The Y chromosome isn't going extinct after all

A few years back, we learned that the Y chromosome was essentially rotting, shedding hundreds of its genes over the last 300 million years. That isn't wrong, but it turns out reports of the Y chromosome's eventual extinction were premature.

The X and Y chromosomes first emerged as the two sex-determining chromosomes - X for female, Y for male - somewhere between 150 and 300 million years ago, depending on which evidence you look at. Whatever the precise timeframe, the story remains much the same. While all the other chromosomal pairs - there are 22 of these in humans, plus a combination of X and Y to make 23 - kept swapping genes with each other over the generations, the X and Y chromosomes stopped this crossing over process.

That's where the trouble started for the Y chromosome. Because it's the lesser partner here - everyone has at least one X chromosome, but only males have a Y chromosome - it suffered more from the cessation of communication between chromosomes. When one of the segments of the X stopped crossing over with the Y, the male chromosome underwent rapid genetic decay. Four more X segments stopped crossing over, and the freefall continued so that today the modern X chromosome possesses just 19 of the original 600 or so genes that made up its ancestor chromosome.

That discovery is what led to reports of the rotting Y chromosome, and it's that notion that David Page, the director of MIT's Whitehead Institute, has spent the last decade trying to refute. Now, he says he and his team have found the most conclusive proof yet that the Y chromosome is in no danger of winnowing away to nothing. It's all a question of timescales.

While the last 150 to 300 million years look pretty bad for the Y chromosome, things look much better when you focus on the more recent past. To demonstrate this, the researchers sequenced the Y chromosome of the Rhesus macaque, a monkey that diverged from our evolutionary ancestors about 25 million years ago. As it turns out, the Rhesus and human Y chromosomes are more or less identical - the Rhesus hasn't lost any ancestral genes on its Y chromosome in the last 25 million years, and the only one we lost in that time comprised a small segment of about 3% of the entire chromosome.

In announcing the results, Page made offered an entertainingly pugnacious take on what this means for the idea of a rotting Y chromosome:

"For the past 10 years, the one dominant storyline in public discourse about the Y is that it is disappearing. Putting aside the question of whether this ever had a sound scientific basis, the story went viral-fast-and has stayed viral. I can't give a talk without being asked about the disappearing Y. This idea has been so pervasive that it has kept us from moving on to address the really important questions about the Y.

"The Y was in free fall early on, and genes were lost at an incredibly rapid rate. But then it leveled off, and it's been doing just fine since. This paper simply destroys the idea of the disappearing Y chromosome. I challenge anyone to argue when confronted with this data."

In case you feel like arguing the point - and I'd be careful, because he is clearly not messing around - you can check out the data for yourself over at Nature.

Via MIT. Image by Creations, via Shutterstock.