RNA virus | classification
Classification of the RNA viruses has proven to be a difficult problem. This is in part due to the high mutation rates these genomes undergo. Classification is based principally on the type of genome (double-stranded, negative- or positive-single-strand) and gene number and organisation. Currently there are 5 orders and 47 families of RNA viruses recognised. There are also many unassigned species and genera.
A study of several thousand RNA viruses has shown the presence of at least five main taxa: a levivirus and relatives group; a picornavirus supergroup; an alphavirus supergroup plus a flavivirus supergroup; the dsRNA viruses; and the -ve strand viruses. The lentivirus group appears to be basal to all the remaining RNA viruses. The next major division lies between the picornasupragroup and the remaining viruses. The dsRNA viruses appear to have evolved from a +ve RNA ancestor and the -ve RNA viruses from within the dsRNA viruses. The closest reltion to the -ve stranded RNA viruses are the
This is the single largest group of RNA viruses with 30 families. Attempts have been made to group these families in higher orders. These proposals were based on an analysis of the RNA polymerases and are still under consideration. To date, the suggestions proposed have not been broadly accepted because of doubts over the suitability of a single gene to determine the taxonomy of the clade.
The proposed classification of positive-strand RNA viruses is based on the RNA-dependent RNA polymerase. Three groups have been recognised:
A division of the alpha-like (Sindbis-like) supergroup on the basis of a novel domain located near the N termini of the proteins involved in viral replication has been proposed. The two groups proposed are: the 'altovirus' group (alphaviruses, furoviruses, hepatitis E virus, hordeiviruses, tobamoviruses, tobraviruses, tricornaviruses and probably rubiviruses); and the 'typovirus' group (apple chlorotic leaf spot virus, carlaviruses, potexviruses and tymoviruses).
Additional work has identified five groups of positive-stranded RNA viruses containing four, three, three, three, and one order(s), respectively. These fourteen orders contain 31 virus families (including 17 families of plant viruses) and 48 genera (including 30 genera of plant viruses). This analysis suggests that alphaviruses and flaviviruses can be separated into two families—the Togaviridae and Flaviridae, respectively—but suggests that other taxonomic assignments, such as the pestiviruses, hepatitis C virus, rubiviruses, hepatitis E virus, and arteriviruses, may be incorrect. The coronaviruses and toroviruses appear to be distinct families in distinct orders and not distinct genera of the same family as currently classified. The luteoviruses appear to be two families rather than one, and apple chlorotic leaf spot virus appears not to be a closterovirus but a new genus of the Potexviridae.
The evolution of the picornaviruses based on an analysis of their RNA polymerases and
Partitiviruses are related to and may have evolved from a totivirus ancestor.
Hypoviruses and barnaviruses appear to share an ancestry with the potyvirus and sobemovirus lineages respectively.
This analysis also suggests that the dsRNA viruses are not closely related to each other but instead belong to four additional classes—Birnaviridae, Cystoviridae, Partitiviridae, and Reoviridae — and one additional order (Totiviridae) of one of the classes of positive ssRNA viruses in the same subphylum as the positive-strand RNA viruses.
One study has suggested that there are two large clades: One includes the Caliciviridae, Flaviviridae, and Picornaviridae families and a second that includes the Alphatetraviridae, Birnaviridae and Cystoviridae, Nodaviridae, and Permutotretraviridae families.
These viruses have multiple types of genome ranging from a single RNA molecule up to eight segments. Despite their diversity it appears that they may have originated in
A number of satellite viruses - viruses that require the assistance of another virus to complete their life cycle - are also known. Their taxonomy has yet to be settled. The following four genera have been proposed for positive sense single stranded RNA satellite viruses that infect plants -
Virtovirus. A family -
Sarthroviridae which includes the genus
Macronovirus - has been proposed for the positive sense single stranded RNA satellite viruses that infect