http://blogs.discovermagazine.com/d-brief/2017/11/06/fungus-...
> Fungi, by contrast, keep it casual. To mate, all a fungus has to do is bump up against another member of its species and let their cells fuse together. S. commune uses a special kind of structure called a clamp connection to do this, and it allows them to exchange their cell’s nuclei, along with the genetic information inside. This keeps reproduction simple and means that a potentially huge number of sexes is possible — other fungi species have dozens or more, though S. commune is certainly an outlier.
> The “sexes” don’t really involve physical differences either, as we might think of when the word “sex” comes to mind. The variations are all in the genome, at two separate loci, or locations, each of which has two alleles, or alternate forms. The loci are called A and B and the alleles are termed “alpha” and “beta.” That makes four possible sexes, but there’s another twist. Every A-alpha/beta and B-alpha/beta can have many different variants, called specificities. It amounts to more than 339 specificities for A and 64 for B. Putting those two together yields thousands of possible unique sexes.
> The fungus can mate with any specificity as long as it’s different somewhere on both A and B. So, two prospective mates could both have the same A-beta and B-alpha, but have different A-alphas and B-betas and they’d be fine to hook up. If they shared A-alpha and A-beta, though, their pheromones wouldn’t be compatible, meaning that they couldn’t carry out the reproductive process. That leaves a ton of options for mating, though, and essentially means that anyone a fungus meets is fair game for sexy time.
But are those real sexes? They do apparently have some sort of impact on reproduction, but the only limitation seems to be that they can't mate with someone who has the exact same structure of the these alleles.
Which makes me wonder if this might have developed not so much as a way to help reproduction, but as a guard against inbreeding.
Yes. Sexes aren't defined as being based on different chromosomes or having different physical anatomy (there are more mainstream examples of outliers for both of these). It's just a classification system for reproduction limitations.