Genetic Regulation of Mating and Dimorphism in Ustilago Maydis

Abstract

The phytopathogenic fungus Ustilago maydis belongs to the Basidiomycetes and is the causal agent of corn smut disease (for review see 1). All green parts of the corn plant can be infected. The initial phase of the disease is characterized by plant tissue chlorosis and anthocyanin pigmentation, later galls or tumors develop which are filled with black masses of teliospores. U. maydis adopts two different morphologies during its life cycle. Haploid sporidia are yeast-like, they grow vegetatively by budding and are nonpathogenic. When two compatible haploid strains fuse they form a dikaryon. This form is filamentous and is able to infect corn plants. The dikaryotic stage is unstable on artificial media and requires the plant for sustained growth; karyogamy and spore formation also occur exclusively in planta. The fusion of haploid cells and the switch from yeast-like to hyphal growth is controlled by two unlinked mating type loci termed a and b. A successful mating reaction requires that the two strains that mate carry different alleles at a and b. Formation of dikaryotic filaments can be assayed on charcoal containing plates where the filamentous dikaryon has a white fuzzy appearance (Fuz⁺ phenotype) while the haploid cells continue their yeast-like growth and have a smooth surface. The a locus, which exists in two alleles, a 1 and a2, controls the fusion of haploid cells and is together with the b locus responsible for maintenance of the filamentous form (2).