Evidence for negative-strand RNA virus infection in fungi

Abstract

Fungal viruses comprise two groups: a major group of five families with double-stranded RNA genomes and a minor group with positive-sense single-stranded (ss)RNA genomes. Although many fungal viruses have been identified, no negative-stranded (−)ssRNA mycoviruses have been reported. Here we present two lines of evidence suggesting the presence of (−)ssRNA viruses in filamentous fungi based on an exhaustive search using extant (−)ssRNA viruses as queries. This revealed (−)ssRNA virus L protein-like sequences in the genome of a phytopathogenic obligate ascomycete, Erysiphe pisi. A similar search for (−)ssRNA viruses in fungal transcriptome shotgun assembly libraries demonstrated that two independent libraries from Sclerotinia homoeocarpa, another phytopathogenic ascomycete, contained several sequences considered to correspond to the entire mononegavirus L gene and likely originating from an infecting (−)ssRNA virus. These results provide strong evidence for both ancient and extant (−)ssRNA virus infections in fungi.