Variation in tolerance and virulence in the chestnut blight fungus-hypovirus interaction

Chestnut blight, caused by the fungus Cryphonectria parasitica, has been ef fectively controlled with double-stranded RNA hypoviruses in Europe for ove r 40 years. The marked reduction in the virulence of C. parasitica by hypov iruses is a phenomenon known as hypovirulence. This virus-fungus pathosyste m has become a model system for the study of biological control of fungi wi th viruses. We studied variation in tolerance to hypoviruses in fungal host s and variation in virulence among virus isolates from a local population i n Italy. Tolerance is defined as the relative fitness of a fungal individua l when infected with hypoviruses (compared to being uninfected); virulence is defined for each hypovirus as the reduction in fitness of fungal hosts r elative to virus-free hosts. Six hypovirus-infected isolates of C. parasiti ca were sampled from the population, and each hypovirus was transferred int o six hypovirus-free recipient isolates. The resulting 36 hypovirus-fungus combinations were used to estimate genetic variation in tolerance to hypovi ruses, in hypovirus virulence, and in virus-fungus interactions, Four pheno types were evaluated for each virus-fungus combination to estimate relative fitness: (i) sporulation, i.e., the number of asexual spores (conidia) pro duced; (ii) canker area on field-inoculated chestnut trees, (iii) vertical transmission of hypoviruses into conidia, and (iv) conidial germination. Tw o-way analysis of variance (ANOVA) revealed significant interactions (P < 0 .001) between viruses and fungal isolates for sporulation and canker area b ut not for conidial germination or transmission, One-way ANOVA among hypovi ruses (within each fungal isolate) and among fungal isolates (within each h ypovirus) revealed significant genetic variation (P < 0.01) in hypovirus vi rulence and fungal tolerance within several fungal isolates, and hypoviruse s, respectively. These interactions and the significant genetic variation i n several fitness characters indicate the potential for future evolution of these characters. However, biological control is unlikely to break down du e to evolution of tolerance to hypoviruses in the fungus because the magnit udes of tolerance and interactions were relatively small.