Why do most isogamous species have so few mating types, yet some have so many?
Mating types determine compatibility between the gametes of sexually reproducing eukaryotes. In anisogamous species (those with gametes of different sizes) mating types are synonymous with the two sexes. In isogamous species (those with morphologically similar gametes) the picture is more complex. Simple evolutionary reasoning suggests that a near infinite diversity of mating type classes should be observed. However, while empirical observations show that such a scenario is possible (the fungus Schizophyllum commune has a staggering 23,328 “sexes”) most species have just two. In this talk I will show how this discrepancy can be largely explained by a null model of mating type dynamics. Analytical results will be presented that support the view that much of the variety in mating type number across isogamous species can be explained by a balance between the mutation and extinction of mating type alleles.