The a Locus Governs Cytoduction in Ustilago maydis

We have developed a cytoduction assay to measure cell fusion quantitatively in the basidiomycete corn smut fungus Ustilago maydis. This assay employs a mutation conferring resistance to oligomycin that exhibits non-Mendelian inheritance and presumably affects the mitochondrial genome. After auxotrophic olir cells are mixed with prototrophic oli? cells, prototrophic olir cells can be detected at a significant frequency after several hours of incubation, reaching a maximum of 10%Y of the total prototrophs in the mixture after 18 h. We demonstrate that this cell fusion event occurs only if the mating partners have different alleles of the a mating-type locus and is not influenced by the b locus. These studies support the view that the a locus but not the b locus controls establishment of the filamentous, pathogenic state. The corn smut fungus Ustilago maydis undergoes a dra-matic change in morphology and growth habit following the conjugation of two haploids to form a dikaryon (for reviews, see references 1 and 6). Whereas the haploid grows by budding and forms yeast-like colonies on solid media, the dikaryon proliferates as an obligately pathogenic filament within its host. Compatibility between haploids is deter