Characterization of three mycoviruses co-infecting the plant pathogenic fungus Sclerotinia nivalis

Abstract

Two dsRNAs of approximately 6.0-kb and 3.0-kb in length were detected in strain SsSn-1 of Sclerotinia nivalis. Genomic analysis showed that the 6.0-kb dsRNA was a victorivirus, named as Sclerotinia nivalis victorivirus 1 (SnVV1). The genome of SnVV1 is 5162bp in length containing two large open reading frames (ORFs), ORF1 and ORF2. ORF1 was deduced to encode a coat protein (CP) showing homology to CPs of viruses belonging to the family Totiviridae. The stop codon of ORF1 overlaps the start codon of ORF2 in the tetranucleotide sequence AUGA. ORF2 was predicted to encode for a RNA-dependent RNA polymerase (RdRp) that was very similar to the RdRps of victoriviruses. The 3.0-kb dsRNA was consisted of two species of mitoviruses, named as Sclerotinia nivalis mitovirus 1/SsSn-1 (SnMV1/SsSn-1) and Sclerotinia sclerotiorum mitovirus 3/SsSn-1 (SsMV3/SsSn-1). The genomes of SnMV1/SsSn-1 and SsMV3/SsSn-1 were 2720nt and 2583nt in length, respectively. Both mitoviruses were AU-rich and deduced to contain a major large ORF encoding a mitoviral RdRp with the fungal mitochondrial codon usages. SnMV1/SsSn-1 was most closely related to Sclerotinia sclerotiorum mitovirus 4 (SsMV4/NZ1) and shared 76.5% and 80.1% identity with SsMV4/NZ1 for nucleotide and RdRp sequences, respectively. In addition, the nucleotide and RdRp sequences of SsMV3/SsSn-1 were 90.6% and 95.9% identical to the nucleotide and RdRp sequences of SsMV3/NZ1, respectively. Considering their nucleotide and RdRp sequence identities with other mitoviruses, SnMV1/SsSn-1 may belong to the genus Mitovirus, whereas SsMV3/SsSn-1 is possibly a strain of SsMV3. Both SnMV1/SsSn-1 and SsMV3/SsSn-1 were transmitted to a recipient virus-free colony faster than was SnVV1 through hyphal anastomosis. Co-infection by these mycoviruses had no apparent effects on growth and pathogenicity of S. nivalis.