Viriforms-A New Category of Classifiable Virus-Derived Genetic Elements

The International Committee on Taxonomy of Viruses (ICTV) recently accepted viriforms as a new polyphyletic category of classifiable virus-derived genetic elements, juxtaposed to the polyphyletic virus, viroid, and satellite nucleic acid categories. Viriforms are endogenized former viruses that have been exapted by their cellular hosts to fulfill functions important for the host's life cycle. While morphologically resembling virions, particles made by viriforms do not package the viriform genomes but instead transport host genetic material. Known viriforms are highly diverse: members of family Polydnaviriformidae (former Polydnaviridae) have thus far been found exclusively in the genomes of braconid and ichneumonid parasitoid wasps, whereas the completely unrelated gene transfer agents (GTAs) are widely distributed among prokaryotes. In addition, recent discoveries likely extend viriforms to mammalian genomes. Here, we briefly outline the properties of these viriform groups and the first accepted and proposed ICTV frameworks for viriform classification.

Automated summary

The International Committee on Taxonomy of Viruses (ICTV) has recognized viriforms as a new category of virus-derived genetic elements, distinct from viruses, viroids, and satellite nucleic acids. Viriforms are former viruses that have been integrated into the genomes of cellular hosts and perform functions important for the host's life cycle. These viriforms resemble viruses in appearance but do not package their own genomes, instead transporting host genetic material. They have been found in the genomes of parasitoid wasps and prokaryotes, and recent discoveries suggest their presence in mammalian genomes. This article outlines the properties of different viriform groups and the proposed classification frameworks by ICTV.