Thursday, August 5, 2010

A Species with 10,000 Sexes???

I once got into a vigorous debate with a computer scientist who simply refused to believe me when I said that some species have more than two sexes. The example I was thinking of was slime molds. Here's a bit about their odd sexuality (from Wapedia):
Q: I seem to remember a high school bio teacher once telling me that slime molds have close to a dozen genders? is that right?

A: even worse, according to [[1]]: "in fact it’s a slime mold (genus Physarum), an otherworldly creature with 29 variants of sex-controlling genes, dispersed among eight different types of sex cells. To ensure genetic diversity, each slime mold sex cell can only fuse with a sex cell that has completely different variants of genes than its own. If you calculate all the possible combinations of genes and sex cells, you will find that Physarum have more than 500 different sexes." (ZooGoer 33(2) 2004. Copyright 2004 Friends of the National Zoo. All rights reserved.)
But fungi as a group are full of the wonders of mind-boggling sex. Here's a bit from a post on the fascinating Cornell Mushroom Blog entitled "A fungus walks into a singles bar":
The trickiest concept for discussing fungal sex is “gender.” In humans, there are men and there are women (from a strictly reproductive point of view), and it takes one of each to make a baby. The male donates some genetic material (a gamete, the sperm), which is received by a female gamete (the egg), and those gametes get together to form a new human. We trust you’re all familiar with this scene.

Among fungi, any individual can donate or receive genetic material–so you can already see we need to let go of the concept of gender. Let’s talk instead in terms of what mycologists call mating types. A fungus simply needs to find a mate of a different mating type. Of the fungi you might be familiar with, hmm, most species have only two mating types (they’re bipolar), and some have four or more possible mating types (they’re tetrapolar). Any particular individual of a species is just one mating type, of course. Most molds have two; many mushrooms and bracket fungi have four or more. A few fungi, like the unassuming split gill, Schizophyllum commune, have more than ten thousand!

In the same way that our genders are controlled by our genetics (Kathie has two X chromosomes; Bradford has an X and a Y), mushroom mating types are determined by genes. In mushrooms, either one or two sets of genes control the ability to mate. Mating type genes in fungi don’t confer secondary sexual traits like facial hair or Adam’s apples; they do control 500 to 1000 genes involved in the development of sexual structures and spores. Shockingly, either fungus partner can get pregnant (by making a mushroom), or be a dad (by delivering a gamete), or both. The whole division of labor thing is an animal quirk.

Now here is where it gets really crazy. If you haven’t shed your attachment to gender, now’s the time. In many large, charismatic fungi, genes at two different locations on the chromosomes control what’s called a tetrapolar mating system. In these fungi, two individuals must differ at both loci to make a good match. Now let’s say you’re one of these fungi. If at location MAT-A you have the A2 mating type allele, and at location MAT-B you have the B1 mating type allele, then your mating type is A2B1. You must find a partner who is different than you at both locations (may I suggest A1B2?). Your beautiful baby spores will be this mix: A1B1; A2B2; A1B2; and A2B1. If your babies should get together and try to mate (perish the thought), they will only succeed 25% of the time. Hello? You with me?

It got a little complex there, didn’t it? The deal is, a fungus just has to find a mate of a different mating type. So actually, when a species has a lot of mating types, it’s EASIER for an individual to find a mate, because the odds go up. In contrast to humans–a human can typically mate successfully (have kids, I mean) with about half the people in the room. But a fungus might find that nearly everybody on the dance floor is a potential mate. See?

Homosexual fungi? Well, not exactly. At least in the human version of homosexuality nobody’s going to get pregnant without some outside input. But there are some fungi that are self-compatible (homothallic)–they can have offspring without a partner. There are two ways to do this. One is to have copies of all the needed mating type alleles in each nucleus (in the majority of mushrooms, a spore contains only a single nucleus, and a haploid one at that). The other way to go is to pack two different, compatible nuclei into a spore, ready to mate. This latter method is how the common supermarket button mushroom does it. I know! It seems like such an ordinary mushroom.
You gotta love those wacky sexy fungi! It is a little mind-blowing to think of the 10,000 mating types of the Schizophyllum commune!

Oh... definitely you must visit the Cornell Mushroom Blog post above to watch the fascinating movie of dog stinkhorn effloresce, so to speak.

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