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Sporidial Mating-Type Ratios of Teliospores from Natural Populations of the Anther Smut Fungus Microbotryum (= Ustilago) violaceum

In the fungal plant pathogen Microbotryum violaceum, infection is achieved by the dikaryotic conjugation product between two haploid sporidia differing in mating type. Each diploid teliospore, the transmission unit, is heterozygous for the mating-type locus; therefore, germination and meiosis should produce haploid sporidia of both mating types. We investigated the mating-type ratios generated by growing sporidial colonies from single teliospores collected from six populations in two regions of northern Switzerland. One-third of the teliospores produced strongly or totally biased mating-type ratios. Usually, teliospores from a single fungal collection showed bias in both directions; however, in one population a consistent bias in one direction was found. Either more rapid replication of one mating type, perhaps on account of deficiency mutations, or a head start in replication can lead to the loss of one of the mating types after several cell cycles on laboratory medium. An experiment on replication rates of sporidia with the two different mating types confirmed that differential replication could lead to mating-type bias, although the observed difference in replication rates did not correlate with mating-type biases recorded from the teliospore from which these sporidial colonies originated. Teliospore collections from which we obtained only totally biased teliospore colonies in our survey formed conjugation hyphae when suspended on leaf tissue. Therefore, we suggest that, early after germination, meiotic products of both mating types must be present, and early self-conjugation is possible. A comparison of mating-type bias from colonies isolated 4 and 24 d after teliospore germination showed a significant change in the degree of bias, indicating that bias develops over time. Mating-type bias may influence the breeding system and the probability of self compared with outcross conjugations. However, infection initiation must take place quickly, before the flowers where spores arrive fall off the plant. Therefore, processes occurring early after spore deposition may be far more important for disease production than those that develop over several cycles of replication of the haploid sporidia.