S-1. Mating Type Mutation in Sclerotinia trifolirum Frikss , the Causal Fungus of the Sclerotial Rot of the Forage Legumes

다국어 초록 (Multilingual Abstract)

Sclerotinia trifoliorum Erikss. displays dimorphism in spore size, with 4 : 4 segregation of large and small ascospores in each ascus. The two sized spores are arranged in six types of first and second meiotic division. The second meiotic division frequency(mean =20.8%) did not vary significantly among isolates of various sources. This indicated that the dimorphism in spore size was regulated by the same allelic pair in all isolates.
The single ascosporic strains derived from the large and small ascospores of an ascus were very similar in morphology, growth rate and pathogenicity. Without exception, however nonpaired cultures from single ascosporic strains derived from small spores (small spore strains) failed to produce apothecia, whereas those from large spores (large spore strains) produced sporulating apothecia but their asci again showed the 4 : 4 segregation for spore size.
Mating experiments revealed that the small spore strains are self-sterile but cross-fertile only with the large spore strains. From the mating behavior of the small spore strains, this fungus which has hitherto been known as homothallism was determined to have bipolar heterothallism. The spore size difference was determined to be a pleiotropic expression of mating type allele, since the factors determining mating type and ascospore size were inseparable. The mating types of large and small spore strains were designated as L and S, respectively.
Apothecial formation by single 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 mutation, however, occurs in one direction only, from L to S, since the small spore strains are invariably selfsterile.
The sexual role of the microconidia was demonstrated for the first time by successful mating in which spermatization of the sclerotia of small spore strains was accomplished using microconidia of large spore strains.