Weird evolution: platypus sex

Say hello to the platypus, one of the strangest animals in the world. It lays eggs but isn't a bird. It produces milk but has no nipples. It has a skeleton like a reptile. And it's one of the only venomous mammals on the planet.  In addition to all of that, it has five times as many sex chromosomes as we humans do. Oh yes. Let's talk about platypus sex.

First, though, a quick overview of sex chromosomes in humans. 

Our DNA is organised within 23 pairs of these little wiggly looking things—our chromosomes. One pair constitutes our sex chromosomes, which determine the sex we're born as. If both are an X, you're female. And if you have one X and one Y, you're male.

When we humans decide to make a baby human, the chromosomes split in order to create a new configuration with half from each parent. The egg from the mother will have 23 of her 46 total chromosomes, including one of the two Xes from her sex chromosomes. 

The sperm from the father will have 23 of his chromosomes, including either an X or a Y.

When the egg and sperm meet, if the sperm carried an X chromosome, it's a girl. If it carried a Y, it's a boy.

So it’s a fairly reasonable mechanism. It doesn't need to get any more complicated than that. But it does. Let's return to the platypus.

They have 21 pairs of ordinary chromosomes, plus 5 pairs of sex chromosomes. That's 8 more total sex chromosomes than us. But there are only two sexes of platypus. So... why. Just, why, platypus? But we'll get to that later. First, let's look at how their sex is determined.

Like humans, their sex chromosomes are either X or Y. And, again, like us, the female will have all Xes, and the male will have half Xes and half Ys.

When it comes time to make a new platypus, the splitting of the female sex chromosome seems straightforward enough. They only have Xes, so no matter which of those Xes they pass on, 100% of the sex chromosomes they pass on will be Xes, just like in humans. 

There are a few caveats here, as you may notice that not all X chromosomes look the same... but in terms of the percentage split of the Xes, there's nothing new here.

But with the males, you can see that the sex chromosomes can’t split at random, because then, you might get a platypus with 3 Ys and 7 Xes, or 1 Y and 9 Xes, and so on. 

So when the male sex chromosomes are forming into sperm, they pay special attention to the order they're in. They start in an alternating chain like this. When they produce sperm, they take every *second* sex chromosome. Depending on which one they take first, they'll produce sperm with five Xes, or sperm with five Ys.

Once the egg and sperm meet, if there are 10 Xes, it’s female—and if there are 5 Xes and 5 Ys, it’s male. There’s nothing in-between.

So there's how the platypus gets around this strange quirk of genetics. But why is it like this in the first place, when two sex chromosomes work just fine? Do five Ys make the male platypus more sexy? Do the enforced combinations of Xes and Ys make the platypus more fit for survival? The answer to these questions is no. In fact, having 10 sex chromosomes is a hazard more than anything else.

190 million years ago, the ancestor of the platypus had a single pair of sex chromosomes, like us. Then, during a single event of platypus reproduction, the pair accidentally swapped pieces with an ordinary pair of chromosomes, and passed the chain of their four sex chromosomes on to future generations. This happened again, making a chain of six, and so on. 

This breaking and rejoining of chromosomes happens all the time. Sometimes, the result is unbalanced, which means it’ll be harder for the offspring to survive. But the result can also be balanced, which means it’ll probably be just as fit as any other platypus, and may well pass that mutation through the species. And when these kinds of mutations happen, they can’t “unhappen”. Chromosome breakage occurs at random places, so another break and join won’t restore the original pattern.

So, ultimately, it was a perfect storm of chance that led to this bizarre configuration in the platypus. It’s not going to fix itself, because the platypus is just fine how it is, and evolution doesn’t have the opportunity to go back to a clean slate. So, platypuses are stuck with this weird artifact of their evolution.

Ah, but isn’t that just the story of us all? If you'd like to read more about the quirky side of evolution, there’s more to discover at Nova.

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