of our children are clearly invisible on the horizon. What is clear, there will be a much more urgent concern in the next 4.6 million years.
Komentar
Posting Komentar