Despite concerns it may be shrinking, male sex chromosome is likely to remain in rude health for many millions of years to come.Ian Sample, science correspondent
The Guardian, Thursday 9 January 2014
One explanation for the lack of genetic variation on the Y chromosome is the Genghis Khan effect, with a small number of men fathering many children. Photograph: AlamyReports 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 that 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 might vanish completely over the next five million years, leaving humans to join the Okinawa spiny male rat on the list of species that make do without a sex chromosome. But that unsettling prospect is dismissed in research published on Friday by scientists at the University of California, Berkeley. Having studied the genetic makeup of 16 men, they conclude that natural selection is not about to cast the shrunken male chromosome on the evolutionary scrapheap.
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. Typically, people have 23 pairs of chromosomes including two that govern sex, which are 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. 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 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, named 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 were the driving force for low genetic variation on the Y chromosome, fewer than one in four men would have fathered children in 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.
"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 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.
Thursday, January 16, 2014
The Y Chromosome is Not Doomed to Shrivel Away to Nothing
Well, dang, I guess that's some good news . . . . From The Guardian (UK):