Thorough characterization of a new curvulavirid from a Japanese strain of Cryphonectria nitschkei

A new curvulavirid was isolated from a Japanese strain of the filamentous ascomycete Cryphonectria nitschkei and thoroughly characterized. The virus termed Cryphonectria nitschkei curvulavirus 1 (CnCvV1) has a bi-segmented dsRNA genome. CnCvV1 dsRNA1 encodes an RNA-dependent RNA polymerase (592 amino acids), while dsRNA2 possesses two ORFs, one that encodes a protein associated with the genomic dsRNA and the other that encodes a hypothetical protein of unknown function. CnCvV1 could be experimentally introduced into another virus-free strain of <i>C. nitschkei</i> and two strains of different fungal species within the genus <i>Cryphonectria</i> (<i>Cryphonectria parasitica</i> and <i>Cryphonectria carpinicola</i>). Based on phenotypic comparison, the virus caused asymptomatic infection in the three newly established fungal strains. However, there was a reduced colony growth rate and increased CnCvV1 accumulation in an RNA silencing-deficient mutant (Δ<i>dcl2</i>), relative to the wt strain EP155 of a model virus host fungus (<i>C. parasitica</i>). These findings suggest that CnCvV1 is targeted by RNA silencing in <i>C. parasitica</i>. This study provides a foundation for further exploration of curvulavirids that have been biologically understudied.