A neo-virus lifestyle exhibited by a (+)ssRNA virus hosted in an unrelated dsRNA virus: Taxonomic and evolutionary considerations.

Recent studies illustrate that fungi as virus hosts provides a unique platform for hunting viruses and exploring virus/virus and virus/host interactions. Such studies have revealed a number of as-yet-unreported viruses and virus/virus interactions. Among them is a unique intimate relationship between a (+)ssRNA virus, yado-kari virus (YkV1) and an unrelated dsRNA virus, yado-nushi virus (YnV1). YkV1 dsRNA, a replicated form of YkV1, and RNA-dependent RNA polymerase, are trans-encapsidated by the capsid protein of YnV1. While YnV1 can complete its replication cycle, YkV1 relies on YnV1 for its viability. We previously proposed a model in which YkV1 diverts YnV1 capsids as the replication sites. YkV1 is neither satellite virus nor satellite RNA, because YkV1 appears to encode functional RdRp and enhances YnV1 accumulation. This represents a unique mutualistic virus/virus interplay and similar relations in other virus/host fungus systems are detectable. We propose to establish the family Yadokariviridae that accommodates YkV1 and recently discovered viruses phylogenetically related to YkV1. This article overviews what is known and unknown about the YkV1/YnV1 interactions. Also discussed are the YnV1 Phytoreo_S7 and YkV1 2A-like domains that may have been captured via horizontal transfer during the course of evolution and are conserved across extant diverse RNA viruses. Lastly, evolutionary scenarios are envisioned for YkV1 and YnV1.