A fungus walks into a singles bar
During a radio interview once the host asked me to explain fungal sexuality. My answer was a bit too intricate and long to make it into the broadcast–I got bogged down in the details. It’s not that you need a PhD to understand how fungi have sex, it’s just that it’s a most unfamiliar kind of system, in which analogies to human sexuality will get you in trouble. People occasionally ask me about this, and I’ve really never come up with a solid, succinct answer. So I thought I’d have a try at a kind of introduction to the sex life of fungi–at least the part that has to do with sexual compatibility. You tell me what’s unclear, and maybe we can collaboratively improve this piece.
My gift to you: 2.5 days worth of emergence by the dog stinkhorn, Mutinus caninus. It is here mainly to provide an outlet for your anthropomorphizing. The spores of this stinkhorn are its sexual offspring. They are produced in the stinky green goop on the head of the thing. Further words elude me, but for more stinkhorn sauciness, have a look at Michael Kuo’s Stinkhorn Hall of Fame and our own other stinkhorn posts. Time lapse video by Kent Loeffler.
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.
The other kink of fungi (best friends forever?) is that even if they are not sexually compatible (because they are of the same mating type), different individuals of a species may be vegetatively compatible. An entirely different genetic compatibility system dictates whether they can fuse their mycelia to share resources, even though they’ll never have sex. Intriguingly, there are no sexually transmitted diseases among fungi, but there are myco-viruses that can be spread through vegetative compatibility. I’m sure there’s a “nonsexual STD” joke trapped inside this paragraph…
It’s all horribly interesting. There’s much more to say. But it is hard to get our minds around some of this stuff, just because we’re so used to the silly but familiar animal model.
p.s. Were you really expecting me to tell the joke? I’m not telling the joke.
Further reading:
- For regular folk: Tom Volk’s Fungus of the Month for February 2000: Schizophyllum commune
- For boffins: J. Heitman, J.W. Kronstad, J.W. Taylor, L.A. Casselton, Editors. 2007. Sex in Fungi: Molecular Determination and Evolutionary Implications. ASM Press. 572p.
- For gene-jocks: go spelunking in Schizophyllum’s genome!
Thank you, 7Song and Lawrence Millman, for your astute and/or cheeky comments.
Thank you Gillian Turgeon, for your sharp eye and for this advanced mindbender:
“Schizophyllum has two mat loci: A and B. Each of these carries an α and an β sublocus therefore we have AαAβBαBβ for an individual. For compatibility need to be different at Aα or Aβ and at Bα or Bβ. For e.g., Aα1Aβ1Bα1Bβ1 X Aα2Aβ1Bα1Bβ2 would be successful I think…”
Great post — I don’t have any suggestions for improvements.
Very good! I understood it.
The punchline is “And I’m a Gemini!”
Great read, throughly entertaining and informational – the way knowledge should be presented!
Will all or most of the spores from a single fruitbody be vegetatively compatible in a tetrapolar species? i.e. will a substrate inoculated with multispore solution eventually consolidate into a single organism. By single organism I don’t mean genetically homogenous, but rather a single organism in the sense that nutrients and signals move around in a single network.
Or rather will the substrate consist of a number of smaller networks of mycelia that do not share nutrients or information?
Also can you please discuss the causes or mechanisms involved when a dikaryote decides to divorce back into it’s parent monokaryotes?
[…] science writer too, and if you’re curious about the subject, check out her explanation “A Fungus Walks into a Singles Bar . . . “ at the Cornell Mushroom Blog. Or you could just go for the gratuitous Mutinus caninus dog stinkhorn […]
Jon, you ask difficult questions. My own understanding of vegetative (somatic) compatibility in basidiomycetes is sketchy (and I think maybe it’s not just me). Not only does the mushroomy practice of living mainly as dikaryons bring us to a whole ‘nother level of trouble, but also the genetic loci responsible for incompatibility aren’t well-characterized yet (but see here). I don’t expect the offspring of a single mushroom to all merge together, no, I think it’s very likely there will be multiple somatically incompatible groups among the progeny. Also, I’m not aware of any divorce mechanism by which a dikaryon reverts to a monokaryon, although I could imagine you could force this via artificial selection in the lab or by rare chance in nature. Now you see why I was so brief when it came to this part of my explication…
Thanks for the elaboration. If you are right that the progeny remain as individuals, do you imagine that these individuals are at antagonistic towards one another?
The article you link says:
“In the first experiment, we also observed other interactions such as hyphal walls in interaction zones, increased exudation of dark-coloured metabolites and increased production of aerial hyphae”
Could the metabolites indicate stress from competition between strains? On agar, strains will sector out, but they don’t make clear lines of separation like the mycelium of 2 different species typically would.
What do you make of the article’s observations? I’ve never noticed multispore substrates to exude more or less metabolites than isolated strains.
Feel free to edit this out if you feel it’s tangential.