|
Type Species |
(NV) |
Virions are nonenveloped, roughly spherical, about 40 nm in diameter and exhibit T = 4 icosahedral shell quasi-symmetry. Distinct capsomers have been resolved by cryo-electron microscopy and image reconstruction (Fig. 3). The genome consists of ssRNA. Viruses in the genus Omegatetravirus have bipartite genomes, whereas those in the genus Betatetravirus have monopartite genomes.
Physicochemical and Physical Properties
Virion Mr is about 16 106. Virion S20w is 194-217S. Virion buoyant density in CsCl is 1.28-1.30 g/cm3 (varies with species). Virions of Helicoverpa armigera stunt virus (HaSV) are stable between pH 3.0-11.0, and at temperatures up to 55°C.
The type species, Nudaurelia capensis virus (NV), contains two, positive-sense, ssRNA segments of 5.3 kb (RNA1, Mr 1.75 106) and 2.45 kb (RNA2, Mr 0.8 106). These genomic RNAs are capped at their 5-ends but their 3-ends are not polyadenylated, nor blocked like nodaviral RNAs, but instead terminate with a distinctive tRNA-like structure. It is likely that omegatetraviruses encapsidate both genomic RNAs within a single particle.
Capsids consist of 240 protein subunits (protomers) arranged on a T = 4 surface lattice. Each protomer consists of the two cleavage products (Mr 62 103 and 7.8 103) of a single CP precursor (Mr 69.8 103). Overall, the CPs of NV and NV share less than 20% amino acid sequence identity, indicating that the beta and omega genera of the tetraviruses diverged in the distant past.
None reported.
None reported.
Genome Organization and Replication
Omegatetraviruses replicate in the cytoplasm. The viral genome consists of two unique molecules of RNA, probably encapsidated in the same virus particle (Fig. 4). RNA1 (Mr 1.75 106; 5.3 kb) encodes the catalytic subunit of the RNA replicase in an ORF of 1704 codons in the case of HaSV. RNA2 (Mr 0.8 106; 2.45 kb) encodes the CP precursor (Mr 70 103). A 157-codon ORF precedes and partially overlaps that for the CP; it encodes a non-structural protein (p17) of unknown function. Separate dsRNAs that correspond to the two genome segments are observed.
The two recognized omegatetraviruses (NV and HaSV) display no serological relationship although their CPs show a high degree of amino acid sequence identity.
The viruses replicate primarily in the cytoplasm of midgut cells of the larvae of several Lepidoptera species. Crystalline arrays of virus particles are often seen within cytoplasmic vesicles. There is a considerable range of pathogenicity with different isolates, and symptoms can vary from inapparent to acutely lethal infections.
Nature: All species were isolated from Lepidoptera species, principally from Saturniid, Limacodid and Noctuid moths.
Laboratory: No infections by members of the genus Omegatetravirus have yet been achieved in cultured cells, but infectious HaSV particles were produced by plant protoplasts transfected with plasmids carrying full-length cDNAs that corresponded to the viral genome segments.
As with the betatetraviruses, oral transmission is implied by the midgut site of viral replication. At high host densities, horizontal spread appears to be the major route of infection, but evidence exists for vertical transmission which might be responsible for the observed persistence of tetraviruses within insect populations.
List of Species Demarcation Criteria in the Genus
The species demarcation criteria of betatetraviruses also apply to omegatetraviruses.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Nudaurelia capensis virus |
RNA2 [S43937] |
(NV) |
|
Helicoverpa armigera stunt virus |
RNA1 [U18246] RNA2 [L37299] |
(HaSV) |
Tentative Species in the Genus
None reported.
List of Unassigned Viruses in the Family
|
Acherontia atropas virus |
(AaV) |
|
Agraulis vanillae virus |
(AvV) |
|
Callimorpha quadripuntata virus |
(CqV) |
|
Eucocytis meeki virus |
(EmV) |
|
Euploea corea virus |
(EcV) |
|
Hyalophora cecropia virus |
(HcV) |
|
Hypocritae jacobeae virus |
(HjV) |
|
Lymantria ninayi virus |
(LnV) |
|
Saturnia pavonia virus |
(SpV) |
|
Setora nitens virus |
(SnV) |
|
Nudaurelia capensis |
(N |
a N
V resembles the tetraviruses in appearance but is serologically unrelated to any known species.
Phylogenetic Relationships within the Family
The overall level of amino acid sequence similarity between NV and NV CPs is 26%.
On the basis of their nonstructural protein sequences, tetraviruses form a distinct lineage within the alpha-like RNA virus superfamily. Structural similarities between the CPs of the tetraviruses and nodaviruses suggest that they had a common ancestor.
Nudaurelia capensis is the emperor pine moth.
Tetra: from Greek ‘tettares’ meaning four, as T = 4.
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