Host susceptibility and spread of disease in a metapopulation of Silene dioica

Hypothesis: In host–pathogen metapopulations, founder events and restricted gene flow cause differentiation in susceptibility among patches and populations of the host. Patterns of disease spread correspond to levels of susceptibility in host populations. Organism: The host-plant Silene dioica and the sterilizing anther-smut fungus Microbotryum violaceum. Study site: An archipelago in northern Sweden where populations of S. dioica constitute a hierarchically age-structured metapopulation and where M. violaceum is common. Methods: Plants from patches within recently diseased island populations were transplanted into an experimental population where they were naturally exposed to the anther-smut. We also included two populations with a long history of disease. Results: We found that susceptibility varied among recently diseased populations, while no significant differentiation in susceptibility was detected among patches within populations. Populations showing an increase in disease prevalence in the in-situ populations were more susceptible in the experiment than populations where the disease has remained at low levels. The more susceptible populations showed similar levels of susceptibility to the populations with a long history of disease. Conclusion: We propose that the combination of restricted host and pathogen dispersal and high turnover rates of host patches within populations maintain variation for resistance and mediate host–pathogen co-existence at the metapopulation level