Isolation of Two Alleles of the b Locus of Ustilago maydis

The basidiomycete Ustilago maydis is the causative agent of the disease corn smut. To be pathogenic, strains of U. maydis must be heterozygous for a locus called ``b,'' which appears to control both pathogenicity and sexual development. Two alleles of the b locus of U. maydis were isolated by complementation and hybridization. The clones have the specificities of b1 and b2 alleles as demonstrated by their effects on the colony morphology and pathogenicity of haploid and diploid strains upon transformation. For example, nonpathogenic haploid and diploid strains of U. maydis carrying b2 alleles became pathogenic when transformed with the cloned b1 allele. Furthermore, an a2 b2 haploid strain could be transformed to an a2 b1 genotype by gene replacement using a DNA fragment containing a b1 allele and a selectable marker. The isolation of b alleles represents an important step toward understanding the control of dicaryon formation, dicaryon maintenance, and pathogenicity in U. maydis.