New insights into reovirus evolution: implications from a newly characterized mycoreovirus

We performed molecular cloning and complete genome sequencing of a novel mycoreovirus, Sclerotinia sclerotiorumreovirus 1 (SsReV1), isolated from an isolate of the phytopathogenic fungus Sclerotinia sclerotiorum. SsReV1 has a genome of 28 055 bp and is composed of 11 double-stranded RNA segments. With a combination of unique molecular features, virion shape and composition, and phylogenetic analysis, SsReV1 is significantly distinct from all known reoviruses and defines a novel genus in the family Reoviridae. Interestingly, two conserved domains, double-stranded RNA binding motif (dsRBM, Pfam 00 035) and reovirus sigma C capsid protein (Reo_σC, pfam04 582), were identified in the genome of SsReV1, which are widespread in diverse virus lineages. Sequence comparison and phylogenetic analysis revealed that multiple cross-family horizontal gene transfer (HGT) events could occur between reoviruses and double-stranded DNA viruses, single-stranded RNA viruses and even cellular organisms. Interestingly, the dsRBM of SsReV1 was phylogenetically related to dsRNA-binding proteins of some insects, but not reoviruses. These results indicated that SsReV1 is a new taxonomic representative in Reoviridae, which provides new insights into the diversity and global ecology of reoviruses and other segmented double-stranded RNA viruses. More importantly, the present results provided evidence indicating that reoviruses indeed have HGT events with other virus lineages on a large scale and that HGT may serve as an important driving factor that plays a key role in the evolution of reoviruses.