Selfing Propensity under Choice Conditions in a Parasitic Fungus, Microbotryum violaceum, and Parameters Influencing Infection Success in Artificial Inoculations

The anther smut fungus Microbotryum violaceum has a mixed mating system, being able to either self or outcross. The infection success may be influenced by selfing or outcrossing. It will affect the amount and distribution of genetic variation within natural populations and, in particular, in this system, may explain the maintenance of common highly deleterious alleles linked to the mating‐type locus. Using artificial inoculations with two different mixtures of strains that have different identifiable genotypes on the host plant Silene latifolia, we estimate infection success and selfing propensity of this fungus. Fungal spores were placed in flowers, mimicking natural spore deposition by pollinators. Successful infection was found in 31.5% of inoculated plants, and infection success depended on the number of inoculations and host plant sex, females having a higher per‐contact risk but males a higher global probability of infection. Many host individuals were infected by more than one fungal genotype, and separate stems of the same plant were infected independently. We found 14.3% and 28.3% outcrossing events for the two mixtures of fungal strains, confirming that M. violaceum is predominantly selfing. Because we inoculated with spore mixtures, this predominant selfing did not result from a lack of available mating partners, which could further reduce outcrossing in nature. We suggest that selfing may largely result from intratetrad mating (=intrameiotic), and we estimated the intratetrad mating rate between 43.3% and 71.4%. This has important consequences for understanding the maintenance of haplolethal alleles in M. violaceum.