ICTV defines viriforms as new category of virus-derived genetic elements

The International Committee on Taxonomy of Viruses (ICTV) recently agreed to recognize viriforms as a new category of virus-derived genetic elements, viroids and satellite nucleic acids, according to an MDPI article published Feb. 3.

The article defines viruses as "a type of MGEs that encode at least one protein that is a major component of the virion encasing the nucleic acid of the respective MGE and therefore the gene encoding the major virion protein itself. Or MGEs that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities."

In contrast to traditional viruses, viriforms are a polyphyletic group of former viruses that have been incorporated by their host cells to serve specific functions crucial to the host's life cycle. Although they have a similar morphology to virions, viriform particles do not contain viriform genomes, instead transporting the genetic material of the host.

Viriforms are incredibly diverse, with members of the Polydnaviriformidae family exclusively found in braconid and ichneumonid parasitoid wasp genomes, and gene transfer agents (GTAs) distributed widely between prokaryotes. 

Recent studies have suggested that viriforms could also be present in mammalian genomes.