Keywords

1.1 Introduction

Viruses are very small (submicroscopic) pathogenic particles (virions) composed of a protein which forms covering (coat) and a nucleic acid core. The nucleic acid, which is DNA or RNA, carries all genetic information required for sustaining. All viruses are obligate parasites and require cellular machinery of hosts for the multiplication. Replication and transcription of viruses to produce more nucleic acid and formation of proteins takes place within the host cell using some of the host’s machinery by reprogramming hosts gene expression (Hanley-Bowdoin et al. 2004). Viruses are not functional outside their host. Therefore all the viruses are obligate parasites. All types of living organisms are hosts for viruses, but most of the viruses are host specific and infect only one type of host. Viruses are usually named on the basis of their host, for example, viruses that infect bacteria are known as bacteriophages, whereas others, those that infect algae, are phycoviruses, protozoa, fungi that are known as mycoviruses .

1.2 Definition

‘Viruses are obligate intracellular parasites that are capable of infecting eukaryotes, bacteria and archaea, as well as other organisms’ (Desnues and Raoult 2012; Desnues et al. 2012; Raoult and Forterre 2008)

According to Roger Hull (2009), a virus is a set of one or more nucleic acid template molecules normally encased in a protective coat or coats of protein or lipoproteins that is able to organise its own replication only within suitable host cells. Within such cells, virus replication is (1) dependent on the host’s protein-synthesising machinery, (2) organised from pools of the required materials rather than by binary fission and (3) located at sites that are not separated from the host cell contents by a lipoprotein bilayer membrane and (4) continually gives rise to variants through several kinds of change in the viral nucleic acid.

1.3 Plant Viruses

Plant viruses are also obligate intracellular parasites as the other viruses that use the molecular machinery of the host for their replication (Ahlquist et al. 2003). These viruses are widely distributed and economically important (Wren et al. 2006). The plant viruses cause many harmful plant diseases and they are responsible for a tremendous loss in crop production and crop quality worldwide. Virus-infected plants show several kinds of symptoms depending on the disease type and host but leaf yellowing is common. Some of the other symptoms of virus infection are whole leaf or in a pattern of stripes or blotches; leaf distortion, like leaf curling, mottling and other growth distortions like stunting of the whole plant; and abnormalities in flower or fruit formation (Giampetruzzi et al. 2012).

1.4 History

Tobacco mosaic virus (TMV) was the first virus to be discovered and studied, which causes mosaic disease in tobacco plants (Soosaar et al. 2005). In 1882, Adolf Mayer (1843–1942) while studying tobacco plant described a condition, which he called ‘mosaic disease’ (Mosaaikkrankheit), and now it is well known to be caused by the tobacco mosaic virus (TMV). The diseased plants had variegated leaves that were mottled (Mayer 1882). He excluded the possibility of a fungal infection and could not detect any bacterium and speculated that a ‘soluble, enzyme-like infectious principle was involved’ (van der Want and Dijkstra 2006). He did not pursue his idea any further and a major observation was made in 1892 by Iwanowski who showed that sap from tobacco plants displaying the disease described by Mayer was still infective after it had been passed through a bacteria-proof filter candle (Roger Hull 2009). However, based on previous studies, it was thought that this agent was a toxin. Iwanowski’s experiment was repeated in 1898 by Beijerinck, who showed that the agent multiplied in infected tissue and called it contagium vivum fluidum (Latin for ‘contagious living fluid’) to distinguish it from contagious corpuscular agents (Beijerinck 1898). Beijerinck and other scientists used the term virus to describe the causative agents of such transmissible diseases to contrast them with bacteria (Roger Hull 2009). Earlier workers used the term ‘virus’ for both bacteria and viruses, but later on with more discoveries, the term ‘filterable viruses’ was used (Roger Hull 2009). With further discoveries the word filterable was dropped and term virus was adopted (Roger Hull 2009).

In the history of plant viruses, the importance of tobacco mosaic virus cannot be underestimated. TMV was the first virus to be studied and also to be crystallised. It was the very first virus to be studied in detail. In 1941 the first X-ray diffraction pictures of TMV was obtained by Bernal and Fankuchen. On the basis of her pictures, Rosalind Franklin discovered the full structure of the virus in 1955 (Creager and Morgan 2008). In the year 1941, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells. Replication of TMV involves -sRNA using + strand RNA as template (Buck 1999; Ishikawa and Okada 2004).

Nowadays at least 3705 viruses are known of which about 1000 are plant viruses. The plant viruses are studied because they have negative impact on crop production. The viruses were considered as a health threat to humans, livestock and crop plants. In recent few decades, research and development in virology has made it possible in understanding virus-host interactions and has transformed viruses into important tools of biomedicine and biotechnology (Rajamaki et al. 2004). For example, many plant viruses are used to produce proteins useful for plants and animals (Pogue et al. 2002), and many animal viruses are used for the development of vaccines against human and animal viruses such as chicken pox, rabies, foot and mouth disease, measles, etc. (Walmsley and Arntzen 2000).

The development of plant virology can be categorised into five major (overlapping) ages as follows according to Roger Hull 2009.

Prehistory age

752 AD Plant virus in Japanese poem written by the Empress Koken and translated by T. Inouye:

In this village

It looks as if frosting continuously

For, the plant I saw

In the field of summer

The colour of the leaves were yellowing

1600–1637 Tulipomania

Recognition of viral entity

1886 Mayer transmission of TMV

1892 Iwanowski filterability of TMV

1898 Beijerink viruses as an entity

Biological age

1900–1935 Descriptions of many viruses

Biochemical/Physical age

1935 Purification of TMV

1936 TMV contains pentose nucleic acid

1939 EM TMV rod-shaped particles

1951 TYMV RNA in protein shell

1956 Virus particles made of identical protein subunit

1955/56 Infectious nature of TMV RNA

1962 Structure of isometric particles

1983 Structure of TBSV to 2.9 Å

Molecular age

1960 Sequence of TMV coat protein

1980 Sequence of CaMV DNA genome

1982 Sequence of TMV RNA genome

1984 Infectious transcripts of multicomponent BMV

1986 Transgenic protection of plants against TMV

1996 Recognition of RNA silencing

1997 Recognition of virus suppressors of silencing

The transmissions of animal and plant viruses use different strategies to move from one host to other host and from one cell to other. The movements of plant viruses from one plant to the other need some vector , i.e., means of transmission such as insects, mites, flies, etc. The movement of viruses from one plant cell to other occurs through the plasmodesmata because viruses cannot pass through the thick cell wall. Plants probably have specialised mechanisms for transporting mRNAs through plasmodesmata, and these mechanisms are thought to be used by RNA viruses to spread from one cell to another (Ivanovski 1892).

1.5 Classification and Nomenclature of Viruses

The arrangement of different living organisms into different taxonomic categories (taxa) on the basis of their similarities and/ or relationships is called as classification, while assigning a particular name to them is called as nomenclature. The classification and nomenclature are studied under broader terminology known as taxonomy. The taxonomy of viruses is somewhat recent exercise. Johnson (1927) was the first virologist for emphasising the importance of the viral taxonomy. The earliest classification of virus was based on only few properties which include ecological and biological properties, basically the pathological property which was given greater emphasis. In 1939, Holmes published his system of classification of viruses, which was based on interaction of host with its pathogen using binomial and trinomial system of nomenclature. With the discovery of electron microscope and biochemical studies, the classification of viruses as a group was done by different virologists such as herpesvirus group, myxovirus group, poxvirus group, etc. During this period several attempts were made to classify viruses but none were perfect. There was a need to develop a universal system of viral classification.

Earlier viruses were classified on the basis of the two developed system, the Linnaean system and Adansonian system (Roger Hull 2009). The Linnaean system was based on monothetic hierarchical system which was developed by Linnaeus for plant and animal taxonomy. The classification based on Linnaean system was not suitable for the classification of viruses due to several shortcomings. The second system, i.e., Adansonian system, was more suitable for the viral classification because this system considers several criteria at once. The Adansonian system used in viral taxonomy is polythetic hierarchical classification system published by Adanson in 1763. A polythetic class can be defined as the class in which all the members share the several properties in common (Adanson 1763). According to this system, the virus species are defined by several common properties which they share. In other words the members of a virus species are defined collectively by a consensus group of properties. Earlier this system was not so feasible due to its complexity of several characters. The problems of Adansonian system were sort out by use of computers and now it is used universally. At present more than sixty characters are used for classifying viruses. Various discoveries in cell and molecular biology have provided many tools and techniques, which helped in comparing nucleic acid sequences. The sequencing of DNA or RNA has helped in creating phylogenetic trees for the viruses (Hull 2009).

Several criteria are used for the classification of viruses. Some of the criterians are virion properties, which include shape, size, presence or absence of envelope and peplomers, molecular mass, buoyant density, sedimentation coefficient, pH stability , thermal stability, cation stability (Mg2+, Mn2+), solvent stability, detergent stability, radiation stability, properties of proteins, genome organisation and replication such as type of nucleic acid, DNA or RNA, single or double stranded, linear or circular, positive or negative sense or ambisense, number of segments, size of genome or genome sequence, presence or absence of 5׳ terminal cap, presence or absence of 5׳ terminal polypeptide, presence or absence of 3׳ terminal poly A tract; nucleotide sequence comparison; number of proteins, size of proteins, functional activities of proteins, presence or absence of lipid nature of lipids, presence or absence of carbohydrate, nature of carbohydrate, genome organisation, strategy of replication of nucleic acids, characteristics of translation and post-translational processing, site of accumulation of virion protein, site of assembly, site of maturation and release, cytopathology, inclusion body formation, antigenic properties such as serological relationship, mapping epitopes and biological properties; host range, natural and experimental, pathogenicity, association with disease, tissue tropisms, pathology, histopathology, mode of transmission in nature, vector relationship, geographic distribution (Roger Hull 2009; Leppard et al. 2007).

The nature (molecular and genetic composition) of the virus genome packaged into the virion particle is one of the major factors in classification of viruses. Possible genome types are:

  • dsDNA

  • ssDNA

  • ssDNA(−)

  • ssDNA(+)

  • ssDNA(+/−)

  • dsDNA-RT

  • ssRNA-RT

  • dsRNA

  • ssRNA(−)

  • ssRNA(+)

  • ssRNA(+/−)

  • Viroid

1.6 Baltimore System of Virus Classification

Developed by David Baltimore (1971). The Baltimore classification has + RNA as its central point. This system of virus classification is based upon the relationship between viral genome and messenger RNA. All viruses must produce mRNA, or (+) sense RNA and a complementary strand of mRNA, or nucleic acid is called (−) sense (strand) (Voyles 2002). According to Baltimore viruses can be grouped into seven classes on the basis of mRNA synthesis:

  1. 1.

    Class 1: dsDNA viruses; mRNA is synthesised normally using negative strand as template.

  2. 2.

    Class 2: ssDNA viruses ; mRNA is synthesised by double stranded DNA intermediate.

  3. 3.

    Class 3: dsRNA viruses; mRNA is synthesised by complementary strand(template strand).

  4. 4.

    Class 4: ssRNA viruses; RNA directly functions as mRNA.

  5. 5.

    Class 5: (−) sense ssRNA viruses; mRNA is synthesised by synthesis of positive strand.

  6. 6.

    Class 6: genome (+) strand RNA viruses; genome is synthesised by reverse transcription.

  7. 7.

    Class 7: DNA reverse transcribing viruses with RNA intermediates.

The international committee on nomenclature of virus was established by a group of 43 virologists from all over the world in 1966 at International Congress for Microbiology held in Moscow to develop a uniform system of classification and nomenclature (Fauquet et al. 2005). The name of ICNV was changed to International Committee on Taxonomy of Viruses in 1974. The ICTV is the main governing body for all matters related to viral taxonomy . At present, International Committee on Taxonomy of Viruses (ICTV) is a committee of the Virology Division of the International Union of Microbiological Societies. The ICTV is made up of an executive committee (EC) with officers of the ICTV, subcommittee chairs and elected members. The officers manage ICTV activities, while the subcommittee chairs are responsible for managing a series of study groups that assess the current virus taxonomy and recommend updates. Elected members assist the subcommittee chairs in managing the process of making taxonomic assignments.

At present the ICTV is composed of six subcommittees. The responsibilities of subcommittee are to classify fungal and algal viruses, plant viruses, invertebrate viruses, prokaryotic viruses and vertebrate viruses. These subcommittees discuss the classification of newly discovered viruses and manage rules accordingly. The last committee, i.e., the sixth subcommittee, is responsible for managing ICTV data and maintaining the ICTV database and websites. There are 76 international study groups (SGs) functioning under ICTV for the study of families and genera. Each SGs is headed by the chairman. The chairman is appointed by the relevant subcommittee chair. Chairman of the SGs is responsible for (1) organising discussions among SG members of emerging taxonomic issues in their field, (2) for overseeing the submission of proposals for new taxonomy and (3) for the preparation, or revision, of relevant chapter(s) in ICTV Reports. Since its inception ICTV has published nine reports. The first report was published in 1971, 2nd in 1976, 3rd in 1979, 4th in 1982, 5th in 1991, 6th in 1995, 7th in 2000, 8th in 2005 and 9th in 2011. In 2015 ICTV has published its virus taxonomy release. According to this taxonomic release, viruses are divided into seven orders, 111 families, 30 subfamilies, 610 genera and 3705 species.

ICTV activities are governed by statutes agreed with the virology division. The statutes define the objectives of the ICTV. These are:

  1. 1.

    To develop an internationally agreed taxonomy for viruses;

  2. 2.

    To develop internationally agreed names for virus taxa

  3. 3.

    To communicate taxonomic decisions to the international community of virologists;

  4. 4.

    To maintain an index of agreed names of virus taxa.

The present universal system of viral taxonomy given by ICTV follows the hierarchical system which includes order, family, subfamily in some, genus and species. Lower hierarchical system is also developed by ICTV. According to ICTV the hierarchical system is as follows:

  • Order: An ‘order’ is the highest taxonomic level of virus classification into which virus species can be categorised. In the present taxonomic system, use of order is optional. Some of the viruses are unassigned during classification. If ‘unassigned’ has been entered, the taxon has not been assigned to an order. The first order to be established was Mononegavirales in 1990. This order comprises non-segmented ssRNA negative-sense viruses, namely the families Filoviridae, Paramyxoviridae and Rhabdoviridae (Fauquet et al. 2005). According to current taxonomic release of ICTV (2015), seven orders have been assigned, while 78 virus families have not been assigned to any orders. The orders are Caudovirales (3 families), Herpesvirales (3 families), Ligamenvirales (2 families), Mononegavirales (5 families), Nidovirales (4 families), Picornavirales (5 families) and Tymovirales (4 families), and 78 virus families have not been assigned to orders.

  • Family: A ‘family’ is a level in the taxonomic hierarchy into which virus species can be classified. If marked ‘unassigned’ (which is rare), the lower taxonomic level of ‘genus’ has not been assigned to a family. A total of 104 families have been described by ICTV 2015.

  • Subfamily: A ‘subfamily’ is a level in the taxonomic hierarchy into which virus species can be classified. Use of the taxonomic level subfamily is optional. If left blank, the lower taxonomic levels of genus and/or species have not been assigned to a subfamily

  • Genus: A ‘genus’ is a level in the taxonomic hierarchy into which virus species can be classified. Viral genus may be defined as ‘a population of virus species that share common characteristics and are different from other population of species’ (Fauquet et al. 2005). If ‘unassigned’ (which is rare), that species has not been assigned to a genus.

  • Species: The 7th ICTV Report formalised for the first time the concept of the virus species as the lowest taxon (group) in a branching hierarchy of viral taxa. As defined therein, ‘a virus species is a polythetic class of viruses that constitute a replicating lineage and occupy a particular ecological niche’ (Van Regenmortel 1990). A polythetic class can be defined as the class in which all the members share the several properties in common. According to this system, the virus species are defined by several common properties which they share. In other words the members of a virus species are defined collectively by a consensus group of properties. Virus species thus differ from the higher viral taxa, which are ‘universal’ classes and as such are defined by properties that are necessary for membership.

One ‘type of species’ is chosen for each genus to serve as an example of a well-characterised species for that genus. If the value in this column is ‘1’, this indicates that this species has been chosen as the type species for its genus.

1.7 Nomenclature of Viruses

The guide line for naming of viruses by ICTV (9th Report) are as follows:

The genus name ends in ‘-virus’, subfamily name ends in ‘-virinae’, family name ends with ‘-viridae’ and order name ends with ‘-virales’ universally in formal taxonomy . In viral taxonomy, the finalised names of virus orders (e.g., Caudovirales), families (e.g., Myoviridae), subfamilies (e.g., Pseudovirineae Peduovirineae) and genera (e.g., Hpunalikevirus) are printed in italics, and the first letters of the names are written in capitals. The names of species are printed in italics with first letter of first word in capital (e.g., Mumps virus). The rest of the words is not capitalised unless they are proper nouns (e.g., West Nile virus), parts of proper nouns (Enterobacteria phage MS2) or alphabetical identifiers (e.g., Enterovirus A). Names of virus strains, on the other hand, are not italicised. The first letter of the first word is not capitalised (e.g., herpes simplex virus) unless it is a proper noun, typically based on the binomial name of the species it infects (Van Regenmortel 1999; Mayo 2000).

The outline of present , (ICTV taxonomic release, 2014) taxonomy of viruses is as follows:

 

Order

 

Family

Genus

Subfamily

Genus

1

Caudovirales

1

Myoviridae

Bcep78likevirus

Eucampyvirinae

Cp220likevirus

 

Bcepmulikevirus

Cp8unalikevirus

Felixounalikevirus

Peduovirinae

Hpunalikevirus

Hapunalikevirus

P2likevirus

I3likevirus

Spounavirinae

Spounalikevirus

Mulikevirus

Twortlikevirus

Pbunalikevirus

Unassigned

Phicd119likevirus

Tevenvirinae

Schizot4likevirus

Phihlikevirus

T4likevirus

Phikzlikevirus

Unassigned

Punalikevirus

 

Viunalikevirus

Unassigned

2

Podoviridae

Bcep22likevirus

Autographivirinae

Phikmvlikevirus

Bppunalikevirus

Sp6likevirus

Epsilon15likevirus

T7likevirus

F116likevirus

Unassigned

Luz24likevirus

Picovirinae

Ahjdlikevirus

N4likevirus

Phi29likevirus

P22likevirus

Unassigned

Phieco32likevirus

Unassigned

3

Siphoviridae

3alikevirus

  

77likevirus

Andromedalikevirus

Barnyardlikevirus

Bignuzlikevirus

Bronlikevirus

C2likevirus

C5likevirus

Charlielikevirus

    

Che8likevirus

  

Che9clikevirus

Chilikevirus

Cjwunalikevirus

Corndoglikevirus

D3112likevirus

D3likevirus

Halolikevirus

Hk578likevirus

Iebhlikevirus

Jerseylikevirus

L5likevirus

Lambdalikevirus

N15likevirus

Omegalikevirus

P23likevirus

Pbiunalikevirus

Pgonelikevirus

Phic3unalikevirus

Phicbklikevirus

Phie125likevirus

Phietalikevirus

Phifllikevirus

Phijlunalikevirus

Psimunalikevirus

Reylikevirus

Sap6likevirus

Sfi1unalikevirus

Sfi21dtunalikevirus

Skunalikevirus

Spbetalikevirus

    

T5likevirus

  

Tm4likevirus

Tp2unalikevirus

Tunalikevirus

Wbetalikevirus

Xp10likevirus

Yualikevirus

2

Herpesvirales

1

Alloherpesviridae

Batrachovirus

  

Cyprinivirus

Ictalurivirus

Salmonivirus

2

Herpesviridae

–

Alphaherpesvirinae

Iltovirus

Mardivirus

Scutavirus

Simplexvirus

Unassigned

Varicellovirus

  

Betaherpesvirinae

Cytomegalovirus

Muromegalovirus

Proboscivirus

Roseolovirus

Unassigned

  

Gammaherpesvirinae

Lymphocryptovirus

Macavirus

Percavirus

Rhadinovirus

Unassigned

Unassigned

3

Malacoherpesviridae

Aurivirus

  

Ostreavirus

3

Ligamenvirales

1

Lipothrixviridae

Alphalipothrixvirus

  

Betalipothrixvirus

Deltalipothrixvirus

Gammalipothrixvirus

  

Rudiviridae

Rudivirus

4

Mononegavirales

1

Bornaviridae

Bornavirus

  

2

Filoviridae

Cuevavirus

Ebolavirus

Marburgvirus

3

Nyamiviridae

Nyavirus

Unassigned

4

Paramyxoviridae

–

Paramyxovirinae

Aquaparamyxovirus

Avulavirus

Ferlavirus

Henipavirus

Morbillivirus

Respirovirus

Rubulavirus

Pneumovirinae

Metapneumovirus

Pneumovirus

5

Rhabdoviridae

Cytorhabdovirus

  

Ephemerovirus

Lyssavirus

Novirhabdovirus

Nucleorhabdovirus

Perhabdovirus

Sigmavirus

Sprivivirus

Tibrovirus

Tupavirus

Unassigned

Vesiculovirus

5

Nidovirales

1

Arteriviridae

Arterivirus

  

2

Coronaviridae

–

Coronavirinae

Alphacoronavirus

Betacoronavirus

Deltacoronavirus

Gammacoronavirus

Torovirinae

Bafinivirus

Torovirus

3

Mesoniviridae

Alphamesonivirus

  

4

Roniviridae

Okavirus

6

Picornavirales

1

Dicistroviridae

Aparavirus

Cripavirus

2

Iflaviridae

Iflavirus

3

Marnaviridae

Marnavirus

4

Picornaviridae

Aphthovirus

  

Aquamavirus

Avihepatovirus

Avisivirus

Cardiovirus

Cosavirus

Dicipivirus

Enterovirus

Erbovirus

Gallivirus

Hepatovirus

Hunnivirus

Kobuvirus

Kunsagivirus

Mischivirus

Mosavirus

Oscivirus

Parechovirus

    

Pasivirus

  

Passerivirus

Rosavirus

Sakobuvirus

Salivirus

Sapelovirus

Senecavirus

Sicinivirus

Teschovirus

Tremovirus

 

5

Secoviridae

 

Comovirinae

Cheravirus

Sadwavirus

Sequivirus

Torradovirus

Unassigned

Waikavirus

6

Unassigned

Bacillarnavirus

–

 

Labyrnavirus

7

Tymovirales

1

Alphaflexiviridae

Allexivirus

  

Botrexvirus

Lolavirus

Mandarivirus

Potexvirus

Sclerodarnavirus

Unassigned

2

Betaflexiviridae

Capillovirus

Carlavirus

Citrivirus

Foveavirus

Tepovirus

Trichovirus

Unassigned

Vitivirus

3

Gammaflexiviridae

Mycoflexivirus

4

Tymoviridae

Unassigned

  

Maculavirus

Marafivirus

Tymovirus

8

Virus families not assigned to an order

1

Adenoviridae

5 genera

  

2

Alphatetraviridae

2 genera

3

Alvernaviridae

1 genus

4

Amalgaviridae

1 genus

5

Ampullaviridae

1 genus

6

Anelloviridae

11 genera

7

Arenaviridae

2 genera

8

Ascoviridae

1 genus

9

Asfarviridae

1 genus

10

Astroviridae

2 genera

11

Avsunviroidae

3 genera

12

Baculoviridae

4 genera

13

Barnaviridae

1 genus

14

Benyviridae

1 genus

15

Bicaudaviridae

1 genus

16

Bidnaviridae

1 genus

17

Birnaviridae

4 genera

18

Bromoviridae

6 genera

19

Bunyaviridae

5 genera

20

Caliciviridae

5 genera

21

Carmotetraviridae

1 genus

22

Caulimoviridae

8 genera

23

Chrysoviridae

1 genus

24

Circoviridae

2 genera

25

Clavaviridae

1 genus

26

Closteroviridae

4 genera

27

Corticoviridae

1 genus

28

Cystoviridae

1 genus

29

Endornaviridae

1 genus

30

Flaviviridae

4 genera

  

31

Fuselloviridae

2 genera

  

32

Geminiviridae

7 genera

33

Globuloviridae

1 genus

34

Guttaviridae

2 genera

35

Hepadnaviridae

2 genera

36

Hepeviridae

2 genera

37

Hypoviridae

1 genus

38

Hytrosaviridae

2 genera

39

Inoviridae

2 genera

40

Iridoviridae

5 genera

41

Leviviridae

2 genera

42

Luteoviridae

3 genera

43

Marseilleviridae

1 genus

44

Megabirnaviridae

1 genus

45

Metaviridae

3 genera

46

Microviridae

1 subfamily

1 subfamily

47

Mimiviridae

2 genera

 

48

Nanoviridae

2 genera

49

Narnaviridae

2 genera

50

Nimaviridae

1 genera

51

Nodaviridae

2 genera

52

Nudiviridae

2 genera

53

Ophioviridae

1 genera

54

Orthomyxoviridae

6 genera

55

Papillomaviridae

39 genera

  

56

Partitiviridae

5 genera

  

57

Parvoviridae

 

2 subfamilies

58

Permutotetraviridae

1 genus

 

59

Phycodnaviridae

6 genera

60

Picobirnaviridae

1 genus

61

Plasmaviridae

1 genus

62

Polydnaviridae

2 genera

63

Polyomaviridae

1 genus

64

Pospiviroidae

5 genera

65

Potyviridae

8 genera

66

Poxviridae

2 genera

67

Pseudoviridae

3 genera

68

Quadriviridae

1 genera

 

69

Reoviridae

 

2 subfamilies

 

70

Retroviridae

 

2 subfamilies

71

Sphaerolipoviridae

3 genera

 

72

Spiraviridae

1 genus

73

Tectiviridae

1 genus

74

Togaviridae

2 genera

75

Tombusviridae

13 genera

76

Totiviridae

5 genera

77

Turriviridae

1 genus

78

Unassigned

14 genera

79

Virgaviridae

6 genera