Why Men Won't Be Going Extinct Any Time Soon
Men can breathe a sigh of relief—they won’t be disappearing any time soon. It turns out that reports of the Y chromosome’s imminent demise have been greatly exaggerated.
In fact, say researchers, despite having lost many of its genes over millions of years of evolution, the Y chromosome persists because it is responsible for more than just giving men their sex characteristics.
In a new paper published this week in the journal Nature, researchers provide more evidence that the Y chromosome’s gene-shedding days are over.
"This paper tells us that not only is the Y chromosome here to stay, but that we need to take it seriously, and not just in the reproductive tract," said study author David Page, director of the Whitehead Institute, in a press release. "There are approximately a dozen genes conserved on the Y that are expressed in cells and tissue types throughout the body. These are genes involved in decoding and interpreting the entirety of the genome. How pervasive their effects are is a question we throw open to the field, and it's one we can no longer ignore."
The Incredible Shrinking Chromosome: In humans, sex is determined by the X and Y chromosomes. Women have two X chromosomes, while men have one X and one Y. When the gene responsible for "maleness" along with sperm production first arose on the Y chromosome, some 320 million years ago, the X and Y chromosome both shared the same set of 600 “ancestral” genes.
Today, only 19 of those original genes remain on the Y chromosome, which is evident by its much smaller size. This rapid gene loss led some cell biologists to believe that the Y chromosome would continue to shrink until it disappeared completely. By comparison, the X chromosome has lost only two percent of its ancestral genes throughout history.
According to the new study, though, the Y chromosome is no longer changing at such a rapid rate, losing only one gene over the past 25 million years. The researchers discovered this by comparing the Y chromosomes of humans with those of chimpanzees and rhesus monkeys, as well as to the chromosomes of more distantly related mammals, such as marmosets, mice, rats, bulls, and opossums.
The Y Chromosome Is Holding Steady: So the Y chromosome seems to be here to stay, and the reason why, say researchers, is that many of the ancestral genes that remain on the Y chromosome are responsible for necessary functions that go beyond sex determination.
"Evolution is telling us these genes are really important for survival," said Winston Bellott, a research scientist at Whitehead Institute and lead author of the new paper. "They've been selected and purified over time."
These genes are involved in protein synthesis, and they are active in areas beyond the reproductive tract—in the lungs, heart, blood, and other tissues throughout the body. Because these genes are used for vital functions, their continued survival is more likely to be favored by evolution.
Not everyone, though, is convinced that the Y chromosome is out of the woods. “Y degradation is clearly not a linear process,” said professor Jennifer Graves of the La Trobe Institute of Molecular Science, in a Nature news report. “The last stages of decay are likely to be subject to great fluctuations.” In fact, she said, two species of spiny rats in Japan have already shed their Y chromosomes—yet the males still survive.Y Chromosome Hints at Future Gender-Based Medicine
It may take millions of years for us to find out whether men will shed their Y chromosomes completely like the Japanese rats, but the researchers believe that the Y chromosome has much to offer us in the short-term.
If the Y chromosome plays a role in regulating cells outside of the reproductive tract, it’s possible that cells that carry an X-Y pair of chromosomes could react differently under certain circumstances than those that carry an X-X pair. This could potentially result in differences between men and women in both disease resistance and severity.
This type of gender-based medical research is nothing new, but the authors call for taking it to the cellular level.
Mammalian Y chromosomes retain widely expressed dosage-sensitive regulators.
"There is a clear need to move beyond a unisex model of biomedical research," said Page, "which means we need to move beyond a unisex model of our understanding and treatment of disease."
The human X and Y chromosomes evolved from an ordinary pair of autosomes, but millions of years ago genetic decay ravaged the Y chromosome, and only three per cent of its ancestral genes survived. We reconstructed the evolution of the Y chromosome across eight mammals to identify biases in gene content and the selective pressures that preserved the surviving ancestral genes. Our findings indicate that survival was nonrandom, and in two cases, convergent across placental and marsupial mammals. We conclude that the gene content of the Y chromosome became specialized through selection to maintain the ancestral dosage of homologous X–Y gene pairs that function as broadly expressed regulators of transcription, translation and protein stability. We propose that beyond its roles in testis determination and spermatogenesis, the Y chromosome is essential for male viability, and has unappreciated roles in Turner’s syndrome and in phenotypic differences between the sexes in health and disease.Genetics: The vital Y chromosome:Comparisons of Y-chromosome sequences in various mammals reveal abundant gene loss early in the chromosome's evolution but remarkable gene stability across the Y chromosomes of extant species.
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