Principles of Classification and Nomenclature

Vivek Prasad Professor Department of Botany University of Lucknow Lucknow

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Principles of Plant and Nomenclature

One look at the universe, and we see the variety there is. Be it animals, be it , be it microbes, be it chemicals, be it planets, be it stars etc. In the days of yore, when knowledge was limited to very few of each of these, there did not seem to be a problem. However, as human curiosity went up, and scientific inputs gathered speed, realization dawned that the constituents making up any area or field have diversity within themselves. Thus, all animals are not the same, all plants are not the same, all microbes, even though microscopic, are not the same. Wherever there is a variety, for the sake proper study, a classification is required. Classification thus provides a uniformity in an otherwise randomly distributed system.

Classification of and nomenclature of became a matter of serious concern when more and more viruses were identified infecting plants, animals, and humans of course, and it was seen that the characteristics were different. The majority of viruses infecting plants possessed RNA as their , while those infecting animals had DNA. Exceptions, of course exist in both types.

Since initially a consensus on a uniform naming system of the viruses was lacking, the International Committee on Nomenclature of Viruses was established in 1966 (ICNV). It was then felt that naming (nomenclature) was not adequate, and hence the ICNV was remodeled in 1971, and called the International Committee on Taxonomy of Viruses. The ICTV came up with a uniform system of virus classification based on specific guidelines formulated by them, and applicable to all branches of .

The ICTV has the following six Subcommittees:

1. Subcommittee for fungal and protest viruses 2. Subcommittee for plant viruses 3. Subcommittee for bacterial and archaeal viruses 4. Subcommittee for animal DNA viruses and 5. Subcommittee for animal ds- and (-) ssRNA viruses 6. Subcommittee for animal (+) ssRNA

The overall organization is that there are small Study Groups with each Subcommittee. They report to the Subcommittee. All Subcommittees report to the Executive Committee, that in turn reports to the ICTV made up of the Executive Committee plus what are called National Members from different countries or regions.

As mentioned earlier, the ICTV defined a series of criteria for classification.

Criteria for Classification

Virion Properties

Morphology: Size, shape, envelope, symmetry etc. Physico-chemical properties: Molecular weight, sedimentation co-efficient particle stability etc. Genome: Type of , size, strandedness, linear/circular, sense, number and size of segments, nucleotide sequence, G+C ratio etc. : Number and size of subunits, structural and non- structural , functional activities of proteins, amino acid sequence, glycosylation etc. Lipids: Content, type Carbohydrates: Content, type

Replication Properties

Genome organization, replication strategy, ORFs, transcriptional and translational characteristics, post-translational processing, site of replication and virion assembly, release etc.

Antigenic Properties

Antigenicity, serological relationships etc.

Biological Properties

Geographical distribution, range, pathogenicity, symptoms, transmission, virus- relationship etc.

The nomenclature of viruses was initially haphazard. Thus the virus causing mosaic of tobacco was said to be the . Later it was known as the tobacco virus 1. Trying to keep in tune with the binomial system of nomenclature for other biological entities, the TMV was proposed to be called Marmor tabaci. Finally however, the ICTV laid down the principles of nomenclature that should aim for stability, do not have names that are confusing, and avoid unnecessary creation of names.

Some essentials of rules for naming are: Existing names to be retained if possible, names of persons not to be used, names should be easy to remember and use, and they must have endings to signify the taxon level (thus the name TMV came back, albeit with one difference – tobacco mosaic virus is now tobacco mosaic tobamovirus. All viruses that are similar will thus have their names ending with tobamovirus to reflect that they belong together).

While writing down the names, the rules for orthography are: Names of virus orders, families, subfamilies and genera (e.g. Tobamovirus) are printed in italics with their first letters capitalized, the species names are printed in italics and the first letter of the first word of the name is capitalized.

References:

1. ICTV on the web: https://talk.ictvonline.org 2. Hull, R, 2014, Plant Virology, 5th Ed., Academic Press, 1118 pp. 3. Verma, HN, 2003, Basics of Plant Virology, Oxford and IBH, 228 pp.