History of the Mating Types inUstilago maydis

This chapter presents some general features of Ustilago maydis and a synopsis of its life cycle. The life cycle of U. maydis is characterized by three cell types: a cigar-shaped haploid, unicellular form that divides by budding and is nonpathogenic; a dikaryotic, filamentous form that grows at the tip cell and is pathogenic; and a diploid spore, the teliospore, formed within the tumors induced by the fungus, that is not capable of vegetative growth but germinates and undergoes meiosis to produce the haploid yeast-like form. Heterothallism and Sex Factors Stakman and Christensen demonstrated for the first time that U. maydis is heterothallic. Six mutants designated bmut (three bGmut, two bDmut, and one bImut) were obtained, all of which induced tumors and produced teliospores when inoculated with their progenitors, with themselves and with each other, and thus behave as “universal” alleles. The b locus of U. maydis was the first multiallelic mating-type locus cloned and understood at the molecular level and provided a framework in which to understand the more complex A multiallelic mating-type locus of other basidiomycetes. Strains with an inactive gene for the pheromone precursor can respond to the pheromone produced by cells of opposite mating type but they cannot induce a response in cells of opposite mating type, and strains with an inactive receptor gene can induce a response in cells of opposite mating type but cannot respond to cells of opposite mating type.