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 C. nitschkei and two strains of different fungal species within the genus Cryphonectria (Cryphonectria parasitica and Cryphonectria carpinicola). 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 (Δdcl2), relative to the wt strain EP155 of a model virus host fungus (C. parasitica). These findings suggest that CnCvV1 is targeted by RNA silencing in C. parasitica. This study provides a foundation for further exploration of curvulavirids that have been biologically understudied.