Eukaryotic Microbial RNA Viruses—Acute or Persistent? Insights into Their Function in the Aquatic Ecosystem

Isolated RNA viruses mainly parasitize eukaryotes. RNA viruses either expand horizontally by infecting hosts (acute type) or coexist with the host and are vertically inherited (persistent type). The significance of persistent-type RNA viruses in environmental viromes (the main hosts are expected to be microbes) was only recently reported because they had previously been overlooked in virology. In this review, we summarize the host-virus relationships of eukaryotic microbial RNA viruses. Picornavirales and Reoviridae are recognized as representative acute-type virus families, and most of the microbial viruses in Narnaviridae, Totiviridae, and Partitiviridae are categorized as representative persistent-type viruses. Acute-type viruses have only been found in aquatic environments, while persistent-type viruses are present in various environments, including aquatic environments. Moreover, persistent-type viruses are potentially widely spread in the RNA viral sequence space. This emerging evidence provides novel insights into RNA viral diversity, host-virus relationships, and their history of co-evolution.