The a and b Loci of Ustilago maydis Hybridize with DNA Sequences from Other Smut Fungi. G. Bakkeren. Biotechnology Laboratory, University of British Columbia, V6T 1Z3, Canada. B. Gibbard(1), A. Yee(1,3), E. Froeliger(4), S. Leong(4), and J. Kronstad(1,2,3). (1)Biotechnology Laboratory, Departments of (2)Microbiology and (3)Plant Science, University of British Columbia, V6T 1Z3, Canada; (4)Plant Disease Resistance Research Unit, U. S. Department of Agriculture-Agricultural Research Service, and Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. MPMI 5:347-355. Accepted 2 April 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

The smut fungi are obligately parasitic during the sexual phase of their life cycle, and the mating-type genes of these fungi play key roles in both sexual development and pathogenicity. Among species of smut fungi it is common to find a bipolar mating system in which one locus with two alternate alleles is believed to control cell fusion and establishment of the infectious cell type. Alternatively, several species have a tetrapolar mating system in which two different genetic loci, one of which has multiple alleles, control fusion and subsequent development of the infection hyphae. Cloned sequences from the a and b mating-type loci of the tetra-polar smut fungus Ustilago maydis were used as hybridization probes to DNAs from 23 different fungal strains, including smut fungi with both tetrapolar and bipolar mating systems. In general, all of the smut fungi hybridized with the mating-type genes from U. maydis, suggesting conservation of the sequences involved in mating interactions. A selection of DNAs from other ascomycete and basidiomycete fungi failed to hybridize with the U. maydis mating-type sequences. Exceptions to this finding include hybridization of DNA from the a1 idiomorph of U. maydis to DNA from one strain of U. violacea and hybridization of both a idiomorphs to DNA from Saccharomyces cerevisiae.