Different a alleles of Ustilago maydis are necessary for maintenance of filamentous growth but not for meiosis.

Two loci (the incompatibility or mating type loci), a and b, govern the life cycle of Ustilago maydis, a fungal pathogen of corn. U. maydis diploids heterozygous at both a and b (a[unk] b[unk]) form mycelial colonies (Fuz(+) phenotype), induce tumors, and produce teliospores able to undergo meiosis. We report here the isolation and characterization of nonmycelial (Fuz(-)) derivatives. These Fuz(-) diploids have allowed us to examine the requirement of a[unk] and b[unk] in maintenance of filamentous growth and tumor-inducing ability. The Fuz(-) diploids are of four classes: two are inferred to be homozygous for b (a[unk] b=); the other two are shown to be homozygous for a (a= b[unk]). These observations confirm the requirement for b[unk] and reveal the requirement for a[unk] in filamentous growth. U. maydis is thus like other basidiomycetes that have two loci governing growth of the dikaryotic filament. The mating behavior of the Fuz(-) diploids indicates that heterozygosity at a or b does not block mating. Although both a= b[unk] and a[unk] b= diploids are nonmycelial, they differ in that a[unk] b= diploids are nonpathogenic, whereas a= b[unk] diploids are pathogenic and produce teliospores able to undergo meiosis. These findings substantiate previous, more limited observations. They demonstrate that ability to induce tumors and produce normal teliospores requires different alleles at b but not at a.