Classification of Viruses
In this appendix selected groups of viruses are briefly described and a sketch of each is included.
The illustrations are not to scale but should provide a general idea of the morphology of each
group. The viruses are separated into five sections based primarily on their host preferences.
This material has been adapted with permission from chapter 22 of Introduction to Modern
Virology, 5th ed. by N. J. Dimmock, A. J. Easton, and K. N. Leppard, copyright © 2001 by
Blackwell Scientific Publications, Ltd.
We have not italicized the names of specific viruses in the text. However, it should be noted that
in formal taxonomic usage the official name of a virus species—just as with other taxonomic
names—is italicized, and the first letter of the first word in the species name is capitalized.
Tentative species names and names of strains or isolates are not italicized. In this appendix, we
will capitalize and italicize the genus name, but not always the species name.
A. VIRUSES OF VERTEBRATE AND INVERTEBRATE ANIMALS
1. Viruses with double-stranded DNA genomes
a. Family: Adenoviridae
Nonenveloped icosahedral particles of 60 to 90 nm with a fiber protein at each vertex. Contains
one molecule of linear double-stranded DNA (dsDNA) of 25,000 to 42,000 base-pairs (bp). A
protein is covalently linked to the 5’ end and forms a pseudocircular genome by noncovalently
linking to the 3’ end. Replicates and is assembled in the nucleus. Infects vertebrates but one
genus infects fungi.
Genera:
Mastadenovirus—infects mammals (e.g., human adenovirus type 2 or type 5)
Aviadenovirus—infects birds (e.g., fowl adenovirus type 1)
Atendovirus—bovine adenoviruses
Siadenovirus (e.g., frog adenovirus 1, turkey adenovirus 3)
b. Family: Baculoviridae
Enveloped rod-shaped particles, each with a nucleocapsid of 300 nm X 30 to 60 nm with one
molecule of circular dsDNA of 80,000 to 180,000 bp. Virion may be occluded in a protein
inclusion body (which makes it very stable). A large and diverse group, common in over 50
species of Lepidoptera; also infects Coleoptera, etc.
Genera:
Granulovirus—may be occluded in a polyhedron containing one particle (e.g., Cydia pomonella
granulovirus).
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Nucleopolyhedrovirus. Virions may be occluded in a polyhedron containing many particles (e.g.,
Autographica californica multiple nucleopolyhedrosis virus).
c. Family: Herpesviridae
Enveloped 120 to 220 nm particle with spikes, enclosing successively a tegument and an
icosahedral nucleocapsid of 125 nm. One molecule of linear dsDNA of 120,000 to
240,000 bp. May bud from nuclear membrane. Very large family that infects vertebrates; latency
is a common feature of the life cycle. Some are oncogenic.
Subfamily: Alphaherpesvirinae
Genera:
Simplexvirus (e.g., human herpes [simplex] virus type 1 and 2—HHV-1 and -2)
Varicellovirus (e.g., HHV-3 or varicella-zoster [chickenpox] virus)
Subfamily: Betaherpesvirinae
Genera:
Cytomegalovirus (e.g., HHV-5 or human cytomegalovirus)
Muromegalovirus (e.g., mouse cytomegalovirus)
Roseolavirus (e.g., HHV-6, HHV-7)
Subfamily: Gammaherpesvirinae (lymphoproliferative viruses; oncogenic)
Genera:
Lymphocryptovirus (e.g., HHV-4 or Epstein-Barr virus)
Rhadinovirus (e.g., ateline herpesvirus 2, herpesvirus saimiri, HHV-8)
Unclassified: Marek’s disease virus, turkey herpesvirus.
d. Family: Iridoviridae
Icosahedral isometric particle of 120 to 200 nm, which may be surrounded by a lipid envelope.
Purified virus pellets are iridescent blue or green. One molecule of linear dsDNA of 140,000 to
300,000 bp, but virions may contain one or more copies. Contains several enzymes. Cytoplasmic
assembly. Includes viruses of insects, fish (flounder, dab, and goldfish) and many frog species
(e.g., frog virus 3).
e. Family: Papillomaviridae
Nonenveloped, 55 nm icosahedral particle with 72 capsomers in a skewed arrangement. One
8,000 bp molecule of double-stranded, covalently closed, circular DNA. Replicates and
assembles in the nucleus. Over 60 human papillomaviruses (HPVs) and others from many
species. Oncogenic.
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f. Family: Polyomaviridae
Nonenveloped 40 nm icosahedral particle with 72 capsomers in a skewed arrangement. One
5,000 bp molecule of covalently closed, circular dsDNA. Replicates and assembles in the
nucleus. Includes murine polyomavirus and simian virus type 40 (SV40). Oncogenic.
g. Family: Poxviridae
Enveloped brick-shaped virions of 220 to 450 nm X 140 to 260 nm X 140 to 260 nm thick.
Complex structure enclosing one or two lateral bodies and a biconcave core, with all enzymes
required for mRNA synthesis. One molecule of linear ds-DNA of 130,000 to 375,000 bp.
Cytoplasmic multiplication. Infects mostly vertebrates but also insects.
Subfamily: Chondropoxvirinae (viruses of vertebrates)
Genera:
Orthopoxvirus (e.g. vaccinia and related viruses)
Avipoxvirus (e.g., fowlpox and related viruses)
Capripoxvirus (e.g., sheeppox and related viruses)
Leporipoxvirus (e.g., myxoma of rabbits and related viruses; spread passively by arthropods)
Molluscipoxvirus (e.g., molluscum contagiosum virus)
Parapoxvirus (e.g., orf virus/milker’s node virus and related viruses)
Suipoxvirus (e.g., swine pox virus)
Yatapoxvirus (e.g., yaba/tanapox and related viruses of monkeys)
Subfamily: Entomopoxvirinae (viruses of insects)
Genera:
Entomopoxvirus A, B, and C
2. Viruses with ssDNA genomes
a. Family: Circoviridae
Nonenveloped small icosahedral particles with circular ssDNA. Another genus infects plants.
Genus:
Circovirus—20 nm virion with one molecule of DNA of 1,800 to 2,300 nucleotides (e.g., chicken
anemia virus; probably also TT [transfusion-transmitted] virus of humans)
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b. Family: Parvoviridae
Nonenveloped icosahedral particles of 18 to 26 nm that contain no enzymes. One 5,000
nucleotide linear molecule of either negative- or positive-sense ssDNA per particle. On extraction,
these form an artifactual double strand. Replication is nuclear.
Subfamily: Parvovirinae (viruses of vertebrates)
Genera:
Parvovirus—(e.g., canine and feline parvovirus, minute virus of mice)
Erythrovirus—B19 virus, which causes fifth disease of children
Dependovirus—these are satellite viruses
Subfamily: Densovirinae (viruses of insects)
3. Viruses with dsRNA genomes and a virion-associated RNA-dependent RNA polymerase
a. Family: Birnaviridae
Nonenveloped icosahedral 60 nm particle with a 45 nm core containing two segments of linear
dsRNA of 2,800 and 3,200 nucleotides. Has a VPg protein covalently linked to 5’ end of each
segment. Cytoplasmic multiplication.
Genera:
Aquabirnavirus (e.g., pancreatic necrosis virus of fish)
Avibirnavirus (e.g., infectious bursal disease of chickens, turkeys, ducks)
Entomobirnavirus (e.g., Drosophila X virus)
b. Family: Reoviridae
Large family with members found in vertebrates, insects, and plants. Nonenveloped 60 to 80 nm
icosahedral particle containing an isometric nucleocapsid with 10 to 12 segments of linear
dsRNA of 18,000 to 30,000 bp. Cytoplasmic multiplication. Within a genus RNA segments in a
mixed infection readily assort to form genetically stable hybrid virus.
Genera:
Orthoreovirus—infects vertebrates (humans, dogs, cattle, birds); 10 dsRNA segments (e.g.,
reoviruses of humans)
Orbivirus—replicates in vertebrates and their insect vectors; 10
dsRNA segments (e.g., bluetongue virus of sheep)
Rotavirus—name comes from its wheel-like particle with spokes; causes life-threatening diarrhea
in very young vertebrates of many species, including humans; 11 dsRNA segments
Aquaerovirus—viruses of fish and Crustacea; 11 dsRNA segments
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Coltivirus—Colorado tick fever virus of vertebrates; 12 dsRNA segments
Cypovirus—cytoplasmic polyhedrosis viruses of insects (including Lepidoptera, Hymenoptera,
and Diptera); 10
dsRNA segments
4. Viruses with positive-sense ssRNA genomes
a. Family: Caliciviridae
Nonenveloped icosahedral 35 to 40 nm particle with 32 calyxlike (cup-shaped) depressions.
Contains one molecule of linear positive-sense ssRNA of 7,500 nucleotides. The 5’ end has a
VPg or a cap structure (hepatitis E virus). Cytoplasmic multiplication with subgenomic mRNA.
Infects many species (e.g., human calicivirus, Norwalk virus, hepatitis E virus, rabbit hemorrhagic
fever virus, swine exanthema virus).
b. Family: Coronaviridae
With the Arteriviridae, forms the order Nidovirales. Enveloped particles of 120 to 160 nm with
club-shaped sparse protein spikes. Contains a helical nucleocapsid with one molecule of positivesense ss-RNA of 28,000 to 31,000 nucleotides. One of the largest RNA genomes. Cytoplasmic
replication with a nested set of subgenomic mRNAs with a common leader sequence. Buds from
Golgi apparatus and endoplasmic reticulum.
Genera:
Coronavirus (e.g., avian infectious bronchitis virus and human coronavirus OC43)
Torovirus—biconcave or toroidal or doughnut-shaped particles
(Berne virus of horses and Breda virus of cattle)
c. Family: Flaviviridae
Enveloped 40 to 60 nm particles with an isometric nucleocapsid of 25 to 30 nm and one molecule
of linear positive-sense ss-RNA of 9,500 to 12,500 nucleotides. Differ from Alphaviridae by the
presence of a matrix protein, the lack of subgenomic mRNAs, and budding from the endoplasmic
reticulum. Cytoplasmic multiplication. Spread by arthropods unless stated.
Genera:
Flavivirus—large group of viruses that multiply in the vertebrate host and insect or tick vector
(e.g., yellow fever virus, tick-borne encephalitis virus group, dengue virus group, Japanese
encephalitis virus group)
Pestivirus—includes border disease virus (sheep), bovine diarrhea virus, classic swine fever (hog
cholera) virus; no vector
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Hepacivirus—hepatitis C virus of humans; no vector
d. Family: Picornaviridae
Nonenveloped viruses of mainly vertebrates. One molecule of linear positive-sense ssRNA of
7,000 to 8,500 nucleotides in 30 nm icosahedral particles. Has a 5’ VPg protein. Cytoplasmic
multiplication.
Genera:
Enterovirus—acid resistant, primarily of the intestinal tract (e.g., polioviruses, most echoviruses,
coxsackieviruses of humans and various nonhuman enteroviruses)
Aphthovirus—economically important foot-and-mouth disease viruses of cattle and other
ruminants
Cardiovirus (e.g., encephalomyocarditis [EMC] virus of mice)
Hepatovirus (e.g., human and simian hepatitis A viruses)
Parechovirus (e.g., human echoviruses 22 and 23)
Rhinovirus—acid labile; infects the upper respiratory tract; includes about 100 common cold
viruses; also equine rhinoviruses and various unclassified viruses of insects
e. Family: Tetraviridae
Nonenveloped 40 nm icosahedral particle with one molecule (6,500 nucleotides) or two
molecules (2,500 and 5,300 nucleotides) of linear positive-sense ssRNA. No infection of cultured
cells. All isolated from Lepidoptera (e.g., Naudaurelia  virus)
f. Family: Togaviridae
Enveloped 70 nm particles containing an icosahedral nucleocapsid and one molecule of linear
positive-sense ssRNA of 9,000 to 12,000 nucleotides. Cytoplasmic multiplication; buds from the
plasma membrane. Has an intracellular subgenomic mRNA.
Genera:
Alphavirus—multiplies in vertebrate host and insect vector (e.g., Sindbis virus and Sernliki forest
virus)
Rubivirus—rubella virus of humans; no vector
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5. Viruses with negative-sense/ambisense ssRNA genomes and a virion-associated RNAdependent RNA polymerase
a. Family: Arenaviridae
Enveloped isometric usually 100 nm particles with club-shaped spikes. Genome is contained in
two circular helical nucleocapsids, the larger with one molecule of linear negative-sense ssRNA
of 7,500 nucleotides and the smaller, also linear, of 3,500 nucleotides. Both are ambisense. May
package more than two nucleocapsids per virion. Virion contains host cell ribosomes for no
known function. Cytoplasmic multiplication; buds from plasma membrane. Divided into the
LCM/LASV subgroup (Old World arenaviruses: lymphocytic choriomeningitis virus, lassa virus
and related viruses) and the Tacaribe complex subgroup (New World arenaviruses: Tacaribe,
Junin, Pichinde and related viruses).
b. Family: Bornaviridae
Enveloped isometric 90 nm virions with a helical nucleocapsid and one molecule of linear
negative-sense ssRNA of 9,000 nucleotides. Nuclear replication. Recently isolated from people
with behavioral/neuropsychiatric problems but not known if causal. Known to infect horses since
the eighteenth century and several other vertebrate species. Natural host unknown. Borna
disease virus.
c. Family: Bunyaviridae
Enveloped isometric 100 nm particle with 10 nm spikes. Contains three helical nucleocapsids,
each with one molecule of linear negative-sense ssRNA—large: 6,000 to 12,000 nucleotides,
medium: 3,500 to 6,000 nucleotides, and small: 1,000 to 2,000 nucleotides. Cytoplasmic
multiplication; buds from the Golgi membranes. Infects vertebrates and spread by arthropods
unless stated. Also a genus that infects plants.
Genera:
Bunyavirus—mosquito and gnat vectors (e.g., Bunyamwera and 150 or so other viruses)
Hantavirus—no arthropod vector (e.g., Hantaan virus)
Nairovirus—tick vector (e.g., Nairobi sheep disease virus)
Phlebovirus—S RNA is ambisense; sandfly, tick and gnat vectors (e.g., sandfly fever virus,
uukuniemi virus)
d. Family: Filoviridae
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Genus:
Filovirus—enveloped long filamentous, sometimes branched, particles 800 to 900 (sometimes
14,000) nm X 80 nm with helical nucleocapsid of 50 nm diameter. One molecule of negativesense ssRNA of 19,000 nucleotides. Buds from plasma membrane. Includes Marburg, Ebola, and
Reston viruses. Highly pathogenic for humans. Natural reservoir not known.
e. Family: Orthomyxoviridae
Enveloped pleomorphic 100 nm (sometimes filamentous) particles with a dense layer of protein
spikes. Six to eight helical nucleocapsids, 9 nm in diameter with transcriptase activity. Each
contains one molecule of linear negative-sense ssRNA, totaling 12,000 to 15,000 nucleotides. All
RNA synthesis is nuclear. Within a genus, RNA segments in a mixed infection readily assort to
form genetically stable hybrid viruses. Plasma membrane budding.
Genera:
Influenzavirus A—genome comprises eight molecules of RNA ranging from 900 to 2,350
nucleotides. Virions have separate hemagglutinin and neuraminidase spike proteins. Undergoes
antigenic shift and drift. Natural reservoir is seashore birds; infects several vertebrate species
including humans.
Influenzavirus B—genome comprises eight molecules of RNA ranging from 900 to 2,350
nucleotides. Virions have separate hemagglutinin and neuraminidase spike proteins. Undergoes
antigenic drift only. Infects humans.
Influenzavirus C—genome comprises seven molecules of RNA ranging from 1,000 to 2,350
nucleotides. Virions have one hemagglutinin–esterase fusion spike protein. The esterase is
a receptor-destroying enzyme. Undergoes minor antigenic variation. Infects humans.
Thogotovirus—carried by ticks and occasionally infects humans. Genome comprises six
molecules of RNA. (Thogoto virus and Dhori virus; Asia, Africa, and Europe.)
Possibly also another genus for infectious salmon anemia virus.
f. Family: Paramyxoviridae
Enveloped pleomorphic particle usually 150 to 200 nm in diameter. Has a dense layer of fusion
protein spikes and attachment protein spikes. Contains one helical nucleocapsid 13 to 18 nm in
diameter with one molecule of linear negative-sense ss-RNA of 13,000 to 16,000 nucleotides.
Filamentous forms of up to 400 nm common. Cytoplasmic multiplication; buds from plasma
membrane. Infects vertebrates. Most, but not all, are respiratory viruses.
Subfamily: Paramyxovirinae
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Genera:
Respirovirus (e.g., human parainfluenzavirus type 1)
Morbillivirus—measles virus, rinderpest virus, and canine distemper virus
Rubulavirus (e.g., mumps virus, Newcastle disease virus of poultry or avian parainfluenzavirus
type 1)
Subfamily: Pneumovirinae
Genera:
Pneumovirus (e.g., human and bovine respiratory syncytial viruses, pneumonia virus of mice)
Metapneumovirus (e.g., turkey rhinotracheitis virus)
g. Family: Rhabdoviridae
Enveloped, bullet-shaped particles 100 to 430 nm X 45 to 100 nm, containing one molecule of
linear negative-sense ssRNA of 11,000 to 15,000 nucleotides. Has 5–10 nm spikes. Inside is a
helical nucleocapsid. Cytoplasmic multiplication and buds from plasma membrane. Infects
vertebrates. Also genera that infect plants.
Genera:
Vesiculovirus (e.g., vesicular stomatitis virus)
Lyssavirus (e.g., rabies virus, unusual as infects all mammals)
Ephemerovirus (e.g., bovine ephemeral fever virus)
Novirhabdovirus (e.g., infectious hemopoietic virus of fish)
6. Viruses with RNA genomes that replicate through a DNA intermediate
a. Family: Retroviridae
Enveloped 80 to 100 nm particles with spikes. Nucleocapsid can be isometric or a truncated
cone; contains two identical copies of a linear positive-sense ssRNA of 7,000 to 11,000
nucleotides. Virions contain reverse transcriptase and integrase enzymes. The DNA provirus is
nuclear and integrated with host DNA. Transmission is horizontal or vertical. Associated with
many different diseases. Not all viruses are oncogenic.
Genera:
“Mammalian type C retroviruses” (e.g., murine leukemia virus); oncogenic
Alpharetrovirus (e.g., avian leukosis virus, Rous sarcoma virus); oncogenic
Betaretrovirus (e.g., Mason-Pfizer monkey virus)
Gammaretrovirus (e.g., mouse mammary tumour virus); oncogenic
Deltaretrovirus (e.g., bovine leukemia virus, human T-cell
lymphotrophic virus types 1–3); oncogenic
Epsilonretrovirus (e.g., walleye dermal sarcoma virus of fish)
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Lentivirus—infects primates (human immunodeficiency virus types 1 and 2, simian
immunodeficiency viruses), horses, sheep, goats, cattle, and cats; all associated with
immunodeficiency diseases
Spumavirus (e.g., human spumaviruses); named after their foamy cytopathogenic effect; no
disease known
7. Viruses with a DNA genome that replicate through an RNA intermediate
a. Family: Hepadnaviridae
A40 to 48 nm enveloped particle containing an isometric nucleocapsid with DNA polymerase and
protein kinase activities. One partially double-stranded circular DNA molecule that is not
covalently closed. This has a complete negative-sense strand of 3,000 nucleotides with a 5’terminal protein, and a variable length positive-sense strand of 1,700 to 2,800 nucleotides. The
circularization overlaps the 3’ and 5’ termini of the negative-sense DNA. These are
“reversiviruses” that have a reverse transcriptase.
Genera:
Orthohepadnavirus (e.g., hepatitis B [HBV] of humans, which is strongly associated with liver
cancer, and woodchuck hepatitis virus)
Avihepadnavirus (e.g., duck hepatitis B virus)
B. VIRUSES THAT MULTIPLY IN PLANTS
Knowledge of plant virus multiplication is harder to obtain than that of animal viruses because
plant cell culture systems are less manageable. Greater emphasis is given to physical properties
and disease characteristics, especially as many are important in agriculture. Differences in virus
proteins, translation strategy (which may not be mentioned here), and vector are important criteria
in plant virus classification. Some plant viruses also multiply in their animal vector (Bunyaviridae,
Marafivirus, Reoviridae, Rhabdoviridae), so the plant/animal virus distinction becomes uncertain.
1. Viruses with ssDNA genomes
a. Family: Geminiviridae
Virions comprise two incomplete icosahedra, 30 nm long X 18 nm, joined as Siamese twins.
Nonenveloped. Has closed circular ssDNA. Nuclear replication. Persistent or not in an insect
vector but does not multiply in the vector.
Genera:
Curtovirus—genome is one DNA molecule of 3,000 nucleotides. Does not infect
monocotyledonous plants (grasses). Whitefly vector (e.g., beet curly top virus).
Mastrevirus—genome is one DNA molecule of 2,600 nucleotides. Infects mainly
monocotyledonous plants (grasses). Leafhopper vector (e.g., maize streak virus).
Begomovirus—most genomes are two molecules of DNA each of 2,500 nucleotides. Whitefly
vector (e.g., bean golden mosaic virus).
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2. Viruses with dsRNA genomes and a virion-associated RNA-dependent
RNA polymerase
a. Family: Reoviridae
Large and widespread family. Genera also found in vertebrates and insects. Nonenveloped 60 to
80 nm icosahedral particles containing an isometric nucleocapsid with 10 to 12 segments of linear
dsRNA of 18,000 to 30,000 bp. Has a transcriptase activity. Cytoplasmic multiplication. Within a
genus RNA segments in a mixed infection readily assort to form genetically stable hybrid virus.
Genera:
Fijivirus—10 dsRNA segments. Multiplies in leafhopper insect vector (e.g., Fiji disease virus).
Oryzavirus—10 dsRNA segments. Multiplies in insect vector (e.g., rice ragged stunt virus).
Phytoreovirus—12 dsRNA segments. Multiplies in leafhopper insect vector (e.g., wound tumour
virus of clover).
3. Viruses with positive-sense ssRNA genomes Isometric virions
a. Family: Comoviridae
Two molecules of linear positive-sense ssRNA, each encapsidated separately in a nonenveloped
30 nm icosahedral particle. Both RNAs needed for infectivity. Cytoplasmic multiplication.
Genera:
Comovirus—RNAs of < 7,000 and 3,981 nucleotides.
Nonpersistent in beetle vector (e.g., cowpea mosaic virus).
Fabavirus—RNAs of < 7,000 and 4,500 nucleotides.
Nonpersistent in aphid vector (e.g., broad-bean wilt virus 1).
Nepovirus—RNAs of > 7,000 and 4,000 nucleotides. Does not multiply in nematode vector or not
vectored (e.g., tobacco ringspot virus).
b. Family: Luteoviridae
Nonenveloped icosahedral 25 to 30 nm particles with one molecule of linear positive-sense ssRNA of 5,500 to 7,000 nucleotides. Does not multiply in aphid vector, but persists.
Genera:
Enamovirus—no VPg protein (e.g., pea enation mosaic virus)
Luteovirus—no VPg (e.g., barley yellow dwarf virus)
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Polerovirus—RNA has a 5’ VPg (e.g., potato leafroll virus)
c. Family: Tombusviridae
Nonenveloped isometric 32 to 35 nm particle with one or two molecules of linear positive-sense
ssRNA totaling 4,000 to 5,000 nucleotides. Cytoplasmic multiplication.
Genera:
Aureusvirus—one RNA. Soil transmitted; no vector (e.g., Pothos latent virus).
Avenavirus—one RNA. Transmitted mechanically from plant to plant and possibly also by fungi
(e.g., oat chlorotic stunt virus).
Carmovirus—one RNA. Soilborne transmission without vector (e.g., carnation mottle virus).
Dianthovirus—two RNAs (4,000 and 1,500 nucleotides) in one particle. Soilborne transmission
without vector (e.g., carnation ringspot virus).
Machlomovirus—one RNA. Infects only grasses. Mechanical or beetle transmission (e.g., maize
chlorotic mottle virus).
Necrovirus—one RNA. Fungal vector (e.g., tobacco necrosis virus A).
Panicovirus—one RNA. Transmitted mechanically (e.g., Panicum mosaic virus).
Tombusvirus—one RNA. Transmitted mechanically or by grafting (e.g., tomato bushy stunt virus).
Isometric virions and virions that are short rods
d. Family: Bromoviridae
Nonenveloped particles. Linear positive-sense ssRNA. Tripartite genome of 2,000 to 3,000
nucleotides (total 8,000 nucleotides). RNA 4 is coat protein mRNA encoded by RNA 3.
Cytoplasmic multiplication.
Genera:
Alfamovirus—four RNAs in four bacilliform particles (short rods) 56, 43, 35, and 30 nm X 18 nm.
Particles each with one molecule of RNA 1 (3,644 nucleotides) or RNA 2 (2,593 nucleotides) or
RNA 3 (2,037 nucleotides) or two molecules of RNA 4 (881 nucleotides). Infectivity requires
RNAs 1 to 3 + coat protein or RNA 4. Cytoplasmic multiplication. Nonpersistent in aphid vector
(e.g., alfalfa mosaic virus).
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Bromovirus—four RNAs in three isometric 27 nm particles. Particles each with one molecule of
RNA 1 (3,234 nucleotides) or RNA 2 (2,865 nucleotides) or RNA 3 (2,117 nucleotides) + RNA 4
(800 nucleotides). Infectivity requires RNAs 1 to 3 + coat protein or RNA 4. Beetle vectors (e.g.,
brome mosaic virus).
Virions that are rigid rods
These genera have not yet been assigned to a family:
e. Tobamovirus—Nonenveloped rod, 300 nm X 18 nm, with helical symmetry. Contains one
molecule of linear positive-sense ssRNA of 6,395 nucleotides. Transmitted mechanically or by
seed (e.g., tobacco mosaic virus).
f. Tobravirus—two nonenveloped rods, 200 nm X 22 nm and 46 to 115 nm X 22 nm, with helical
symmetry. Contains, respectively, one molecule of linear positive-sense ssRNA of 7,000 or 2,000
to 4,000 nucleotides. The larger RNA alone is infectious and the smaller one specifies the coat
protein. Both are needed for synthesis of new virions. Cytoplasmic multiplication. Nematode
vector (e.g., tobacco rattle virus).
Virions that are flexuous rods (i.e., with bends but not necessarily flexible)
g. Family: Potyviridae
Nonenveloped flexuous rod(s) with helical symmetry. There are one or two molecules of linear
positive-sense ss-RNA. Cytoplasmic multiplication. Genera have different vectors.
Genera:
Potyvirus—particle of 700 to 900 nm X 12 nm with one RNA molecule of 10,000 nucleotides. A
VPg at the 5’ end. Nonpersistent in aphid vector (e.g., potato virus Y).
Ipomovirus—one particle of 800 to 950 nm X 12 to 16 nm with one RNA molecule of 11,000
nucleotides. Whitefly vector (e.g., sweet potato mild mottle virus).
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Macluravirus—nonenveloped flexuous rod, 700 nm X 13 to 16 nm, with one molecule of RNA of
8,000 nucleotides. Nonpersistent in aphid vector (e.g., maclura mosaic virus).
Rymovirus—one particle of 700 nm X 11 to 15 nm with one molecule of RNA of 8,500 to 10,000
nucleotides. Mite vector (e.g., ryegrass mosaic virus).
Tritimovirus—one particle of 700 nm _ 13 nm with one molecule of RNA of 9,672 nucleotides.
Persistent in mite vector (e.g., wheat streak mosaic virus).
Bymovirus—two particles of 500 to 600 and 250 to 300 nm X 13 nm. The larger with an RNA of
7,632 nucleotides and the smaller with an RNA of 3,585 nucleotides. Fungal vector (e.g., barley
yellow mosaic virus).
4. Viruses with negative-sense/ambisense ssRNA genomes and a virion-associated RNAdependent RNA polymerase
a. Family: Bunyaviridae
Enveloped isometric 100 nm particle with 10 nm spikes. Each contains three helical
nucleocapsids with linear negative-sense ssRNA. The largest nucleocapsid comprises an RNA of
6,000 to 12,000 nucleotides, the medium of 3,500 to 6,000 nucleotides, and the smallest of 1,000
to 2,000 nucleotides. Cytoplasmic multiplication; buds from the Golgi membranes. Spread by
arthropods unless stated. Also genera that infect animals.
Genus:
Tospovirus—M and S RNAs are ambisense. Transmitted by insect (thrips) vector. Multiplies in
vector (e.g., tomato spotted wilt virus).
b. Family: Rhabdoviridae
Enveloped, bullet-shaped particles 100 to 430 nm long and 45 to 100 nm in diameter, containing
a helical nucleocapsid with one molecule of linear negative-sense ssRNA of 11,000 to 15,000
nucleotides. Has 5 to 10 nm spikes. Also genera that infect animals.
Genera:
Cytorhabdovirus—plant viruses that mature in the cytoplasm. Spread by (and multiply in) insect
vector (e.g., lettuce necrotic yellows virus).
Nucleorhabdovirus—plant viruses that mature in the nucleus.
Spread by (and multiply in) insect vector (e.g., potato yellow dwarf virus).
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5. Viruses with DNA genomes that replicate through an RNA intermediate
a. Family: Caulimoviridae
A nonenveloped particle containing one partially double-stranded circular DNA molecule that is
not covalently closed (like that of hepadnaviruses), with a complete negative-sense strand of
7,000 to 8,000 nucleotides and an incomplete positive-sense strand with one to three
discontinuities. The circularization, overlaps the 3’ and 5’ termini of the negative-sense DNA.
These are “reversiviruses” that have a reverse transcriptase. Replication is nuclear and the
genome remains episomal and does not integrate.
Genera:
Caulimovirus—icosahedral 50 nm particles. Nonpersistent in aphid vectors (e.g., cauliflower
mosaic virus).
Badnavirus—rod-shaped particles 130 nm X 30 nm. Nonpersistent in beetle larvae (e.g.,
commelina yellow mottle virus).
C. VIRUSES MULTIPLYING IN ALGAE, FUNGI, AND PROTOZOA
1. Viruses with dsDNA genomes
a. Family: Adenoviridae
Nonenveloped icosahedral particles of 60 to 90 nm with a fiber protein from each vertex. Contains
one molecule of linear ds-DNA of 25,000 to 42,000 bp. Replicate and assembled in the nucleus.
Other genera infect animals.
(Putative) genus:
Rhizidiovirus—60 nm virion with one molecule of linear dsDNA of 25,000 bp. Infects fungi (e.g.,
Rhizidiomyces virus).
b. Family: Phycodnaviridae
Infects algae. Nonenveloped, isometric, 130 to 190 nm, multilayered particles containing one
molecule of linear dsDNA of 160,000 to 380,000 bp. Contain internal lipid. Infects Paramecium,
Chlorella, and Hydra spp. May have a use in controlling algal blooms.
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2. Viruses with dsRNA genomes and a virion-associated RNA-dependent RNA polymerase
a. Family: Hypoviridae
Genus:
Hypovirus—vesicles of 50 to 80 nm with one molecule of linear dsRNA of 9,000 to 13,000 bp, no
structural proteins and polymerase activity. Surrounded by rough endoplasmic reticulum in
infected cells. Causes hypovirulence of host chestnut blight fungus (e.g., Cryphonectria hypovirus
1).
b. Family: Partitiviridae
Two molecules of linear dsRNA of 1,400 to 3,000 bp packaged in one or several nonenveloped
isometric particles that lack structural detail. Have a transcriptase. Cytoplasmic multiplication.
Also genera that infect plants.
Genera:
Partitivirus—particles 30 to 35 nm. Infects fungi, (e.g., Gaeumannomyces graminis virus 019/6A).
Chrysovirus—particles 35 to 40 nm. Infects fungi (e.g., Penicillium chrysogenum virus).
c. Family: Totiviridae
Nonenveloped particles of 30 to 40 nm with one molecule of linear dsRNA of 5,000 to 7,000 bp.
One major capsid protein. Single shell. Cytoplasmic multiplication.
Genera:
Totivirus—some viruses also have other satellite RNA segments that encode killer proteins.
Infects the fungus Saccharomyces cerevisiae.
Giardiavirus—infects the protozoan Giardia lamblia.
Leishmaniavirus—one RNA. Infects protozoa: Leishmania spp.
3. Viruses with positive-sense ssRNA genomes
a. Family: Barnaviridae
Nonenveloped icosahedral short rod, 50 nm X 19 nm, with one molecule of linear ssRNA of 4,009
nucleotides. Infects fungi (e.g., mushroom bacilliform virus).
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b. Family: Narnaviridae
One molecule of linear positive-sense ssRNA of 2,500 nucleotides as a ribonucleoprotein with
polymerase activity. Infects fungi.
Genera:
Narnavirus (e.g., Saccharomyces cerevisiae 20S narnavirus)
Mitovirus (e.g., Cryphonectria parasitica mitovirus 1 NB631)
D. VIRUSES (PHAGES) MULTIPLYING IN ARCHAEA, BACTERIA, MYCOPLASMA, AND
SPIROPLASMA
The well-known molecular biology of phages is based on a detailed study of a few
representatives, and surprisingly little is known of their comparative biology.
1. Viruses with dsDNA genomes
Viruses that have some head-tail structure, here ordered by decreasing tail length
a. Family: Siphoviridae
Nonenveloped particle with a long noncontractile tail up to 570 nm X 8 nm. Icosahedral head of
60 nm. Contains one molecule of linear dsDNA of 48,500 bp. Causes no host DNA breakdown.
Infects bacteria including enterobacteria, mycobacteria, Lactococcus, Methanobacterium, and
Vibrio. Includes coliphage T1, lambda (λ), chi (), and phi () 80.
b. Family: Myoviridae
Nonenveloped particle with a complex contractile tail 80 to 455 nm X 16 nm. Contraction requires
ATP. Head separated from tail by a neck. Head isometric (as shown) or elongated, 110 nm X 80
nm. Has one molecule of linear dsDNA of 40,000 to 170,000 bp. Infects enterobacteria, Bacillus,
and Halobacterium spp. (Archaea). Includes the T-even coliphages T2, T4, and T6, and PBS1,
SP8, SP50, P1, P2, 21, 34, and Mu.
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c. Family: Podoviridae
Nonenveloped particle with a short noncontractile tail of 20 nm X 8 nm. Icosahedral head of 60
nm. One molecule of linear dsDNA of 19,000 to 44,000 bp. Causes host DNA breakdown.
Includes the coliphages T3 and T7, enterobacteria phage P22, and bacillus phage phi () 29.
Viruses that do not have a head-tail structure
d. Family: Fuselloviridae
Enveloped lemon-shaped particle 100 nm X 60 nm with short tail fibers at one pole. One
molecule of circular dsDNA of 15,465 bp. Infects Archaea such as Desulfolobus and
Methanococcus (e.g. Sulfolobus virus 1).
e. Family: Tectiviridae
Nonenveloped icosahedral 63 nm particle. Unusual enveloped nucleoprotein surrounded
by lipid. One molecule of linear dsDNA of 150,000 bp. On attachment forms a tail-like tube 60 nm
X 10 nm. Infects gram-negative bacteria (e.g., enterobacteria phage PRD1). Has similarities with
adenoviruses.
f. Family: Corticoviridae
Nonenveloped, nontailed icosahedral 60 nm particle formed of several layers, including one of
lipid. Spikes at vertices. One molecule of circular dsDNA of 9,000 bp. Infects Pseudomonas (e.g.,
Alteromonas phage PM2).
g. Family: Plasmaviridae
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Enveloped pleomorphic 80 nm particle with small dense core; 50 and 125 nm particles also
produced. One molecule of circular dsDNA of 12,000 bp. Infects Mycoplasma (e.g.,
Acholeplasma phage L2).
h. Family: Rudiviridae
Nonenveloped rigid rod of 500 to 780 nm X 23 nm with helical symmetry, with a plug and three
tail fibers at each end. One molecule of linear dsDNA of 33,000 bp. Nonlytic. Infects thermophilic
Archaea, Sulfolobus (e.g., Sulfolobus virus SIRV-1).
i. Family: Lipothrixviridae
Enveloped, rigid rod of 400 nm X 40 nm with protrusions at both ends that participate in cell
attachment. One molecule of linear dsDNA of 16,000 bp. Virions stable at 100°C. Infects Archaea
(e.g., Thermoproteus virus 1).
2. Viruses with ssDNA genomes
a. Family: Inoviridae
Nonenveloped rod with one molecule of circular positive-sense ssDNA of 5,000 to 10,000
nucleotides. Virion length depends on the length of the DNA and its conformation.
Genera:
Inovirus—flexuous rods of 700 to 2,000 nm X 7 nm with DNA of 4,400 to 8,500 nucleotides. Host
bacteria not lysed. Includes enterobacteria phage M13 and enterobacteria phage fd.
Plectrovirus—nearly straight rods of 85 or 280 nm X 10 to 15 nm with DNA of 4,400 to 8,500
nucleotides. Infects Mycoplasma (85 nm) and Spiroplasma (280 nm) (e.g. Acholeplasma phage
MV-L51).
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b. Family: Microviridae
Icosahedral particle with large knobs on the 12 vertices. Minimum and maximum diameters are
22 and 23 nm, respectively. One molecule of circular ssDNA of 4,400 to 5,400 nucleotides.
Genera:
Microvirus—infects bacteria (e.g., enterobacteria phage phi () X 174)
Spiromicrovirus—infects Spiroplasma (e.g., Spiroplasma phage 4)
Chlamydiamicrovirus (e.g., Chlamydia phage 1)
Bdellomicrovirus (e.g., Bdellovibrio phage MAC 1)
3. Viruses with dsRNA genomes and a virion-associated RNA-dependent RNA polymerase
a. Family: Cystoviridae
Enveloped icosahedral 85 nm particle with 8 nm spikes. Has a 58 nm nucleocapsid and a 43 nm
core. Each particle contains three molecules of linear dsRNA of 6,374, 4,057, and 2,948 bp.
Infects Pseudomonas (e.g., Pseudomonas phage phi [] 6).
4. Viruses with positive-sense ssRNA genomes
a. Family: Leviviridae
Nonenveloped 26 nm icosahedral particles with one molecule of linear positive-sense ssRNA of
3,400 to 4,300 nucleotides. Include enterobacteria phages R17, MS2, and Q.
E. SATELLITE VIRUSES AND SATELLITE NUCLEIC ACIDS OF ANIMALS, PLANTS, FUNGI,
AND BACTERIA
The replication of satellite viruses and satellite nucleic acids depends upon coinfection of a host
cell with a helper virus. The satellite genome has no significant homology with that of the helper
and hence differs from other types of dependent nucleic acid molecules. Here, we use a broad
definition of a satellite virus—one that is incapable of independent production of progeny virus
particles, yet may be able to replicate itself. Satellite viruses encode their own coat protein,
whereas satellite nucleic acids do not and use the coat protein of their helper virus. Some
satellite viruses/satellite nucleic acids modulate the replication of the helper virus and exacerbate
or diminish disease.
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1. Satellite nucleic acids with dsDNA genomes
Nonenveloped isometric virions with a Myoviridae helper (enterobacteria phage P2). One
molecule of linear dsDNA of 11,627 bp. Interesting that the head-tail helper virion protein can
form an isometric virion (e.g., enterobacteria P4 satellite).
2. Satellite viruses with ssDNA genomes
a. Family: Parvoviridae
Nonenveloped icosahedral particle of 18 to 26 nm that contains no enzyme. One 5,000 nucleotide
linear molecule of either negative- or positive-sense ssDNA per particle. On extraction these form
an artifactual double strand. Nuclear replication. There are also independently replicating
(autonomous) parvoviruses of animals. Subfamily: Parvovirinae
Genus:
Dependovirus—dependent on helper adenovirus or herpesvirus for efficient replication, but in
some cell cultures replicate independently. Particles package 90% negative-sense DNA. Encode
their own coat protein. Adeno-associated viruses. Infects vertebrates.
3. Satellite nucleic acids with ssDNA genomes
A genome of 682 nucleotides with no ORF. Geminiviruses helpers (e.g., tomato leaf curl virus
satellite DNA). Infects plants. Particle morphology not yet determined.
4. Satellite nucleic acids with dsRNA genomes
Nonenveloped isometric particles with a Totiviridae helper virus. Linear dsRNA of 500 to 1,800 bp
inside a virion made of helper virus coat protein (e.g., satellite of Saccharomyces cerevisiae M
virus). Infects fungi.
5. Satellite viruses with positive-sense ssRNA genomes
Isometric particles with one linear ssRNA.
Tobacco necrosis virus satellite virus subgroup:
Particles of 17 nm containing one molecule of RNA of 1,239 nucleotides. Fungal vector.
Infects plants.
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Chronic bee-paralysis virus-associated satellite virus subgroup:
Particles of 12 nm containing three molecules of RNA totaling 1,000 nucleotides. Infects
animals.
6. Satellite nucleic acids with positive-sense ssRNA genomes
Large linear mRNA satellites:
Encode a non-structural protein. Rarely modify disease. Linear RNA of 800 to 1,500
nucleotides (e.g., arabis mosaic virus large satellite RNA). Infects plants.
7. Satellite nucleic acids with negative-sense ssRNA genomes
Deltavirus—hepatitis delta virus. Enveloped isometric particle of 36 nm with one circular RNA
molecule of 1,700 nucleotides. Dependent on hepatitis B virus, which contributes the envelope,
but expresses two delta antigen proteins that form the core. RNA has selfcleaving (ribozyme)
activity. May exacerbate hepatitis B virus infection. Infects humans.
8. Satellite nucleic acids with ssRNA genomes (unclassified as make no mRNA)
Have no functional ORF. Small linear RNA satellites: Linear RNA of 340 nucleotides (e.g.,
cucumber mosaic virus satellite RNA). Infects plants.
Circular RNA satellites: Covalently closed circular RNA of usually 350 nucleotides. Smallest is
220 nucleotides. All form a double-stranded rodlike viroid RNA. This is encapsidated. Also called
virusoids. Have self-cleaving (ribozyme) activity. Five types, all with sobemovirus helpers (e.g.,
velvet tobacco mottle virus satellite RNA). Infects plants.
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