Thursday, May 3, 2012
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!
According to Tom Volk at the University of Wisconsin, Schizophyllum commune can have 28,000 different mating strains! It sounds like exactly the same sort of frequency dependent selection that leads to hundreds of self-sterility alleles is responsible.
In Schizophyllum commune there are more than 300 alleles at the A locus and more than 90 known for the B locus. Thus there are more than 28,000 different combinations of A and B, or 28,000 different sexes! Each individual is compatible with 27,997 of the others in the worldwide population (99.98% outbreeding) compared with being compatible with only 1/4 of its siblings. Thus the enormous number of sexes in fungi is meant to encourage non-sibling mating and non-relative mating, which ensures genetic diversity in the population. This seems to have worked quite well in the widely distributed Schizophyllum.