Mating between haploid, budding cells of the dimorphic fungus Ustilago maydis results in the formation of a dikaryotic, filamentous cell type. Mating compatibility is governed by two mating-type loci called a and b; transformation of genes from these loci (e.g. a1 and b1) into a haploid strain of different mating type (e.g. a2 b2) allows filamentous growth and establishes a pathogenic cell type...
