Reports of the imminent death of the male Y chromosome have, to quote Mark Twain, been greatly exaggerated, according to researchers.
Source: Creative images
Some experts believe the dwindling Y chromosome, which determines male characteristics, is on the way out - and even that men could be extinct in five million years.
But now it might be time for chaps to break open the beer and breath a sigh of relief. A new comparison of Y chromosomes in 16 African and European men indicates that male genes are here to stay.
“The Y chromosome has lost 90% of the genes it once shared with the (female) X chromosome, and some scientists have speculated that the Y chromosome will disappear in less than five million years,” said US evolutionary biologist Dr Melissa Wilson Sayres, from the University of California at Berkeley.
“Our study demonstrates that the genes that have been maintained, and those that migrated from the X to the Y, are important, and the human Y is going to stick around for a long while.”
Patterns of Y chromosome variation in the 16 men were found to be consistent with natural selection acting to maintain their gene content, much of which plays a role in male fertility.
Chromosomes are the wrapped bundles of DNA found in the nuclei of every cell. Along the DNA strands are chemical sequences which provide the coded instructions for making proteins - these are the genes.
In relative terms, the Y chromosome is puny, containing just 27 unique genes compared with thousands on other chromosomes.
Over time, it has been stripped down to its bare essentials - but that does not mean it is destined for the scrap heap, said the scientists.
The study, reported in the online journal Public Library of Science Genetics, focused on the low variation of the Y chromosome, which was best explained by the effects of intense natural selection.
Evolutionary pressure to weed out bad mutations had trimmed the chromosome down and turned it into a lean machine.
“We show that a model of purifying selection acting on the Y chromosome to remove harmful mutations, in combination with a moderate reduction in the number of males that are passing on their Y chromosomes, can explain low Y diversity,” said Dr Wilson Sayres.
Prior to 200 million years ago, when mammals were still relatively new on Earth, early versions of the X and Y sex chromosomes existed as interactive pairs. With each generation, they swapped a few genes so that offspring were a genetic mix of their parents.
Then the gene for male features became fixed on the Y chromosome, and attracted other male-specific genes such as those controlling development of the testes, sperm and semen.
Many of these turned out to be harmful for females, so the X and Y stopped swapping genes and started to evolve separately.
Today women have a matched pair of X chromosomes while men have an XY pair.
Because the X and Y cannot swap genes it is more difficult for mistakes in the Y chromosome to be corrected.
This has made it more sensitive to natural selection, and led to its degradation over time, scientists believe.
The Y chromosome’s 27 genes include 17 still retained after 200 million years and 10 more recently acquired but poorly understood.
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