Realm (virology)

In virology, realm is the highest taxonomic rank established for viruses by the International Committee on Taxonomy of Viruses (ICTV), which oversees virus taxonomy. Six virus realms are recognized and united by specific highly conserved traits:

• Adnaviria, which contains archaeal filamentous viruses with A-form double-stranded (ds) DNA genomes encoding a unique alpha-helical major capsid protein;
• Duplodnaviria, which contains all dsDNA viruses that encode the HK97-fold major capsid protein;
• Monodnaviria, which contains all single-stranded DNA (ssDNA) viruses that encode a HUH superfamily endonuclease and their descendants;
• Riboviria, which contains all RNA viruses that encode RNA-dependent RNA polymerase and all viruses that encode reverse transcriptase;
• Ribozyviria, which contains hepatitis delta-like viruses with circular, negative-sense ssRNA genomes;
• and Varidnaviria, which contains all dsDNA viruses that encode a vertical jelly roll major capsid protein.

The rank of realm corresponds to the rank of domain used for cellular life, but differs in that viruses in a realm do not necessarily share a common ancestor based on common descent nor do the realms share a common ancestor. Instead, realms group viruses together based on specific traits that are highly conserved over time, which may have been obtained on a single occasion or multiple occasions. As such, each realm represents at least one instance of viruses coming into existence. While historically it was difficult to determine deep evolutionary relations between viruses, in the 21st century methods such as metagenomics and cryogenic electron microscopy have enabled such research to occur, which led to the establishment of Riboviria in 2018, three realms in 2019, and two in 2020.