Genetical and Functional Organization of the a Mating Type Locus of Ustilago Maydis

The phytopathogenic fungus Ustilago maydis belongs to the Basidiomycetes and is the causal agent of corn smut (for review see Banuett, 1992). U. maydis can infect leaves, stems, tassels and ears of corn plants. Infection is followed by the formation of galls or tumors, which are filled with black masses of teliospores. The life cycle of U. maydis is characterized by a change in morphology: haploid sporidia are yeast-like, they grow vegetatively by budding and are nonpathogenic. The dikaryon, on the other hand, 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 in planta. only The fusion of haploid cells and the switch between yeast-like and hyphal growth is controlled by two unlinked genetic loci, a and b. A successful mating reaction occurs only between strains carrying different alleles at both mating type loci. The mating reaction can be assayed on charcoal containing plates: only the filamentous growing dikaryon has a white fuzzy (Fuz+ phenotype) appearance. The b locus exists in many different alleles and each allele codes for a pair of homeodomain proteins which are assumed to act as transcriptional regulators when appropriately combined (Gillissen et al., 1992). The a locus, which exists in two alleles (al and a2), controls the fusion of haploid cells and, together with the b locus, the maintenance of filamentous growth (Banuett, 1989).