Herpesvirales
| Herpesvirales | |
|---|---|
| Virus classification | |
| Group: | Group I (dsDNA) |
| Order: | Herpesvirales |
| Families | |
The Herpesvirales is an order of viruses all sharing the same overall morphology.
Virology
All herpes viruses have a capsid structure that consists of a DNA core surrounded by an icosahedral capsid consisting of 12 pentavalent and 150 hexavalent capsomeres (T = 16). The capsid has a diameter of ~110 nanometers (nm) is embedded in a proteinaceous matrix called the tegument, which in its turn is enclosed by a glycoprotein-containing lipid envelope with a diameter of about 200 nm.
The DNA genome is linear and double stranded and ~134 kilobases in length. The genome contains terminal and internal reiterated sequences.
Species in this order infect animals - both vertebrate and invertebrate.
History
The genus Herpesvirus was established in 1971 in the first report of the International Committee on Taxonomy of Viruses (ICTV). This genus consisted of 23 viruses and 4 groups of viruses. In the second ICTV report in 1976 this genus was elevated to family level - the Herpetoviridae. Because of possible confusion with viruses derived from reptiles this name was changed in the third report in 1979 to Herpesviridae. In this report the family Herpesviridae was divided into 3 subfamilies (Alphaherpesvirinae, Betaherpesvirinae and Gammaherpesvirinae) and 5 unnamed genera: 21 viruses were listed. In 2009 the family Herpesviridae was elevated to the order Herpesvirales. This elevation was necessitated by the discovery that the herpes viruses of fish and molluscs were only distantly related to those of birds and mammals. Two new families were created - the family Alloherpesviridae which incorporates bony fish and frog viruses and the family Malacoherpesviridae which contains those of molluscs.
This order currently has 3 families, 3 subfamilies plus 1 unassigned, 17 genera, 90 species and plus 48 as yet unassigned viruses.[1]
Naming system
The system of naming herpesviruses was originated in 1973 and has been elaborated considerably since. The recommended naming system specified that each herpes virus should be named after the taxon (family or subfamily) to which its primary natural host belongs. The subfamily name is used for viruses from members of the family Bovidae or from primates (the virus name ending in –ine, e.g. bovine), and the host family name for other viruses (ending in –id, e.g. equid). Human herpes viruses have been treated as an exception (human rather than hominid). Following the host-derived term, the word herpes virus is added, followed by an Arabic number (1,2,3,...). These last two additions bear no implied meaning about taxonomic or biological properties of the virus.
Some exceptions to this system exist. A number of viruses names (e.g. Epstein–Barr virus) are so widely used that it is impractical to attempt to insist on their replacement. This has led to the a dual nomenclature in the literature for some herpes viruses. All herpes viruses described since this system was adopted have been named in accordance with it.
Phylogenetics
The branching order suggests that Alloherpesviridae is the basal clade and that Herpesviridae and Malacoherpesviridae are sister clades.[1] Given the phylogenetic distances between vertebrates and molluscs this suggests that herpesviruses were initially fish viruses and that they have evolved with their hosts to infect other vertebrates.