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 on ly one mating type from germinated teliospores of one fungal individua l) was observed to be a common and widespread feature of the anther-sm ut fungus, Ustilago violacea, collected from natural populations of it s host, Silene alba. The bias was usually to mating type A1, but the f requency of bias and its spatial distribution varied from region to re gion. Populations with high frequencies of bias still showed high rate s of disease transmission. Crosses between A1 mating type sporidial li nes from completely biased individuals and A2 mating types from unbias ed individuals showed no bias in the progeny. During teliospore germin ation, biased individuals often showed conjugation among adjacent cell s of the promycelium, suggesting that both mating types are present in the germinating teliospore but one mating type is unable to grow as f ree-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 dist ances.