The family Totiviridae currently contains five genera Totivirus, Victorivirus, Leishmavirus, Trichomonasvirus, and Giardiavirus. Members in this family generally have a set of two-open reading frame (ORF) elements in their genome with the 5′-proximal ORF (ORF1) encoding a capsid protein (CP) and the 3′-proximal one (ORF2) for RNA-dependent RNA polymerase (RdRp). How the downstream open reading frames (ORFs) are expressed is genus-specific. All victoriviruses characterized thus far appear to use the stop/restart translation mechanism, allowing for the expression of two separate protein products from bicitronic genome-sized viral mRNA, while the totiviruses use a −1 ribosomal frame-shifting that leads to a fusion product of CP and RdRp. We report the biological and molecular characterization of a novel victorivirus termed Alternaria alternata victorivirus 1 (AalVV1) isolated from Alternaria alternata in Pakistan. The phylogenetic and molecular analyses showed AalVV1 to be distinct from previously reported victoriviruses. AalVV1 appears to have a sequence signature required for the −1 frame-shifting at the ORF1/2 junction region, rather than a stop/restart key mediator. By contrast, SDS–polyacrylamide gel electrophoresis and peptide mass fingerprinting analyses of purified virion preparations suggested the expression of two protein products, not a CP-RdRp fusion product. How these proteins are expressed is discussed in this study. Possible effects of infection by this virus were tested in two fungal species: A. alternata and RNA silencing proficient and deficient strains of Cryphonectria parasitica, a model filamentous fungus. AalVV1 showed symptomless infection in all of these fungal strains, even in the RNA silencing deficient C. parasitica strain.

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 (Δ dcl2 ), 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.