Hypovirulence-Associated Traits Induced bya Mycovirus of Cryphonectria parasitica AreMimicked byTargeted Inactivation ofa HostGene

Expression ofthe Vr2 geneofCryphonectria parasitica isdown-regulated instrains ofthefungus containing adouble-stranded RNAgenetic element that reduces fungal virulence (W.A.Powell andN.K.VanAlfen, Mol. Cell. Biol. 7:3688-3693, 1987). We havesequenced the Vr2 geneandcharacterized itsstructure; themRNA contains a short openreading framewhoseproduct hasstructural similarities toseveral fungal pheromones. A null mutant was constructed byhomologous recombination todetermine thefunction ofthe Vr2 geneand whether its disruption resulted inany ofthealtered phenotypes exhibited bymany hypovirulent strains, such asreductions invirulence, pigmentation, andsporulation. TheVr2 null mutant(18dm) exhibited awild-type phenotype withrespect togrosscolony morphology, growth rate, pigmentation, asexual sporeviability, and virulence inapple fruit andchestnut trees. However, numbers ofasexual fruiting bodies (pycnidia) andconidia were reduced significantly incomparison withthewild-type strain EP155/2. Insexual crossesof18dmwith a wild-type strain oftheopposite mating type, perithecia (sexual fruiting bodies) developed butwere barren. Deletion oftheVr2 generesults ina phenotype thatmimicsthatofmany double-stranded-RNA-containing hypovirulent strains; i.e., thenull mutantexhibits significant reductions inasexual sporulation andpycnidium production aswell asimpaired sexual crossing ability. Toourknowledge, this isthefirst report ofthepartial reproduction ofa virus-induced phenotype bydeletion ofavirus-perturbed host gene.