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Genetics

Heterothallism and Mating Type Mutation in Sclerotinia trifoliorum. J. Y. Uhm, Former research assistant, Plant Pathology, Tokyo University of Agriculture, Tokyo, 156, Japan, Present address: Assistant professor, Department of Agricultural Biology, College of Agriculture, Kyungpook National University, Daegu, 635, Korea; H. Fujii, professor, Plant Pathology, Tokyo University of Agriculture, Tokyo, 156, Japan. Phytopathology 73:569-572. Accepted for publication 28 October 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-569.

Each ascus of Sclerotinia trifoliorum contains four large and four small ascospores. The small-spore strains (strains derived from single small spores) are self-sterile, but cross-fertile only with those from large-spore strains (strains derived from large spores). On the other hand, the large-spore strains are always self-fertile, but their asci again show 4:4 segregation by spore size. From the mating behavior of small-spore strains, this fungus was determined to have bipolor heterothallism. Spore size difference is the pleiotropic expression of mating type. The mating types of large- and small-spore strains were designated as L and S, respectively. Formation of apothecia by large-spore strains is due to mutation for mating type and subsequent pairing of parental type and mutated nuclei in the same thallus; therefore, the resulting asci show segregation for spore size as well as mating type. However, the mating-type mutation occurs in one direction only, from L to S, since the S cultures are always self-sterile. The sexual role of microconidia was also demonstrated by successful mating in which spermatization of the sclerotia of small-spore strains was accomplished with microconidia of large-spore strains.