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Type Species |
(OuMV) |
The bacilliform virions constitute a series of particles with conical ends (apparently hemi-icosahedra) and cylindrical bodies 18 nm; in diameter. The bodies of the particles are composed of a series of double disks, the commonest particles having 2 disks (particle length 30 nm;), a second common particle having 3 disks (particle length 37 nm;) with rarer particles having 4 disks (particle length 45.5 nm;) and 6 disks (particle length 62 nm;). There is no envelope (Fig. 1 and 2).
Physicochemical and Physical Properties
The Mr of virions and their sedimentation coefficients are not known. The buoyant density in CsCl of all particle sizes is 1.375 g/cm3. The particles are stable near pH 7. Thermally, they are relatively stable; infectivity survives in crude sap after heating for 10 minutes at 70°C but not 80°C, and is retained after at least one cycle of freeze-thaw. Cation stability is not known, except that the particles survive in CsCl density gradients. The particles survive treatment with chloroform but not n-butanol, and survive treatment with 1% Triton X-100 detergent.
The nucleic acid is linear positive-sense ssRNA with an estimated Mr of 1.58 106, divided into 3 segments with estimated Mr of 0.91, 0.35 and 0.32 106. It is not known whether there is a 5-terminal cap, a 5-terminal covalently-linked polypeptide or a 3-terminal poly(A) tract. No nucleotide sequences are available.
There is one capsid protein (CP) of size estimated as Mr 25.2 103. In addition, one nonstructural protein of unknown function accumulates abundantly in the cytoplasm of infected plants, forming fibers or tubules.
None.
None.
Genome Organization and Replication
The CP is encoded by RNA3. The fibrous nonstructural protein is encoded by RNA1 or 2. Particles accumulate to high levels in the cytoplasm of parenchyma cells and become associated with the fibrous protein.
The native virions and the nonstructural protein are good immunogens. They do not cross-react.
The type species can easily be mechanically transmitted to a rather wide range of dicotyledonous plants (34 species in 14 families reported), usually inducing systemic ringspots, mosaic and necrosis, with local lesions on some hosts. There is no particular tissue tropism. No vector has been identified. No experimental transmission was obtained with several aphid species, the whiteflies Trialeurodes and Bemisia or a Tetranychus mite. Experimental seed-transmission rates are 1-2% in Nicotiana benthamiana and N. megalosiphon. Different species in the genus are reported to occur in geographically diverse areas and on widely differing hosts, though there are experimental hosts in common.
List of Species Demarcation Criteria in the Genus
The species demarcation criteria are:
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No evident sequence homologies between the respective RNA2s and RNA3s (by hybridization); | |
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CPs of distinctly different size (Mr 25 or 21 103); | |
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No serological relationships between CPs (by gel-diffusion or EM decoration tests) or distant serological (in Western blots); | |
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Natural host ranges non-overlapping (cucurbits; cherry; cassava). |
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, primary hosts names { }, and assigned abbreviations ( ) are:
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Cassava virus C |
{cassava, Euphorbiaceae} |
(CsVC) |
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Epirus cherry virus |
{cherry, Rosaceae} |
(EpCV) |
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Ourmia melon virus |
{melon, cucumber and pumpkin, Cucurbitaceae} |
(OuMV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
The virion morphology, in particular the architecture of double disks and conical ends, forming particles of different lengths, is unique, differing clearly from that of other known “bacilliform” viruses of similar size. The virions are notably resistant to treatment with solvents, detergent and heat. There are 3 ssRNAs of Mr about 0.9, 0.35 and 0.32 106 combined with a single CP of Mr 21-25 103. A fibrous nonstructural protein accumulates conspicuously in the cytoplasm. The genus may have affinities with other ssRNA positive-strand multipartite viruses such as the members of the family Bromoviridae.
The generic name derives from the district of Ourmia in Northern Iran where the type species was found.
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