Selfing versus outcrossing propensity of the fungal pathogen Microbotryum violaceum across Silene latifolia host plants

In the fungal pathogen Microbotryum violaceum mating (i.e, conjugation betw een cells of opposite mating type) is indispensable for infection of its ho st plant Silene latifolia. Since outcrossing opportunities are potentially rare, selfing may be appropriate to ensure reproduction. On the other hand, outcrossing may create genetic variability necessary in the coevolutionary arms race with its host. We investigated the propensity of M. violaceum to outcross vs, self in diff erent host environments. We used haploid sporidia from each of three strain s from five fungal populations for pairwise mixtures of opposite mating typ e, representing either selfing or outcrossing combinations. Mixtures were e xposed to leaf extract from seven S. latifolia plants. The proportion of co njugated sporidia quantified mating propensity. The identity of both fungal strains and host influenced conjugation. First, individual strains differed in conjugation frequency by up to 30%, and str ains differed in their performance across the different hosts. Second, self ing combinations produced, on average, more conjugations than did outcrossi ng combinations. Selfing appears to be the predominant mode of reproduction in this fungus, and selfing preference may have evolved as a mechanism of reproductive assurance. Third, individual strains varied considerably in co njugation frequency in selfing and outcrossing combinations across differen t hosts. This indicates that conjugation between outcrossing partners could be favoured at least in some hosts. Since the dikaryon resulting from conj ugation is the infectious unit, conjugation frequency may correspond with i nfection probability. This assumption was supported by an inoculation exper iment, where high infectious sporidial dosage resulted in higher infections success than did low dosage. We therefore predict that sexual recombinatio n can provide this pathogen with novel genotypes able to infect local resis tant hosts.