The name given to each organism is given using its genus name followed by its species name. The first letter of the name is usually capitalized and the whole name is either italicized or underlined, for instance humans are named: Homo sapiens or Homo sapiens.
Virus Classification
Viruses only display characteristics of living entities only when they are within a host cell and possess unique structural and genetic features. As a result it is not classified in the same way that typical living organisms are.
There are two schemes of classification of viruses; they are Lwoff’s Scheme and Baltimore’s Scheme. These two schemes are discussed briefly below.
Lwoff’s Scheme for Classification
This scheme developed by Lwoff, RW Horne and P Tournier in 1962 make use of shared properties of viruses rather than the host cell or organism they infect. The classification for viruses under this scheme consist of phylum, class, order, family, sub family, genus, species and strain/type.
Four main characteristics used to group the viruses are:
1) Nature of the nucleic acid (DNA or RNA)
2) Capsid symmetry
3) Presence or absence of an envelope
4) Viron and Capsid Dimensons
The international committee on Taxonomy of Viruses (ICTV) has accepted and is using certain principles in this scheme in addition to that of Baltimore’s scheme.
Baltimore’s Scheme for Classification
This scheme developed by David Baltimore uses mechanisms of viral genome replication and the principle that all virus genomes need to construct positive strand RNA from their genomes in to produce virus proteins for replication to categorize viruses.
Various types of virus genomes have different mechanisms for replication and these genomes are grouped into 7 groups which are shown in the diagram below:
(click image to enlarge)
Baltimore’s classification uses the way the different groups of genetic material are processed into RNA. The diagram below shows how the virus’s genetic materials are converted to mRNA and the intermediate processes and products involved.
(click image to enlarge)
Fig 1: Flowchart showing the conversion of different groups of genetic materials to mRNA and then to proteins
ICTV Taxonomy
The ICTV uses both Baltimore’s and Lwoff’s schemes of classifications for viruses and classifies viruses based on the following:
1) virus genome and sequence relatedness,
2) structure of the virus,
3) natural host range,
4) cell and tissue tropism,
5) pathogenicity and cytopathology,
6) mode of transmission,
7) physiochemical properties
8) and antigenic properties of viral proteins
Presently virus classification is still under development as a result some viruses may not have been assigned to an order, family or genus yet. As a result, informal grouping based on ICTV taxonomic ranking, genome structure and composition is used to classify them.
Type Species
Under the International Code of Virus Taxonomy and Nomenclature classification, each genus is typified using a species, in other words, each genus name will be linked to one species name. This species name linked to the genus is known as the type species.
In this system, each genus will contain a type species assigned to it. The link between the type species and the genus will not always be based on characteristics, some maybe be historical, defining the origins of the genus name. As a result, type species may not be a typical member of the genus and may not characterize the genus.
The table below gives examples of the type species of certain genus.
(click image to enlarge)
---------------------------------------------------------------------
