Mating in fungi is controlled by the loci that determine the mating type of an individual, and only individuals with differing mating types can mate. Basidiomycete fungi have evolved a unique mating system, termed tetrapolar or bifactorial incompatibility, in which mating type is determined by two unlinked loci; compatibility at both loci is required for mating to occur. The multi-allelic tetrapolar mating system is considered to be a novel innovation that could have only evolved once, and is thus unique to the mushroom fungi. This domain is C-terminal to the homeodomain transcription factor region.

1: The divergence-homogenization duality in the evolution of the b1 mating type gene of Coprinus cinereus. Badrane H, May G; Mol Biol Evol 1999;16:975-986. PMID:10406114