The Y chromosome disappears - so what happens to men?

The Y chromosome disappears - so what happens to men?

Darren Griffin, University of Kent and Peter Ellis, University of Kent

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The Y chromosome may be a symbol of masculinity, but it is becoming increasingly clear that it is anything but strong and stable. Although it carries the "master switch" gene SRY, which determines whether an embryo develops as a male (XY) or female (XX), it contains very few other genes and is the only chromosome that is not necessary for life. After all, women get along quite well without it.

In addition, the Y chromosome degenerates rapidly, so that women have two completely normal X chromosomes, but men only have one. That may sound like a long time, but it isn't when you consider that life has existed on Earth for 3.5 billion years.

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The Y chromosome wasn't always like this. If we turn the clock back to 166 million years ago, to the very first mammals, the story is completely different. The early “proto-Y” chromosome was originally the same size as the X chromosome and contained the same genes. However, Y chromosomes have a fundamental flaw. Unlike all other chromosomes, of which we have two copies in each of our cells, Y chromosomes are only ever present in a single copy and are passed on from father to son.

This means that genes on the Y chromosome cannot undergo genetic recombination, the “mixing” of genes that occurs every generation and helps eliminate harmful gene mutations. Without the benefits of recombination, Y-chromosomal genes degenerate over time and are eventually lost from the genome.

Chromosome Y in red, next to the much larger X chromosome.
National Institute for Human Genome Research

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Still, recent research has shown that the Y chromosome has evolved some pretty compelling mechanisms to "put the brakes on" and slow gene loss to the point where it may come to a halt.

For example, a recent Danish study published in PLoS Genetics sequenced parts of the Y chromosome from 62 different men and found that it is prone to large-scale structural rearrangements that enable “gene amplification” – the acquisition of multiple copies of genes that promote health, improve sperm function and mitigate gene loss.@media(min-width:0px){#div-gpt-ad-healthy_holistic_living_com-large-leaderboard-2-0-asloaded{max-width:336px!important;max-height:280px!important;}}

The study also showed that the Y chromosome has developed unusual structures called "palindromes" (DNA sequences that read forwards and backwards - like the word "kayak") that protect it from further degradation. They recorded a high rate of "gene conversion events" within the palindromic sequences on the Y chromosome - essentially a "copy and paste" process that allows damaged genes to be repaired using an undamaged backup copy as a template.

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Looking at other species (Y chromosomes exist in mammals and some other species), there is increasing evidence that Y chromosome gene amplification is a general principle. These amplified genes play a crucial role in sperm production and (at least in rodents) in regulating the sex ratio of offspring. Recently, researchers in Molecular Biology and Evolution provided evidence that this increase in gene copy numbers in mice is a result of natural selection.

When it comes to the question of whether the Y chromosome will actually disappear, the scientific community, as is currently the case in Great Britain, is divided into the “Leavers” and the “Remainers”. The latter group argues that their defense mechanisms are doing an excellent job and have saved the Y chromosome. But the losers say they are just allowing the Y chromosome to hang on by its fingernails before it finally falls off the cliff. The debate therefore continues.

A leading proponent of the Leave argument, Jenny Graves of La Trobe University in Australia, argues that if you take a long-term perspective, Y chromosomes are inevitably doomed - even if they sometimes last a little longer than expected. In a 2016 paper, she points out that Japanese spiny rats and mole voles have completely lost their Y chromosomes - and argues that the process of losing or creating genes on the Y chromosome inevitably leads to fertility problems. This, in turn, can ultimately drive the formation of entirely new species.

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The downfall of men?

As we argue in a chapter of a new e-book, the disappearance of the Y chromosome in humans does not necessarily mean that men themselves are on the way there. Even in those species that have actually completely lost their Y chromosomes, both males and females are necessary for reproduction.

In these cases, the SRY "master switch" gene that determines genetic maleness is relocated to a different chromosome, meaning these species produce males without the need for a Y chromosome. However, the new sex-determining chromosome - the one to which SRY transitions - should begin the process of degeneration again due to the same lack of recombination that doomed the previous Y chromosome.

The interesting thing about humans, however, is that although the Y chromosome is required for normal human reproduction, many of the genes it carries are not necessary when you use assisted reproductive techniques. This means that genetic engineering could soon be able to replace the gene function of the Y chromosome and give same-sex couples or infertile men the opportunity to become pregnant. But even if everyone were able to get pregnant this way, it would be highly unlikely that fertile people would simply stop reproducing naturally.

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Although this is an interesting and hotly debated area of ​​genetic research, there is no need to worry. We don't even know if the Y chromosome will disappear at all. And even if this is the case, as we have shown, we will most likely still need men for normal reproduction to continue.

In fact, the prospect of a “farm animal”-like system where a few “lucky” men are chosen to father the majority of our children is certainly not in the cards. In any case, there will be far more pressing concerns in the next 4.6 million years.

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Darren Griffin, Professor of Genetics, University of Kent and Peter Ellis, Lecturer in Molecular Biology and Reproduction, University of Kent

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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