The Distribution of Mating-Type Bias in Natural Populations of the Anther-Smut Ustilago violacea on Silene alba in Virginia

Complete individual-wide mating-type bias (retrieval of sporidia of only one mating type from germinated teliospores of one fungal individual) was observed to be a common and widespread feature of the anther-smut fungus, Ustilago violacea, collected from natural populations of its host, Silene alba. The bias was usually to mating type Al, but the frequency of bias and its spatial distribution varied from region to region. Populations with high frequencies of bias still showed high rates of disease transmission. Crosses between Al mating type sporidial lines from completely biased individuals and A2 mating types from unbiased individuals showed no bias in the progeny. During teliospore germination, biased individuals often showed conjugation among adjacent cells of the promycelium, suggesting that both mating types are present in the germinating teliospore but one mating type is unable to grow as free-living sporidia. The complete bias was most readily interpreted as evidence of "haploid lethals" linked to mating type that cause poor survival or growth of the sporidial stage. The results show that such "haploid lethals" may be a common occurrence in natural populations, and that fungal mating systems may vary considerably over short distances.