HETEROZYGOSITY AT THE B-MATING-TYPE LOCUS ATTENUATES FUSION IN USTILAGO-MAYDIS

Mating and pathogenesis of the corn smut fungus, Ustilago maydis, are controlled by two unlinked mating-type loci, a and b. Yeast-like haplo ids that differ at both loci are compatible and fuse to establish a pa thogenic dikaryon. Mating is assayed in vitro by co-inoculation on cul ture medium containing activated charcoal; compatible combinations hav e a characteristic ''fuzzy'' appearance caused by the growth of aerial hyphae. In general, this test has not been useful for assaying the ma ting ability of strains that are already mycelial (e.g., those heteroz ygous at b or at both mating-type loci). Using an assay for cytoductio n involving transfer of a mitochondrial marker during transient cell f usion, and engineered strains with defined genotypes, we examined the mating abilities of strains heterozygous or hemizygous at the mating-t ype loci. The data (which have not been available from conventional pa thogenicity or plate mating tests) show that heterozygosity at b atten uates fusion in haploid and diploid strains, whereas strains heterozyg ous at a retain the ability to fuse with a compatible haploid partner. It appears, therefore, that subsequent fusion events are attenuated o nce fusion has occurred to establish the U. maydis dikaryon.