Inbreeding effects on resistance and transmission-related traits in the Silene-Microbotryum pathosystem

Inbreeding in local host populations will be a common phenomenon in host-pa thogen systems that are characterized by metapopulation dynamics, i.e., fre quent extinction and recolonization of local host populations by small numb ers of founding individuals. As an example of a pathosystem with metapopula tion dynamics we investigated the impact of inbreeding in the host plant Si lene alba on its interaction with the anther-smut fungus Microbotryum viola ceum. Seeds from eight populations of S. alba were sampled, and five genera tions of sib mating resulted in 65 inbred lines, with inbreeding coefficien ts of f = 0, 0.25, 0.375, 0.5, and 0.59 per line. In a first experiment the se lines were tested for active, biochemical resistance against fungal infe ction, by artificially inoculating individuals. The percentage of infected individuals differed significantly among populations, lines, and inbreeding levels, and both population-by-inbreeding level and line-by-inbreeding lev el interactions were significant. The most striking result was the strong v ariance in inbreeding effects among lines; inbreeding resulted in increased resistance in some lines and decreased resistance in others. In a second e xperiment for 12 inbred lines, originating from one population, active resi stance and Rower traits associated with passive resistance (avoidance) to t his insect-vectored, florally transmitted disease were measured. Significan t inbreeding depression was demonstrated for petal size and nectar volume. Thus inbreeding might enhance avoidance of spore transmission by insects. F or both active resistance and all flower traits, significant line-by-inbree ding level interactions were found. The results indicate that the effect of inbreeding on the interaction between host and pathogen in this pathosyste m is unpredictable at the local population level, because: (1) strong genot ypic differences in inbreeding effect exist for both active and passive res istance, making the effect of inbreeding at the population level dependent on the genotypic composition of the (founder) population (2) effects of inb reeding on active and passive resistance were not correlated, making the ne t effect of inbreeding on field resistance unpredictable: and (3) in severa l lines, evidence for epistatic effects was round, making the effect of inb reeding dependent on the actual inbreeding level of the genotype. The resul ts underscore that most progress in the study of host-path from an integrat ed ecological and genetic approach.