Nick Lane

They don't really have – they still have two sexes, but they have mating types as well.

And you can have 27,000 mating types in some fungi, which is all about outbreeding so you can mate with just about anything.

Oh, yeah.

It's becoming fungal.

Yes.

So two sexes then in that sense is the worst of all possible worlds you can only mate with.

If you had only one sex, if everyone was a hermaphrodite, you could mate with everybody.

And if you had three sexes, you could mate with two-thirds of the population and so on.

So why two?

Well, this fundamental difference that one is passing on the mitochondria and the other is not.

Beyond that, if you've got multiple mating types, you still have one passes on the mitochondria and the other one doesn't.

So in these fungi that have all of these mating types, there's a kind of a pecking order

that the dominant one will pass on the mitochondria and the less dominant one doesn't pass on the mitochondria.

So you end up with really complex systems.

You can imagine that it's pretty hard to enforce this.

Stuff can go wrong.

The more complex the system is, the more it will go wrong.

So I guess in that sense, why do you end up with two sexes?

It's partly minimization of error.

Yeah, so as soon as you've got this fundamental difference, even in single-celled critters, that one of the sexes passes on the mitochondria and the other one doesn't.