Male Chromosome is Not Dying Out: Y's Demise Not Inevitable

Is the Y chromosome doomed to disappear? That's not the case, according to a new study. Scientists have found that the common notion that the Y's genes are mostly unimportant and that the chromosome is destined to dwindle and disappear is actually untrue.

"The Y chromosome has lost 90 percent of the genes it once shared with the X chromosome, and some scientists have speculated that the Y chromosome will disappear in less than five million years," said Melissa Wilson Sayres, one of the researchers, in a news release.

The idea of a disappearing Y chromosome doesn't mean that males will cease to exist. Some mammals have actually already lost their Y chromosome and still have males and females that reproduce normally. In fact, researchers have managed to create Y-less male mice that still produce normal offspring. Yet this new study reveals that the Y chromosome isn't as superfluous as previous research may indicate.

Before about 200 million years ago, early versions of the sex chromosomes were just like other pairs of chromosomes; with each generation, they swapped a few genes so that offspring were a mix of their parents' genes. Fertilized eggs that got two proto-Xs became females and eggs with a proto-X and proto-Y became males.

For some reason, though, the gene that triggers the cascade of events that results in male features became fixed on the Y chromosome and attracted other male-specific genes. The X and Y do not swap DNA over most of their lengths these days. This means that the Y cannot efficiently fix mistakes, so it has degraded over time.

In order to examine this phenomenon a bit more closely, the scientists examined variation throughout human history; more specifically, they looked at the low variation in the Y chromosomes. They demonstrated that if fewer males were the only cause of the low variability, it would mean that fewer than one in four males throughout history had passed on their Y chromosome each generation. Variations in other human chromosomes make this an unlikely scenario. Instead, it's likely that the low variation can be explained by intense natural selection.

In fact, the researchers found that patterns of variation on the Y chromosome are consistent with natural selection acting to maintain the gene content there, much of which has been shown to play a role in male fertility. Because there is so much natural selection working on the Y chromosome, there has to be a lot more function on the chromosome than researchers originally thought.

"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," said Sayres in a news release.

The findings are published in the journal PLOS Genetics.