Resistance variation within and among host populations in a plant–pathogen metapopulation: implications for regional pathogen dynamics

1 Studies conducted on natural host-pathogen metapopulations have revealed considerable diversity of host resistance phenotypes within populations. The resistance structure of non-infected populations has, however, been largely ignored and the role of among-population variation in resistance profiles in the dynamics of natural pathogen populations is poorly understood. 2 The Plantago lanceolata – Podosphaera plantaginis pathosystem in the Åland Islands in south-west Finland is characterized by the highly fragmented distribution of the host. Only a small fraction of the host populations is infected at one point in time and pathogen turnover rate is high. I studied a sample of these populations to find out whether adjacent host populations are differentiated in their resistance structure and whether variable levels of disease resistance among host populations could be linked to disease incidence patterns. 3 Results show striking differences in the resistance structure both within and among host populations. Sixteen resistance phenotypes were identified in a sample of 64 host individuals. Populations varied from one in which all sampled individuals represented a different resistance phenotype, to one in which half showed identical resistance responses. 4 There was no association between local resistance composition and the geographical distance between populations, suggesting that within-population processes, such as founder effects and genetic drift, largely determine local resistance structure. 5 Non-infected populations showed significantly higher mean levels of resistance than infected populations, suggesting that differences in the mean level of resistance among host populations may retard the spread of the fungus and thereby decrease the probability of regional epidemics.