Mating behaviour of Nannizzia otae (=Microsporum canis).

One hundred and ninety-eight isolates of Microsporum canis, obtained from humans and animals in 12 countries, were paired with the two Japanese tester strains of Nannizzia otae (= M. canis), VUT 74037 (CDC B-2094+) and VUT 74039 (CDC B-2095-). One hundred and forty-one (72%) produced either gymnothecia or pseudogymnothecia in crosses with VUT 74037. Fifty-seven (28%) were nonreactive. None of the paired isolates reacted with VUT 74039. The number of nonreactive isolates decreased to 17% when 104 of the 198 isolates were paired with one additional tester strain of each mating type. All sexually reacting strains, however, belonged to the (-) mating type. Crosses between nonreactive isolates did not result in ascocarp formation. The F1 generations from three different strongly reactive crosses were all characterized by poor ascospore germination. Most of the monoascospore progeny that germinated to form mature colonies were nonreactive in crosses to determine their mating type. Others reacted predominantly as the (+) mating type, thereby precluding the likelihood of an associated lethal factor accounting for the lack of this mating type in our clinical isolates. Several explanations for this phenomenon are presented.