ZAP // Envato Elements; Biology Online
Whether the X chromosome is a decadent old man, doomed to disappear, or a resilient survivor, finally safe and stable, the future of Humanity is assured. Something will emerge, in the next million years, to replace it in the role of determining our sex.
In a published in 2002 in the magazine Naturewith the title “Human sperm: the future of sex“, the evolutionary biologist Jenny Graves reached a controversial conclusion. “The human Y chromosome you’re running out of time”, wrote the researcher, two years later, in another.
As Graves concluded, the sex chromosome responsible for male sex determination lost 97% of its genes ancestors over the last 300 million years. If this rate continues, the Australian biologist calculated, may disappear in another few million years.
O fate of the damned chromosome Y quickly took over the media, often without the meaning that Jenny Graves intended to convey. Your reflections They did not intend to predict the ‘end of men’nor the extinction of the human species; they were a simplified calculation, made on a sheet of paper, in an academic article that, even so, provoked a “hysterical reaction”.
ZAP did not escape the wave of interest in the fate of the Y chromosome, and over the years we have been reporting on it: that it is in men’s bloodstream, that it is in men’s blood (and in men too), and even that there was one that “kills” Y chromosomes.
“I’m really surprised that anyone is worried about the extinction of men 5 or 6 million years from now“, says Graves to . “After all, only we have been human for 0.1 million years. I think we’ll be lucky if we can survive the next century!”
But if Jenny Graves’ calculation is correct, what does this mean for the Y chromosome — and for the future of men?
The good news is that similar chromosomes in other mammalsas well as in fish and amphibians, have already lost their sex-determining role in genetic rearrangements, and species continue to exist to tell the story.
In some rodents, for example, it was completely and silently replaced. Three species of “Y-less moles”, such as Ellobius talipinus, Ellobius tancrei e Ellobius alaicusnow have only X chromosomes. Sex-determining genes, once in the Y, moved to other locations.
Hedgehog rats (Tokudaia osimensis), lost the Y chromosome, which now exists in a new version, which serves as a sexual determinant.
“If it arises a new variant that works better than our poor old Y, it could take over very quickly,” Graves predicted. “It may have even happened in some human population – How would we know?”
After all, sex-determining variants are not routinely screened for in genomic studies, and if Y chromosome function were to shift to another chromosome in a population, there would be no obvious differences. There would still be men, and would continue to be fertile.
The fate of the Y chromosome has captured worldwide attention for years, but behind the sensationalist headlines, many don’t realize that an intense scientific debate, which places two incompatible views of evolution in direct confrontation.
A current of thoughtto which Graves belongs, views the sex chromosome as a decadent old man, condemned to disappear and which can be replaced at any time. Another school of thought views the Y chromosome as a resilient survivor, finally safe and stable.
The evolutionary biologist Jenn Hughesfrom MIT’s Whitehead Institute, agrees with this last interpretation. There is more than a decade that Jenn Hughes and Jenny Graves respectfully disagree about how to interpret the same evidence, participating in open academic debates.
In a published in 2012 in the magazine NatureHughes and his colleagues found that very few essential Y genes have been lost in the human lineage for the last approximately 25 million years.
More recent evidence reinforces this argument, suggesting that there is a strong conservation of essential Y genes in primatesin contrast to fish and amphibians, where a gradual degradation of the Y chromosome is observed. Some scientists, such as Hughesinterpret this as evolutionary stability long-term mutation of the Y chromosome in primates.
“Our work, comparing the genetic content of Y in various mammals, showed that gene loss was rapid at firstbut it stabilized quickly, and practically stopped occurring”, explains Hughes to Science Alert.
“The genes that remain and perform crucial functions throughout the body, so the selective pressure to maintain them is too strong for them to be lost,” says Jenn Hughes.
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For a decade, Jenny Graves (left) and Jenn Hughes (right) have “respectfully disagreed” about the Y chromosome
Jenny Graves disagrees with these interpretations. Just because a gene is highly conserved, Doesn’t mean it can’t be replacedhe defends. Furthermore, the additional genes identified in the human Y sequence in recent years are largely repeated copiessome of which may be inactive.
In the past, Graves called the Y chromosome “scrap DNA”. Creating multiple copies of a gene can increase the chances of at least one surviving, Graves explains, but also can lead to evolutionary ‘failures’ by accident.
Why is the Y chromosome like this? Evolution is to blame.
“In the ancestor of placental mammals, the X and Y chromosomes were identical and had around 800 genes,” explains Hughes. “When Y specialized in male sex determination (about 200 million years ago), X and Y stopped recombining in malesand the Y began to lose genes. However, the continued to recombine in XX femalesremaining practically unchanged.”
Currently, the human Y chromosome retains only 3% of the genes it shared with X. But these genes were not lost at a steady rate. That is the biggest mistake, argues Hughes.
Graves agrees.
Your prediction for extinction of Y, in about 6 million yearsis based on a linear and constant deterioration, but considers this to be very unlikely, which means that the estimate has a very wide margin of error.
Although at times it may appear that the Y chromosome is stabilizing, Graves argues that These ‘photographs’ don’t lasteven though they have apparently persisted for 25 million years.
“I see no reason to suppose that Y degradation has stopped, or can stop, in primates or any other group of mammals,” Graves said. “It is slow and progresses bumpilyfor reasons that we understand well.”
After a public debate between Hughes and Gravesduring an international conference in 2011, on whether the Y chromosome is stable or doomed, the audience voted 50/50, split exactly in halfabout which hypothesis would be correct. Let’s hope it doesn’t take 6 million years to break the tie.
