THE PHEROMONE CELL SIGNALING COMPONENTS OF THE USTILAGO A MATING-TYPELOCI DETERMINE INTERCOMPATIBILITY BETWEEN SPECIES

The MAT region of Ustilago hordei, a bipolar barley pathogen, harbors distinct mating functions (a and b loci). Here, we show that the b loc us is essential for mating and pathogenicity, and can induce pathogeni city when introduced into a strain carrying a b locus of opposite spec ificity. Transformation experiments using components of the al locus a nd analysis of resulting dual mating phenotypes revealed that this loc us harbors a pheromone receptor gene (Uhpra1) and a pheromone gene (Uh mfa1). These U. hordei al genes, when introduced by transformation, ar e necessary and sufficient to make U. maydis, a tetrapolar corn pathog en, intercompatible with U. hordei MAT-2 but not MAT-I, strains. U. ho rdei strains transformed with the U. maydis al locus also become inter compatible with U. maydis a2, but not al, strains. The interspecies hy brids produced dikaryotic hyphae but were not fully virulent on either corn or barley. Partial, natural intercompatibility was shown to exis t between the sugarcane smut U. scitaminea and both U. hordei and U. m aydis. These results show that the signal transduction pathway for mat ing responses is conserved between different smut species. We conclude that, apart from intraspecies compatibility, the Ustilago a locus als o dictates intercompatibility in this group of fungi.