Abstract
Hyperparasites can affect the evolution of pathosystems by influencing the stability of both pathogen and host populations. However, how pathogens of perennial hosts evolve in the presence of a hyperparasite has rarely been studied. Here, we investigated temporal changes in genetic diversity of the invasive chestnut blight pathogen Cryphonectria parasitica in the presence of its parasitic mycovirus Cryphonectria hypovirus 1 (CHV1). The virus reduces fungal virulence and represents an effective natural biocontrol agent against chestnut blight in Europe. We analysed genome‐wide diversity and CHV1 prevalence in C. parasitica populations in southern Switzerland that were sampled twice at an interval of about 30 years. Overall, we found that both pathogen population structure and CHV1 prevalence were retained over time. The results suggest that recent bottlenecks have influenced the structure of C. parasitica populations in southern Switzerland. Strong balancing selection signals were found at a single vegetative incompatibility ( vic ) locus, consistent with negative frequency‐dependent selection imposed by the vegetative incompatibility system. High levels of mating among related individuals (i.e., inbreeding) and genetic drift are probably at the origin of imbalanced allele ratios at vic loci and subsequently low vc type diversity. Virus infection rates were stable at ~30% over the study period and we found no significant impact of the virus on fungal population diversity. Consequently, the efficacy of CHV1‐mediated biocontrol was probably retained.