Novel Double-Stranded RNA Viruses Discovered within Saccharomyces cerevisiae

Saccharomyces yeasts harbor many different viruses and parasitic genetic elements, including double-stranded RNA (dsRNA) viruses from the family Totiviridae. The identification of novel dsRNA viruses in yeasts has been constrained by the lack of effective protocols for the unbiased preparation and sequencing of dsRNAs. We have developed a next-generation sequencing method that enables the amplification and sequencing of yeast dsRNAs. Using this method, we have performed a metagenomic screen of more than 600 strains of Saccharomyces cerevisiae for the presence of novel dsRNA viruses. Surprisingly, we have identified several novel bipartite dsRNA viruses from the family Partitiviridae within different strains of S. cerevisiae. Partitiviruses have never been described within Saccharomyces yeasts or within the wider Saccharomycotina taxonomic subdivision of the Ascomycota phylum. We confirmed the presence of these partitivirus dsRNAs using reverse transcriptase-PCR and have been successful in purifying viral particles from infected yeasts. After deep sequencing of purified dsRNAs, we have identified three species of virus that have weak homology (20-35% amino acid identity) to viruses of the genus Cryspovirus, which replicate within the protozoan pathogen Cryptosporidium parvum. Partitiviruses can modulate the virulence and fecundity of plant and human pathogens, respectively. The discovery of partitiviruses in S. cerevisiae will enable the study of how partitiviruses interact with the host cell environment and alter cellular physiology.