In this article we will discuss about the classification of viruses.
It is still in a fluid state. Whether viruses are plants or animals is purely an academic question and difficult to answer with any assurance. Perhaps they are neither. According to the commonly accepted view, the viruses represent the transition stage between the living and non-living world.
In other words, they stand at the very threshold of life and thus may be considered as a hypothetical model of an early stage in the evolution of living organisms (plants and animals). All biologists, at present agree to place viruses in the phylum Vira.
Holmes (1948) placed them in the order Virales. He divided the order virales on the basis of host specificity into the following three suborders:
Suborder 1. Phytophagineae:
It includes the viruses causing plant diseases.
Suborder 2. Phagineae:
It comprises the viruses which attack bacteria and are known as the bacteriophages.
Suborder 3. Zoophagineae:
It includes viruses causing diseases to men and animals.
On the basis of the genetic material (RNA or DNA), Dwoff and Tausnier (1966) divided the phylum Vira into two subphyla namely, (i) Ribovira containing RNA and (ii) Deoxyvira containing DNA.
On the basis of the symmetry they further sub-divided subphylum Ribovira into two classes, Ribohelica and Ribocubica whereas deoxyvira was split into 3 classes, Deoxyhelica, Deoxycubica and Deoxybinala (with a combination of both helical and cuboidal symmetry).
The classes are divided into orders, as detailed below:
In 1968 Thomberry proposed a system of classification in which all the viruses were placed under the phylum Thallophyta, class Schizomycota, and order Biovirales. The order was further classified into three families Phytoviraceae, Zooviraceae and Rickettsiaceae.
The family Phytoviraceae contained five genera on the basis of the type of hosts attacked viz. Phytovirus, Pteridovirus, Bryovirus, Thallovirus and Bacteriophagus.
Gibbs (1969) presented a computerized hierarchial classification of about 140 viruses and grouped them into two major and five minor clusters or groupings.
On the basis of the behaviour of virus in the hosts, vector relations, particle properties and particle composition, Harrison et al. (1971) divided plant Viruses into 16 groups, as detailed below:
1. Tobravirus group. Type member:
Tobacco rattle virus.
2. Tobamovirus group. Type member:
Tobacco mosaic virus
3. Potexvirus group. Type member:
Potato virus X
4. Caria virus group. Type member:
Carnation latent virus
5. Poty virus group. Type member:
Potato virus Y
6. Cucumovirus group. Type member:
Cucumber mosaic virus.
7. Tymovirus group. Type member:
Turnip yellow mosaic virus.
8. Comovirus group. Type member:
Cowpea mosaic virus
9. Nepovirus group. Type member:
Tobacco ringspot virus.
10. Bromovirus group. Type member:
Brome mosaic virus.
11. Tombus virus group. Type member:
Tomato bushy stunt virus.
12. Caulimovirus group. Type member:
Cauliflower mosaic virus.
13. Alfalfa mosaic virus. Type member:
Alfalfa mosaic virus.
14. Pea Enation Mosaic virus. Type member:
Pea enation mosaic virus.
15. Tobacco necrosis virus. Type member:
Tobacco necrosis virus A strain.
16. Tomato spotted wilt virus. Type member:
Tomato spotted wilt virus.
The greatest drawback of this classification is that only the well-studied viruses have been placed in the above mentioned 16 groups and many little known viruses have been left. In addition, viruses resembling wound tumour and lettuce necrotic yellows have also not been placed in these groups.