It Looks Like the Y Chromosome (and men) are Here to Stay

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x-chromosomeThe male-sex defining Y chromosome, has only 19 of the approximately 600 genes it once shared with the female-defining X chromosome over 200 million years ago, making it much smaller than the X and our 21 other chromosome pairs. In fact, the Y has so few genes left that there was some concern that the loss of a few more could push it to extinction, and with it the male sex. Two independent studies, one led by Daniel W. Bellot and David C. Page of the Whitehead Institute of Biomedical Research at M.I.T. and the other by Diego Cortez and Henrik Kaessmann of the University of Lausanne in Switzerland indicate that this fear is unfounded. Their results were published in the journal Nature.

Page and Bellot and their colleagues looked at the evolutionary history of the Y, decoding the DNA sequence of the Y chromosome of eight mammals, including the rhesus monkey and humans. Their analysis shows that the rhesus monkey’s Y chromosome has about the same number of genes as the human Y, suggesting that the Y had stopped losing genes 25 million years ago when the two species diverged from a common ancestor. “We were actually brought up short by how stable the Y has been during the most recent 25 million years,” Page says. The Cortez and Kaessmann group came to a similar conclusion after studying 15 animals.

It’s believed that the reason for the Y’s stability is that many of its remaining genes are necessary for more than sex determination and are necessary for the survival of all humans. Some of the genes affect protein synthesis, how active a gene is, and others splice RNA segments together. They are found throughout the body, including the heart, the blood, and the lungs. Page says, “These are powerful players in the central command room of cells. It puts this notion of the ‘rotting Y’ to rest.” The Swiss team also found that Y chromosome genes had a broader influence throughout the body than just sex determination and development.

However, not all scientists are convinced that the “rotting Y” idea should be put to rest. Jennifer Graves, a geneticist at the Australian National University in Canberra thinks there just may be a lull in the long-term trend of Y chromosome degradation and that the stability may be temporary. She says “At least two rodent groups have managed to dispense with it. Y degradation is clearly not a linear process. The last stages of decay are likely to be subject to great fluctuations.”

Page believes that the core Y genes are stable in so many species, that further loss is unlikely. “It could happen, but I just don’t see it,” he says.

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