CLASSIFICATION OF VIRUSES

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          According to the commonly accepted view, the viruses represent the transition stage between the living and non-living world. 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 sub-orders –
            Sub-order 1. Phytophagineae – It includes the viruses causing plant diseases.
            Sub-order 2. Phagineae – It includes the viruses which attack bacteria and are known as bacteriophages.
            Sub-order 3. Zoophagineae –  It includes viruses causing diseases to men and animals.
            On the basis of genetic material (RNA or DNA), Dwoff and Tausnier (1966) divided the phylum Vira into two sub-phyla –
SUB-PHYLUM 1. RIBOVIRA (Virus with ribonucleic acid). Ribovira is divided into two classes –
            Class I. Ribohelica (Helical viruses containing RNA). It consists of two orders –
                        Order 1. Rhabdodovirales (Rod-like viruses without envelop). It is further sub                            divided into two sub-orders – Rigidovirales (rigid forms) and Flexivirales (flexible                                     forms).
                        Order 2. Sagovirales (Viruses with envelop).
            Class II. Ribocubica (Cubical viruses containing RNA). It consists of two orders –
                        Order 1. Gymnovirales (Viruses without envelop).
                        Order 2. Togovirales (Viruses with envelop).
SUB-PHYLUM 2. DEOXYVIRA (Virus with deoxyribonucleic acid). Deoxyribovira is divided into three classes –
            Class I. Deoxyhelica (Helical viruses containing DNA). It consists of one order –
                        Order 1. Chaetovirales (Viruses with envelop).
            Class II. Deoxycubica (Cubical viruses containing DNA). It consists of two orders –
                        Order 1. Haplovirales (Viruses without envelop).
                        Order 2. Peplovirales (Viruses with envelop).
            Class III. Deoxybinala (Binal viruses containing DNA). It consists of one order –
                        Order 1. Urovirales (Viruses without envelop).
            On the basis of behavior of virus in the host, vector relations, particle properties and particle composition, Harison et al. (1971) divided the plant viruses into 16 groups, as follows –
            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. Carla 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 drawbacks of this classification is that only well studied viruses have been placed in the above mentioned 16 groups and many little known viruses have been discarded. In addition viruses resembling wound tumour and lettuce necrotic yellows have also not been placed in this group.

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