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Sunday, January 29, 2012

The sex life of fungi

Fungi don’t have physical differences in gender the way animals do.  Instead, they have specific regions of their genomes that code for sexual identity.  These sites determine with which other fungi they are compatible.  But here’s the rub:  there can be more than just two forms of mating type genes (alleles), so fungi can have more than two sexes.

Ronny Kellner and Dominik Begerow from Ruhr-Universität Bochum and Evelyn Vollmeister and Michael Feldbrügge from Heinrich-Heine University Düsseldorf have been studying a type of parasitic fungus called a ‘grass smut’.  This fungus parasitizes grass crops such as wheat, but only after mating.  Thus, grass smuts must find compatible sexual partners before they can proceed through their life cycle.  Fortunately for them, they have three distinct mating types.  To be clear, this does not mean that the fungi require two partners to procreate, but rather that each fungus can successfully mate with either of the other two sex types.

And that's not a grass smut's only reproductive advantage.  They can also successfully mate across species lines.  That may not seem unusual until you note that the species in question diverged over a hundred million years ago, which is about how long ago humans diverged from elephants.  This loose sexual selection gives the fungus access to more genetic variety, which can allow it to parasitize more freely and to evade biological counterattacks.

In case you think having three sexes is extreme, you may want to consider the fungus Schizophyllum commune.  Because S. commune has close to 400 different mating genes spread between two locations on its genome, this little organism comes in about 28,000 sexual forms, each of which can mate with any other form.