Pathogen relatedness affects the prevalence of within-host competition.

Although the evolutionary consequences of within-host competition among pathogens have been examined extensively, there exists a critical gap in our understanding of factors determining the prevalence of multiple infections. Here we examine the effects of relatedness among strains of the anther-smut pathogen Microbotryum violaceum on the probability of multiple infection in its host, Silene latifolia, after sequential inoculations. We found a significantly higher probability of multiple infection when interacting strains were more closely related, suggesting mechanisms of competitive exclusion that are conditional on genotypic characteristics of the strains involved. Pathogen relatedness therefore determines the prevalence of multiple infection in addition to its outcome, with important consequences for our understanding of virulence evolution and pathogen population structure and diversity.