Despite concerns it may be shrinking, male sex chromosome is likely to remain in rude health for many millions of years to come
Reports of the coming death of the male sex chromosome are greatly exaggerated, say scientists, whose work will raise a collective cheer from at least half the population.
The fate of the Y chromosome, which carries the genetic switch that sends a developing embryo down the route to maleness, has been questioned since scientists first discovered it had lost more than 90% of its genes over millions of years of evolution.
The steady withering of the Y has led some to claim that it may vanish completely over the next five million years, leaving humans to join the Okinawa spiny rat on the list of species that make do without a male sex chromosome.
But the unsettling prospect is dismissed in research published today by scientists at the University of California, Berkeley. Having studied the genetic make-up of 16 men, they conclude that natural selection is not about to cast the shrunken male chromosome on the evolutionary scrapheap.
“Natural selection is acting on the Y chromosome and has maintained the genes pretty well,” said Melissa Wilson Sayres, an evolutionary biologist. “All the evidence points toward it not disappearing.”
The Y chromosome has shrunk over time because unlike every other chromosome in the human body, it has no partner. This means it cannot easily be repaired when harmful mutations occur. Most people have 23 pairs of chromosomes including two that govern sex, which are typically X and Y in males, or two Xs in females. The numbers vary in some genetic disorders.
Most chromosomes can repair damage that arises from mutations by swapping DNA with their opposite number, a process called recombination. But the Y is always inherited alone, so has no partner to swap with. As such, the damage builds up until the DNA is discarded, leaving the chromosome that much smaller.
The process has taken its toll on the Y chromosome. Today, the Y carries only 27 genes that are used to make proteins, compared with around 800 on the X chromosome. A few hundred million years ago, early versions of the X and Y were the same size.
Writing in the journal, Plos Genetics, the Berkeley researchers describe the genetic diversity of Y chromosomes in eight European and eight African men. The variation was tiny, and suggests that the Y chromosome has been pared down to its bare essentials by “purifying selection”, an evolutionary process that removes bad mutations from the gene pool.
An alternative explanation for the low genetic variation of Y chromosomes is that a minority of men had a high proportion of children, passing on fewer Y chromosome variants to each successive generation. At the extreme is the Genghis Khan effect, after the Mongol leader who fathered so many sons, his Y chromosome lives on in around 0.5% of the male global population. But the study found that if this was the driving force for low genetic variation on the Y chromosome, fewer than one in four men must have fathered children throughout the course of human history.
According to the researchers, all 27 genes on the Y chromosome, nearly half of which are poorly understood, are acted on by purifying selection. The fact they’re still here suggests they have a valuable role to play in successful breeding.
The Berkeley team is not the first to dismiss the idea that the Y chromosome may go extinct. In work published in 2012, a team of scientists at MIT backed the survival of the Y chromosome after finding that it had lost only one gene in the 25m years since humans and old world monkeys split from their common evolutionary path.
The loss of the Y chromosome would not spell the end of men, but genes for maleness would have to hitch a ride on another chromosome.
“It’s quite clear that purifying selection is operating enormously strongly on these genes. If you can’t make sperm you might as well not exist so far as evolution is concerned because you cannot pass your genes on to the next generation,” said Prof Mark Jobling, a geneticist at Leicester University. But he added that other researchers will want to run their own simulations to verify the work.
Prof Brian Charlesworth, an evolutionary biologist at Edinburgh University, said: “It must be the case that natural selection acting on genes carried on the Y is removing variability across the whole chromosome – the so-called hitchhiking effect, which is especially effective when there is no recombination. This is certainly consistent with the idea that selection is acting on Y chromosome genes.”
“The process is not likely to be ongoing, since once a lot of genes have gone, the degeneration processes more or less grind to a halt,” he added.
The Berkeley team now hopes to study more Y chromosomes to learn whether the genes are subjected to “positive selection”, whereby beneficial mutations spread through the population.
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