Is the Y chromosome dying out?

The sex we're assigned at birth depends largely on a genetic flip of the coin: X or Y? Two X chromosomes and you (almost always) develop ovaries. An X and a Y chromosome? Testes. These packages of genetic material don't just differ in terms of the body parts they give us. With 45 genes (in comparison to around 1,000 on the X), the Y chromosome is puny. And research suggests it has shrunk over time — a proposition that some have, in turns, glumly or gleefully interpreted as predicting the demise of men.

So is the Y chromosome really dying out? And what might that mean for men?

To begin to answer these questions, we have to go back in time. "Our sex chromosomes weren't always X and Y," said Melissa Wilson, an evolutionary biologist at Arizona State University. "What determined maleness or femaleness was not specifically linked to them." 

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When the very first mammals evolved between 100 and 200 million years ago, they didn't have any sex chromosomes at all. Instead, the X and Y were just like any other set of chromosomes — identical in size with corresponding structures, Wilson said.

It's important to note that animals don't need sex chromosomes. That was true then, and it's still true now, said Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia. All of our chromosomes are a cocktail of sex-related and non-sex-related genes. The only special feature of the Y chromosome is one gene, SRY, which acts as an on-off switch for the development of testes, Graves added. In the case of alligators and turtles, an on-off switch isn't even necessary — the temperature in which embryos develop determines their sex. Our mammalian ancestors likely shared this characteristic, Graves wrote in a 2006 review on the subject, published in the journal Cell. But at some point, a plain old, non-sex chromosome in one of these ancestors developed a gene with an on-off switch like this. And that was it: you suddenly needed a Y to develop male reproductive parts.

But as soon as the Y chromosome existed, it was primed to shrink. Over time, genes develop mutations, many of which are harmful, Wilson said. Chromosomes can avoid passing on these mutations by recombining with one another. During meiosis, when our bodies produce sperm and eggs, paternal and maternal chromosomes randomly mix and match their arms. This genetic dance breaks up variants of genes — harmful and beneficial alike — and makes it more likely that only functional copies will get passed on. All the chromosomes do this: chromosome 1 from mom swaps arms with chromosome 1 from dad, and so on. The Y, however, does not have a swapping companion. Although X chromosomes can recombine with one another, Y chromosomes and X chromosomes aren't similar enough to recombine. And because you rarely have two Y chromosomes in an individual, Y can't recombine with itself. 

"If a bad mutation occurs, usually you'd be able to swap with your partner. But the Y can't do that," Wilson said. So Y chromosomes accumulated harmful mutations; over time, those mutations were weeded out by natural selection until the Y got smaller and smaller. 

Graves's research suggests that 166 million years ago, the Y chromosome had 1,669 genes — "same as the X-chromosome" at that time, she said. "So it doesn't take a great brain to realize that if the rate of loss is uniform — 10 genes per million years — and we've only got 45 left, the whole Y will disappear in 4.5 million years." 

Uniform is the key word here. More recent research suggests that the rate of degradation has slowed over time. In a 2005 study published in the journal Nature, researchers compared the human Y chromosome with that of a chimpanzee. Then in 2012, the same team of researchers sequenced the Y chromosome of a rhesus monkey, again publishing the results in Nature. The researchers found that the human Y chromosome has lost only one gene since humans and rhesus monkeys diverged evolutionarily 25 million years ago. It hasn't lost any genes since the divergence of chimpanzees 6 million years ago. These results suggest that decay has not occurred in the linear fashion that Graves originally suggested, in which 10 genes are lost per million years.

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Loss of the Y chromosome isn't off the table — it's happened to other species, Graves pointed out. Two species of underground rodents called mole voles have independently lost their Y chromosomes. So have three endangered species of spiny rats living on several small islands in Japan.

But as those species demonstrate, the loss of the Y chromosome doesn't doom survival; both spiny rats and mole voles still have males and females. "People think that sex is sort of a very determined thing," said Rasmus Nielsen, a geneticist at the University of California, Berkeley, "That if you have a Y chromosome, then you're a man, or you don't have [a] Y chromosome, then you're female. But it doesn't work like that." 

In fact, 95% of genes that are expressed differently between males and females don't actually live on the X and Y chromosomes, Wilson said. For instance, ESR1, a gene that encodes for estrogen receptors, is found on chromosome 6. These receptors are vital for female growth and sexual development. 

"Losing the Y chromosome doesn't mean losing the male," Nielsen added. Instead, the loss of the Y chromosome would likely mean that another gene would take over the job as the main determinant of sex — the on-off switch, Graves said. "There are heaps of genes out there that will do a perfectly good job." 

But how likely is that to happen? "It's possible," Wilson said, "but not in our lifetime."

Originally published on Live Science.