Key to the Placement of Viruses in Taxa
Adapted from the version on ICTV Home Page by
Ed Rybicki
1. Genome DNA 2
Genome RNA
2. Virion DNA is continuous; reverse transcriptase
not used during replication 3
Virion DNA contains discontinuities; reverse transcriptase
used during replication 46
3. DNA double-stranded 4
DNA single-stranded 35
The ds DNA Viruses
4. Host a prokaryote 5
Host a eukaryote 12
5. Virion tailed 6
Virion not tailed 8
6. Tail contractile > 15 nm in diameter :Myoviridae / "
T4-like phages"
Tail not contractile < 12 nm in diameter 7
7. Tail long (65 - 600 nm) : Siphoviridae /
"lambda-like
phages"
Tail short (10 - 20 nm) : Podoviridae /
"T7-like phages"
8. Virion not enveloped 9
Virion enveloped 10
9. DNA linear > 10 kbp;
inner capsid can form a tail-like
appendage : Tectiviridae / Tectivirus
DNA circular < 10 kbp;
no tail-like appendage: Corticoviridae / Corticovirus
10. Host a mycoplasma : Plasmaviridae / Plasmavirus
Host an archaebacterium 11
11. Virion rod-shaped : Lipothrixviridae / Lipothrixvirus
Virion lemon-shaped : Fuselloviridae / Fusellovirus
12. Virion contains one or more fusiform or cylindrical
nucleocapsids and multiple DNA molecules : (Polydnaviridae) 13
Virion contains a single DNA molecule 14
13. Nucleocapsid 85 x 330 nm with 2 envelopes : Polydnaviridae /
Ichnovirus
Nucleocapsid cylindrical, 40 nm diameter x 30-150 nm,
with 1 envelope : Polydnaviridae / Bracovirus
14. DNA > 90 kbp 15
DNA < 90 kbp 31
15. DNA > 300 kbp; virion not enveloped;
host an alga : Phycodnaviridae / Phycodnavirus
DNA usually < 300 kbp; virion enveloped; host an animal 16
16. Genome covalently closed circular DNA;
nucleocapsid rod-shaped (Baculoviridae) 17
Genome linear DNA; nucleocapsid not rod-shaped 18
17. Inclusions typically contain
numerous virions : Baculoviridae / Nucleopolyhedrovirus
Inclusions typically contain
a single virion : Baculoviridae / Granulovirus
18. Virion ovoid or brick-shaped : (Poxviridae) 19
Virion not ovoid or brick-shaped 25
19. Host a vertebrate : (Poxviridae / Chordopoxvirinae) 20
Host an invertebrate : (Poxviridae / Entomopoxvirinae) 24
20. Virion ovoid : Poxviridae / Chordopoxvirinae / Parapoxvirus
Virion brick-shaped
21. Largest virion dimension > 320 nm;
DNA > 250 kbp; host a bird : Poxviridae /
Chordopoxvirinae / Avipoxvirus
Largest virion dimension < 290 nm;
DNA < 250 kbp : Poxviridae /
Chordopoxvirinae /Orthopoxvirus
Largest virion dimension > 290 nm; DNA < 250 kbp 22
22. DNA 175 kbp; largest virion dimension 300 nm : Poxviridae /
Chordopoxvirinae / Suipoxvirus
DNA 188 kbp; largest virion dimension 320 nm : Poxviridae /
Chordopoxvirinae / Molluscipoxvirus
DNA < 170 kbp 23
23. Virion 300 x 270 x 200 nm; DNA about 145 kbp : Poxviridae /
Chordopoxvirinae / Capripoxvirus
Virion 300 x 250 x 200 nm; DNA 160 kbp;
GC content about 40% : Poxviridae /
Chordopoxvirinae / Leporipoxvirus
Virion 300 x 250 x 200 nm; DNA 146 kbp,
GC content about 33% : Poxviridae /
Chordopoxvirinae / Yatapoxvirus
24. Virion ovoid, 450 x 250 nm; host from Coleoptera : Poxviridae /
Entomopoxvirinae / Entomopoxvirus A
Virion ovoid, 350 x 250 nm; DNA about 225 kbp;
host from Lepidoptera or Orthoptera : Poxviridae /
Entomopoxvirinae / Entomopoxvirus B
Virion brick-shaped, 320 x 230 x 110 nm;
DNA > 240 kbp; host from Diptera : Poxviridae /
Entomopoxvirinae / Entomopoxvirus C
25. Virion icosahedral with 70 - 100 nm diameter cores;
virus multiplies in ticks and swine : "African
swine fever-like viruses"
Virion icosahedral; genome circularly permutated and
terminally redundant;
multiplies only in poikilothermic animals : (Iridoviridae) 26
Virion quasi-spherical with 100 - 110 nm diameter cores;
genome not circularly permutated; multiplies only
in vertebrates (Herpesviridae) 30
26. Host an invertebrate 27
Host a vertebrate 28
27. Virion 120 nm in diameter : Iridoviridae / Iridovirus
Virion 180 nm in diameter : Iridoviridae / Chloriridovirus
28. Host an amphibian : Iridoviridae / Ranavirus
Host a fish 29
29. Virion > 200 nm in diameter : Iridoviridae / Lymphocystivirus
Virion < 200 nm in diameter : Iridoviridae /
"Goldfish virus 1-like viruses"
30. Reproductive cycle short, spread in culture rapid;
infection often induces epithelial
lesions; gene complement characteristic
of human herpesvirus 1 : Herpesviridae / Alphaherpesvirinae
/ Simplexvirus / Varicellovirus
Reproductive cycle long, spread in culture slow;
gene complement characteristic of
human herpesvirus 5 : Herpesviridae / Betaherpesvirinae /
Cytomegalovirus / Muromegalovirus / Roseolovirus
Infection often latent in lymphocytes and may cause
lymphoproliferative disease; gene complement
characteristic of human herpesvirus 4 : Herpesviridae /
Gammaherpesvirinae / Lymphocryptovirus /
Rhadinovirus
31. DNA < 30 kbp : (Papovaviridae) 32
DNA > 30 kbp 33
32. Virion 45 nm in diameter; DNA about 5 kbp
with proteins encoded on both strands : Papovaviridae /
Polyomavirus
Virion about 55 nm in diameter; DNA about 8 kbp
with proteins encoded on one strand : Papovaviridae /
Papillomavirus
33. Host a fungus : Rhizidiovirus
Host a vertebrate : (Adenoviridae) 34
34. Host a mammal : Adenoviridae / Mastadenovirus
Host a bird : Adenoviridae / Aviadenovirus
The ssDNA Viruses
35. Host a prokaryote 36
Host a eukaryote 39
36. Virion has helical symmetry : (Inoviridae) 37
Virion icosahedral : (Microviridae) 38
37. Virion filamentous, 700 - 2000 nm in length : Inoviridae /
Inovirus
Virion short, rod-shaped, 70 - 280 nm in length : Inoviridae /
Plectrovirus
38. Host an enterobacterium : Microviridae / Microvirus
Host Spiroplasma sp. : Microviridae / Spiromicrovirus
Host Bdellovibrio bacteriovorus : Microviridae /
Bdellomicrovirus
Host Chlamydia psittaci : Microviridae / Chlamydiamicrovirus
39. Host a plant 40
Host not a plant 41
40a. Particles single
Particles doubled or geminate
40b. Genome monopartite
Genome multipartite
40c. sscDNA +/- 1.3 kb : Circoviridae /
eg: coconut foliar decay virus
40d. sscDNA, >5 +/- 1 kb segments : Plant
"circovirus-like viruses", eg: banana bunchy top virus
40e. Genome monopartite; host graminaceous;
vector a leafhopper : Geminiviridae /
"Subgroup I Geminivirus"
Genome monopartite; host dicotyledonous;
vector a leafhopper : Geminiviridae /
"Subgroup II Geminivirus"
Genome mono or bipartite;
vector a whitefly : Geminiviridae /
"Subgroup III Geminivirus"
41. DNA circular : Circoviridae / Circovirus
DNA linear : (Parvoviridae) 42
42. Host a vertebrate : (Parvoviridae / Parvovirinae) 43
Host an invertebrate : (Parvoviridae / Densovirinae) 45
43. A helper virus (adenovirus or herpesvirus)
needed for productive multiplication : Parvoviridae /
Parvovirinae / Dependovirus
Virus multiplies autonomously 44
44. DNA contains 2 mRNA promoters : Parvoviridae /
Parvovirinae / Parvovirus
DNA contains 1 mRNA promoter : Parvoviridae /
Parvovirinae / Erythrovirus
45. DNA 6 kb, structural and non-structural proteins
encoded on different strands : Parvoviridae /
Densovirinae / Densovirus
DNA 5 kb; proteins all encoded on one strand; virion
contains similar amounts of each sense DNA : Parvoviridae /
Densovirinae / Iteravirus
DNA 4 kb; proteins all encoded on one strand;
virion contains mainly negative sense DNA : Parvoviridae /
Densovirinae / Contravirus
The DNA and RNA Reverse Transcribing Viruses
46. DNA < 5 kbp; host a vertebrate : (Hepadnaviridae) 47
DNA > 7 kbp; host a plant 48
47. Virion < 45 nm in diameter; nucleocapsid about
27 nm in diameter; host a mammal : Hepadnaviridae /
Orthohepadnavirus
Virion > 45 nm in diameter; nucleocapsid
about 35 nm in diameter; host a bird : Hepadnaviridae /
Avihepadnavirus
48. Virion bacilliform : Badnavirus
Virion icosahedral : Caulimovirus
49. Genome encodes reverse transcriptase;
DNA copies integrate in host genome : (Retroviridae) 50
Genome does not encode reverse transcriptase;
virus genome does not integrate 56
50. RNA > 8.5 kb 51
RNA < 8.5 kb 53
51. RNA < 10 kb; nucleocapsid bar-shaped or cone-shaped :
Retroviridae / Lentivirus
RNA > = 10 kb; nucleocapsid not bar- or cone-shaped 52
52. RNA 10 kb; nucleocapsid spherical and centrally located;
gag, pro and pol encoded in different reading frames :
Retroviridae / "Mammalian type B retroviruses"
RNA 11 kb; nucleocapsid eccentric; gag, pro and pol
encoded in the same reading frame : Retroviridae /
Spumavirus
53. RNA < 8 kb; LTR about 350 nt in length 54
RNA 8.3 kb; LTR about 600 nt in length 55
54. RNA 7.2 kb; gag and pol encoded in the same reading frame;
host a bird : Retroviridae /
"Avian type C retroviruses"
RNA 8 kb; gag and pro encoded in different reading frames;
host a mammal : Retroviridae /
"Type D retroviruses"
55. gag, pro and pol encoded in the same reading frame;
R sequence in the LTR about 60 nt : Retroviridae /
"Mammalian type C retroviruses"
pro encoded in a reading frame different from that
encoding gag and pol; R sequence in the LTR >130 nt :
Retroviridae / "HTLV-BLV retroviruses"
56. RNA double-stranded 57
RNA single-stranded 77
The dsRNA Viruses
57. Host a prokaryote : Cystoviridae / Cystovirus
Host a eukaryote 58
58. Genome in > 9 segments : (Reoviridae) 59
Genome in < 9 segments 67
59. Host an animal 60
Host a plant 65
60. Genome in 10 segments 61
Genome in > 10 segments 63
61. Virion lacks an outer capsid and is <
70 nm in diameter : Reoviridae / Cypovirus
Virion comprises cores and outer capsid and is
> 70 nm in diameter 62
62. Outer capsid distinct; virion sediments at
> 600 S : Reoviridae / Orthoreovirus
Outer capsid indistinct; virion sediments
at < 600 S : Reoviridae / Orbivirus
63. Genome in 12 segments : Reoviridae / Coltivirus
Genome in 11 segments 64
64. Virion appears wheel-like; 9 RNA segments are > 2 kbp;
host a mammal or a bird : Reoviridae / Rotavirus
Virion not wheel-like; 6 RNA segments are
> 2 kbp; host a fish or a shellfish : Reoviridae /
Aquareovirus
65. Genome in 12 segments;
virion lacks spikes : Reoviridae / Phytoreovirus
Genome in 10 segments;
virion bears spikes 66
66. Virion 65 - 70 nm in diameter,
with an outer capsid : Reoviridae / Fijivirus
Virion 57 - 65 nm in diameter,
lacks an outer capsid : Reoviridae / Oryzavirus
67. Host an animal : (Birnaviridae) 68
Host not an animal 70
68. Host an invertebrate : Birnaviridae / Entomobirnavirus
Host a vertebrate 69
69. Host an aquatic animal,
usually a fish : Birnaviridae / Aquabirnavirus
Host a bird : Birnaviridae / Avibirnavirus
70. No virions are formed in diseased tissue : Hypoviridae /
Hypovirus
RNA is encapsidated 71
71. Genome monopartite : (Totiviridae) 72
Genome multipartite : (Partitiviridae) 74
72. Virion 40 - 43 nm in diameter; host a
fungus / insect : Totiviridae / Totivirus
Virion < 40 nm in diameter; host a protozoa 73
73. RNA > 6 kbp; host Giardia sp. : Totiviridae /
Giardiavirus
RNA < 6 kbp; host Leishmania sp. : Totiviridae /
Leishmaniavirus
74. Host a fungus 75
Host a plant 76
75. Virions 30 - 35 nm in diameter;
genome bipartite : Partitiviridae / Partitivirus
Virions 35 - 40 nm in diameter;
genome tri- or quadripartite : Partitiviridae /
Chrysovirus
76. Virion 30 nm in diameter : Partitiviridae / Alphacryptovirus
Virion 38 nm in diameter : Partitiviridae / Betacryptovirus
77. RNA negative sense or ambisense 78
RNA positive sense 96
The Negative Sense ssRNA Viruses
78. RNA circular; productive multiplication is
helper virus-dependent : Deltavirus
RNA linear 7
79. Genome monopartite : (order Mononegavirales) 80
Genome multipartite 87
80. Virion filamentous and/or pleomorphic;
RNA 18-19 kb : Filoviridae / Filovirus
Virion pleomorphic, usually spherical;
RNA 15-16 kb : (Paramyxoviridae) 81
Virion bullet-shaped or bacilliform, not pleomorphic;
RNA 11 to 15 kb : (Rhabdoviridae) 84
81. RNA contains 10 transcriptional elements : Paramyxoviridae /
Pneumovirinae / Pneumovirus
RNA contains < 10 transcriptional elements :
(Paramyxoviridae / Paramyxovirinae) 82
82. Virion lacks a neuraminidase : Paramyxoviridae /
Paramyxovirinae / Morbillivirus
Virion contains a neuraminidase 83
83. RNA encodes a C protein : Paramyxoviridae /
Paramyxovirinae / Paramyxovirus
RNA does not encode a C protein : Paramyxoviridae /
Paramyxovirinae / Rubulavirus
84. Host an animal 85
Host a plant 86
85. RNA about 11 kb; virion assembles by budding
from the plasma membrane : Rhabdoviridae / Vesiculovirus
RNA about 12 kb; virion assembles by budding
from intracytoplasmic membranes : Rhabdoviridae / Lyssavirus
RNA > 13 kb : Rhabdoviridae / Ephemerovirus
86. Virions accumulate in the cytoplasm : Rhabdoviridae /
Cytorhabdovirus
Virions accumulate in the
perinuclear space : Rhabdoviridae / Nucleorhabdovirus
87. Genome in > 5 segments : (Orthomyxoviridae) 88
Genome in < 5 segments 90
88. Genome in 8 segments : Orthomyxoviridae /
Influenzavirus A, B
Genome in 7 segments 89
89. Nucleoprotein Mr 64 x 103; infects
only vertebrates : Orthomyxoviridae /
Influenzavirus C
Nucleoprotein Mr 54 x 103; infects
ticks and vertebrates : Orthomyxoviridae /
"Thogoto-like viruses"
90. Virion about 8 nm filaments, host
a plant : Tenuivirus
Virion not filamentous 91
91. Genome bipartite; virion contains
host ribosomes : Arenaviridae / Arenavirus
Genome tripartite; virion does not
contain host ribosomes : (Bunyaviridae) 92
92. All RNA segments negative sense 93
S RNA ambisense 95
93. S RNA < 1 kb; S RNA encodes
NSS protein + N protein : Bunyaviridae / Bunyavirus
S RNA > 1 kb; S RNA encodes
only N protein 94
94. L RNA > 10 kb; G2 protein Mr
< 50 x 103 : Bunyaviridae / Nairovirus
L RNA < 10 kb; G2 protein Mr
> 50 x 103 : Bunyaviridae / Hantavirus
95. Host an animal : Bunyaviridae / Phlebovirus
Host a plant : Bunyaviridae / Tospovirus
The Positive Sense ssRNA Viruses
96. Host a prokaryote : (Leviviridae) 97
Host a eukaryote 98
97. RNA < 4 kb; genome encodes a protein
for cell lysis : Leviviridae / Levivirus
RNA > 4 kb; genome does not encode
a cell lysis protein : Leviviridae / Allolevivirus
98. No specific virions identified; RNA can be
encapsidated in heterologous coat protein;
host a plant : Umbravirus
Virus-specific capsids formed in infected cells 99
99. Virion not enveloped 100
Virion enveloped 127
100. Coat protein(s) are expressed by proteolysis
of a large (Mr > 100 x 103) polyprotein 101
Coat protein(s) expressed by translation of a
small genome segment or a sub-genomic RNA 112
101. Host an animal; structural proteins formed from
the sequence at or within about 300 residues of
the N-terminus of the polyprotein : (Picornaviridae) 102
Host a plant; structural proteins preceded upstream
in the polyprotein by > 400 residues of
non-structural protein 106
102. Polyprotein contains a 'leader' protein 103
Polyprotein does not contain a 'leader' protein 104
103. Virion buoyant density in CsCl < 1.35 g/cm3;
'leader' protein is not a protease : Picornaviridae / Cardiovirus
Virion buoyant density in CsCl > 1.35 g/cm3;
'leader' protein is a protease : Picornaviridae / Aphthovirus
104. Virion not stable at acid pH; virion buoyant density
in CsCl > 1.35 g/cm3 : Picornaviridae / Rhinovirus
Virion stable at acid pH; virion buoyant density
in CsCl < 1.35 g/cm3 105
105. Protein 1A (VP4) small (< 2 kDa) or absent : Picornaviridae /
Hepatovirus
Protein 1A > 3 kDa : Picornaviridae / Enterovirus
106. Virion filamentous : (Potyviridae) 107
Virion isometric 108
107. Genome monopartite; vector an aphid : Potyviridae / Potyvirus
Genome monopartite; vector a mite : Potyviridae / Rymovirus
Genome monopartite; vector a mite : Potyviridae / Whestrevirus
Genome monopartite; vector a whitefly : Potyviridae / Ipomovirus
Genome genome bipartite; vector a fungus : Potyviridae / Bymovirus
108. Genome monopartite : (Sequiviridae) 109
Genome bipartite : (Comoviridae) 110
109. Virus transmitted by aphids : Sequiviridae / Sequivirus
Virus phloem-limited, not mechanically transmissible :
Sequiviridae / Waikavirus
110. Larger RNA species > 7 kb; virion usually contains
1 coat protein with Mr of about 57 x 103;
virus usually transmitted by nematodes : Comoviridae / Nepovirus
Larger RNA species < 7 kb; virion contains
2 coat proteins 111
111. Vector a beetle : Comoviridae / Comovirus
Vector an aphid : Comoviridae / Fabavirus
112. Host a vertebrate 113
Host an invertebrate 114
Host a plant or a fungus 116
113. Virion 30 nm or more in diameter and with
cup-shaped depressions; virion contains
one structural protein : Caliciviridae / Calicivirus
Virion 30 nm or less in diameter, often appearing star-shaped;
virion contains 2 or 3 structural proteins : Astroviridae /
Astrovirus
114. Structural protein Mr < 40 x 103 : Nodaviridae /
Nodavirus
Structural protein Mr > 60 x 103 : (Tetraviridae) 115
115. Genome monopartite : Tetraviridae / "Nudaurelia
capensis b-like viruses"
Genome bipartite : Tetraviridae / "Nudaurelia
capensis w-like viruses"
116. Virus circulates in the bodies of the vectors 117
No vector known or transmission non-circulative 119
117. Vector a leafhopper : Marafivirus
Vector an aphid 118
118. Virion contains 1 RNA; virus not transmissible
mechanically : Luteovirus
Virion contains 2 RNA; virus readily transmissible
mechanically : Enamovirus
119. Virion isometric or bacilliform 120
Virion has helical symmetry 138
120. Host a fungus; virion bacilliform : Barnaviridae / Barnavirus
Host not a fungus 121
121. RNA about 6 kb; coat protein Mr about 20 x 103;
vector a beetle : Tymovirus
RNA < 5.5 kb; coat protein Mr > 20 x 103 122
122. Genome bipartite; virion contains
both genome segments : Dianthovirus
Genome monopartite 123
Genome multipartite; genome contained in > one virion 134
123. Coat protein Mr > 35 x 103 : (Tombusviridae) 124
Coat protein Mr < 35 x 103 125
124. RNA 4 kb; coat protein Mr < 40 x 103 : Tombusviridae /
Carmovirus
RNA > 4 kb; coat protein Mr > 40 x 103 : Tombusviridae /
Tombusvirus
125. RNA < 4 kb; vector a fungus : Necrovirus
RNA > 4 kb; vector an insect 126
126. RNA has a VPg at the 5'-end; coat protein
Mr 30 x 103 : Sobemovirus
RNA is 5'-capped; coat protein Mr 25 x 103 : Machlomovirus
127. Genome expressed as a polyprotein, no sub-genomic RNA
are formed in infected cells : (Flaviviridae) 128
Sub-genomic RNA are formed in infected cells 130
128. RNA > 12 kb; RNA encodes 3 envelope proteins and
1 nucleocapsid protein : Flaviviridae / Pestivirus
RNA < 12 kb; RNA encodes 2 envelope proteins
and 1 core protein 129
129. RNA > 10 kb; host a vertebrate and often
also an invertebrate : Flaviviridae / Flavivirus
RNA < 10 kb; man is the only host : Flaviviridae /
"Hepatitis C-like viruses"
130. Infected cells contain 1 species of
sub-genomic RNA : (Togaviridae) 131
Infected cells contain > 1 species
of sub-genomic RNA 132
131. Virion 70 nm in diameter; infects
vertebrates and insects : Togaviridae / Alphavirus
Virion 60 nm in diameter; host a vertebrate : Togaviridae /
Rubivirus
132. RNA < 20 kb; virion spherical : Arterivirus
RNA > 20 kb; virion pleomorphic : (Coronaviridae) 133
133. Virion spherical or pleomorphic with
club-shaped surface projections : Coronaviridae / Coronavirus
Virion biconcave disk-, kidney- or rod-shaped
with a peplomer-bearing envelope : Coronaviridae / Torovirus
134. Genome bipartite; largest RNA > 5 kb : Idaeovirus
Genome tripartite; largest RNA < 4kb : (Bromoviridae) 135
135. Virions isometric, sedimenting as 1 component 136
Virions not isometric, sedimenting as > 1 component 137
136. Coat protein Mr about 20 x 103;
virus not aphid-transmitted : Bromoviridae / Bromovirus
Coat protein Mr > 24 x 103;
virus aphid-transmitted : Bromoviridae / Cucumovirus
137. Some virions bacilliform;
virus aphid-transmitted : Bromoviridae / Alfamovirus
Virions slightly pleomorphic;
virus not aphid-transmitted : Bromoviridae / Ilarvirus
138. Virion rod-shaped 139
Virion filamentous 142
139. Genome monopartite : Tobamovirus
Genome multipartite 140
140. Virion > 20 nm in diameter;
vector a nematode : Tobravirus
Virion < 20 nm in diameter 141
141. Some virions > 250 nm in length;
largest RNA > 5 kb; vector a fungus : Furovirus
Virions < 200 nm long; largest RNA
< 5 kb : Hordeivirus
142. Virion > 700 nm in length : Closterovirus
Virion < 700 nm in length 143
143. Virion < 600 nm; coat protein
Mr < 25 x 103 : Potexvirus
Virion > 600 nm; coat protein
Mr > 25 x 103 144
144. Virion with prominent banding;
genome lacks a triple gene block 145
Virion without obvious banding;
genome contains a triple gene block : Carlavirus
145. Replicase and coat protein encoded in
the same open reading frame : Capillovirus
Non-structural proteins and the coat protein encoded
in different open reading frames : Trichovirus