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Family	00.070.  Tetraviridae
	Taxonomic Structure of the Family
	Family         00.070. Tetraviridae Genus                      00.070.0.01. Betatetravirus Genus                      00.070.0.02. Omegatetravirus Genus                      00.070.0.00. unassigned

List of Species Demarcation Criteria in the 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.
6. RNA electrophoretic mobilities. In the absence of sequence information, the electrophoretic mobilities of the viral genomic RNAs should be compared with those of other tetraviruses.
7. RNA hybridization properties. In the absence of differences in RNA electrophoretic mobilities, the molecular hybridization properties of the viral genomic RNAs should be compared with those of other tetraviruses.
8. Genome sequence characteristics. The nt sequences of the genomic RNA(s) should be compared with those of other tetraviruses.

Application of these criteria. In practice, while criteria 1 - 5 above may be suggestive of a new species, definitive demarcation is based on the nt sequence of the viral CP gene.

List of Species in the Genus

The ICTVdB virus code and the viruses. Official virus species names are in italics. Tentative virus species names, alternative names ( ), isolates, strains, serotypes, subspecies, or rejected names are not italicized.
 
Virus codes, virus names, genome sequence accession numbers [ ] and assigned abbreviations ( ), are:

Species, their serotypes, strains and isolates

Tentative Species in the Genus

None reported.

List of Species Demarcation Criteria in the Genus

List of Species in the Genus

The ICTVdB virus code and the viruses. Official virus species names are in italics. Tentative virus species names, alternative names ( ), isolates, strains, serotypes, subspecies, or rejected names are not italicized.
 
Virus codes, virus names, genome sequence accession numbers [ ] and assigned abbreviations ( ), are:

Species, their serotypes, strains and isolates

Tentative Species in the Genus

None reported.

List of Unassigned Viruses in the Family

Similarity with other Taxa

The tetravirus CPs form a monophyletic group with the jelly-roll fold (3 subunits being distantly related to the CPs of nodaviruses having the T=3 capsids. It has been speculated that the tetravirus capsid might have evolved from a nodavirus-like ancestor through a process that included insertion of an immunoglobulin-like protein domain coding sequence (either acquired or evolved through sequence duplication) within the CP gene.

In contrast, comparative analysis of currently available non-structural protein sequences split tetraviruses into at least two distinct lineages, prototyped by NβV/HaSV and TaV/EeV respectively, within two different virus superclusters. The replicases of NβV and HaSV resemble those of the "alphavirus-like" supercluster, having the distinct Mtr-HelRdRp domain organization and through phylogenetic clustering of these three domains. The replicases of TaV and EeV lack both Mtr and Hel domains. Furthermore, their RdRp domain has a unique C-A-B motif arrangement in the palm subdomain of the active site that differs from the canonical A-B-C arrangement found in the other tetraviruses, all "alphavirus-like" viruses and indeed almost all known template-dependent polynucleotide polymerases (viral and cellular) carrying the palm sub-domain. Interestingly, the same CAB permutation of the motif arrangement is also found in replicases of all dsRNA birnaviruses. This motif rearrangement is a result of migration of ~22 as residues encompassing motif C between two internal positions, separated by ~110 aa, in a conserved region of ~400 aa. The permuted TaV, EeV and birnavirus enzymes form a minor, deeply separated cluster in the RdRp tree that also includes viruses of the "picornavirus-like supercluster" and order Nidovirales. Thus, TaV/EeV and birnaviruses may represent their own virus supercluster. The RdRp of PrV clusters with viruses of other families and therefore appears to belong to yet another lineage.

These complex and incongruent relationships of CP and replicase proteins imply that viruses currently classified as tetraviruses on the basis of their CP and other properties form a polyphyletic group. It is likely that immediate ancestors of TaV /EeV and PrV have independently acquired CP genes from ancestral tetraviruses resembling NβV and HaSV, respectively. Future revision of tetravirus taxonomy will need to address these complexities.

Derivation of Names

Nudaurelia capensis is the emperor pine moth. Tetra: from Greek 'tettares' meaning four, as T = 4.

References

Collated from VIIIth ICTV Report

Contributed by

Hanzlik, T.N., Gordon, K.H.J., Gorbalenya, A.E., Hendry, D.A., Pringle, F.M., Ward, V.K. and Zeddam, J.-L.

Additional References

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