|
Type Species |
(NV) |
Virions are nonenveloped, roughly spherical, about 40 nm in diameter and exhibit T = 4 icosahedral shell quasi-symmetry (Fig. 1). Distinct capsomers have been resolved by cryo-electron microscopy and image reconstruction. The genome consists of ssRNA. Viruses in the genus Betatetravirus have monopartite genomes, whereas those in the genus Omegatetravirus have bipartite genomes.
Physicochemical and Physical Properties
Virion Mr is about 18 
106. Virion S20w is 194-217S. Virion buoyant density in CsCl is usually 1.28-1.30 g/cm3 but occasionally as high as 1.33 g/cm3 (varies with species). Virions are stable over a broad range of pH and their infectivity can resist desiccation and protease treatment.
Virions of the type species, Nudaurelia capensis virus (NV), contain a single, positive-sense, ssRNA segment of about 6.5 kb (Mr 1.8 106) which represents about 10% of the particle mass. This genomic RNA is not polyadenylated at its 3
-end, nor blocked like nodaviral RNAs, but terminates instead with a distinctive tRNA-like structure. A subgenomic message for the capsid proteins (CP), which is derived from the 3-end of the genomic RNA, can also be encapsidated in some species.
Capsids consist of 240 protein subunits (protomers) arranged on a T = 4 surface lattice. Each protomer consists of the two cleavage products (Mr 58.4 103 and 8 103), of a single CP precursor (Mr 66.4 103). Minor amounts of the uncleaved product may be found in virions.
None reported.
None reported.
Genome Organization and Replication
The viruses replicate in the cytoplasm. The 6625 nts NV genomic RNA contains two ORFs that overlap for 1517 nts: that nearer the 5-end contains 1821 codons and encodes the RNA replicase (Mr 204 103), whereas that nearer the 3-end contains 612 codons and encodes a precursor (Mr 66.4 103) to the two CPs. During RNA replication, a sgRNA which represents the 3 2.5 kb of the genome is synthesized and this serves as the mRNA for the CP precursor (Fig. 2). In some betatetravirus species, including NV, the subgenomic mRNA can be encapsidated in virus particles, which complicates the distinction between the monopartite genome betatetraviruses and the bipartite genome omegatetraviruses.
Most of the members of the group are serologically interrelated but distinguishable. The majority of the isolates were identified on the basis of their serological reaction with antiserum raised against NV.
The viruses replicate primarily in the cytoplasm of gut cells of several Lepidoptera species. Crystalline arrays of virus particles are often seen within cytoplasmic vesicles. Different isolates vary considerably in pathogenicity; symptoms can vary from inapparent to acutely lethal infections.
Nature: All virus species were isolated from Lepidoptera species (moths and butterflies), principally from Saturniid, Limacodid and Noctuid moths and no replication in other animals has been detected. In larvae, virus replication is restricted predominantly to the cells of the midgut.
Laboratory: No infections by members of the betatetravirus genus have been achieved in cultured cells, even when genomic RNA was transfected directly into cells.
Oral transmission of NV to Antherea eucalypti (the emperor gum moth) has been demonstrated experimentally. Oral transmission is implied by the midgut site of viral replication and by reports of some tetraviruses being used as sprayed insecticides in Malaysia (e.g., Darna trima virus). At high host densities, horizontal spread appears to be the major route of infection. Suggestive evidence exists for vertical transmission, which could be responsible for the observed persistence of tetraviruses within insect populations.
List of Species Demarcation Criteria in the Genus
The following criteria can be applied to the demarcation of species within the betatetravirus genus:
1. |
Biological properties (host range, vectors, mode of transmission). Since the natural host-ranges of individual recognized tetravirus species appear to be narrow, virus isolation from a new host can provide suggestive evidence of a new tetravirus species. | ||||
2. |
Antigenic properties. Antisera raised against different isolates or strains of a single tetravirus species should exhibit high levels of cross-reactivity in Western blot and/or neutralization analyses. Lower levels of cross-reactivity in these assays using antisera against previously recognized tetraviruses can provide evidence of a new tetravirus species. | ||||
3. |
Virion physical/physicochemical characteristics. In the absence of more definitive criteria, significant (>5%) differences in virion sedimentation coefficient or buoyant density from those of all previously recognized tetravirus species can provide evidence of a new virus species. | ||||
4. |
Structural protein characteristics. The electrophoretic mobilities in SDS-PAGE of the CP precursor or its cleavage products should be compared with those of other tetravirus species. | ||||
5. |
Genome molecular characteristics.
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6. |
Genome sequence characteristics. The nucleotide sequences of the genomic RNA(s) should be compared with those of other tetraviruses. Because the genome of omegatetraviruses is segmented, reassortment is possible and the two genome segments may have different evolutionary lineages. |
Application of these criteria. In practice, while criteria 1-5 above may be suggestive of a new species, definitive demarcation is based on the nucleotide sequence of the viral CP gene.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names and assigned abbreviations ( ) are:
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Calliteara pudibunda virus |
(CpV) |
|
(Dasychira pudibunda virus) |
(DpV) |
|
Darna trima virus |
(DtV) |
|
Nudaurelia capensis virus |
(NV) |
|
(Antheraea eucalypti virus) |
(AeV) |
|
Philosamia cynthia x ricini virus |
(PxV) |
|
Pseudoplusia includens virus |
(PiV) |
|
Setothosea asigna virus |
(SaV) |
|
(Thosea asigna virus) |
(TaV) |
|
Trichoplusia ni virus |
(TnV) |
Tentative Species in the Genus
None reported.
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