Heterothallism and Mating Type Mutation inSclerotinia trifoliorum

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Abstract
Each ascus of S. trifoliorum contains 4 large and 4 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 bipolar 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 1 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.