What domain is viruses classified under?

Question

Viruses are not derived from preexisting cells, and thus oppose the Cell Theory, and are not cells; they are simply a cluster of protein and genetic material. An organism is defined as "consisting of monomeric units known as cells". If viruses are not made up of one or more cells, they obviously cannot be considered as an organism, if they are not organisms, how can they share the same system of classification as all other organisms on Earth (specifically speaking: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species)? There is also a dispute going over whether viruses are alive or not (since they cannot metabolize and reproduce independently).

I have browsed through many sites on any hint on which Domain viruses are classified under, but all they tell me is the families and genus of some viruses.

Which Domain do viruses belong to?

Answer 1

A good question,

A short answer is- Viruses do not fall under any domain. They are neither prokaryotes nor eukaryotes. They are not considered a part of the classical living world. Therefore, their system of classification is entirely different.

Here is my answer in detail:

This question first arose with the discovery of viruses. When the world was classifying everything under either Animal or Plant kingdoms (Two Kingdom Classification), there came these strange "living chemicals"; they could be crystallized like salt and sugar. When dry crystallized viruses were dissolved in water, they would again regain their ability to infect the host cells. They had genetic material, could evolve and adapt but, if left all by themselves, they were nothing but chemicals! Thus, came the need to revamp and redefine the whole system.

In the new Five-Kingdom classification by R H Whittaker, viruses were not included. Monera, Protista, Fungi, Plantae and Animalia were the five kingdoms of the Living World while the poor viruses were left alone.

They are still considered "living chemicals" which were the "borderline" between living and non-living.

Now, coming to your question-

Their method of classification is separate from the Five-Kingdom system because of their exclusion from the whole system but they are also classified under the Hierarchy of Kingdom, phylum, class, order, family, genus and species.

Here is how they are classified on the basis of the LHT System of Classification by Lwoff, Horne and Tournier (1962):

PHYLUM: Vira

TWO SUBPHYLA: Deoxyvira (DNA viruses) and Ribovira (RNA Viruses)

Now, Deoxyvira has three classes:
a) Deoxyhelica (helical symmetry)
b) Deoxycubica (cubical symmetry)
c) Deoxybinala (viruses with head and tail)

Now, the class Deoxyhelica houses the Order: Chitovirales (enveloped) wherein lies the Family: Poxviridae - poxviruses

In class Deoxycubica, there are two orders- Order: Haplovirales (no envelope) and Order: Peplovirales (mantle viruses).

Haplovirales has five families based on the different number of capsomers (sub-units of the protective protein coat: capsid)

Family: Microvidae has 12 capsomeres, Family: Parvoviridae- 32, Family: Paplloviridae- 72, Family: Adenoviridae- 2 and Family: Iridoviridae- 812 number of capsomers.

Order Peplovirales has Family: Herpesviridae with 162 capsomeres (herpes viruses).

Class Deoxybinala (head-tail, remember?) contains the order Urovirales which in turn has the important family of Phagoviridae (bacteriophages).

Coming to the SUBPHYLUM- RIBOVIRA:

There are two classes: Class Ribohelica (helical symmetry) and Class Ribocubica (cubical symmetry).

Class Ribohelica has two Orders: Order Rhabdovirales (rod-like viruses) and order Sagovirales.

There are two sub-orders to Rhabdovirales:

a) Rigidovirales Plant viruses and b) Flexiviridales Plant viruses

Suborder : Rigidovirales Plant viruses has three families:
Family: Dolichoviridae- 12-13 nm, Family :Protoviridae- 15 nm and Family : Pachyviridae 20 nm.

The suborder Flexiviridales P.V. also has three families:
Family: Leptoviridae- 10-11 nm, Family: Mesoviridae 12-13 nm, Family: Adroviridae 15 nm.

The last order Sagovirales in the class Ribohelica also has three families:
Family: Myxoviridae, Family: Paramyxoviridae, Family: Stomatoviridae

I hope the above info along with all the details helped to clear up your doubts. Please feel free to ask any further questions on the topic. I will try and answer with the best of my capability.

Thanks for asking.

Answer 2

2

Answer 3

Most virologists will tell you viruses are alive. Viruses are not derived from preexisting cells, but they are derived from preexisting viruses. The problem isn't with viruses, it's with the arbitrary definition of life.

Viruses don't belong to any of the existing domains of "life". Yet they haven't achieved enough support to have their own domain. As such, the International committee on virus taxonomy (the governingo body that makes determinations about viral groupings) has developed their own organization system, that includes viral species, genera, families and orders. Check out the website below....