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Type Species |
(CPsV) |
The virions are naked filamentous nucleocapsids about 3 nm in diameter, forming kinked (probably internally coiled) circles of at least two different contour lengths, the shortest length about 760 nm. The circles can collapse to form pseudolinear duplex structures about 9 nm in diameter (Figs. 1 and 2).
Physicochemical and Chemical Properties
The Mr and sedimentation coefficients of virions are not known. The particles are not stable in CsCl but the buoyant density in cesium sulfate (for Ranunculus white mottle virus, RWMV) is 1.22 g/cm3. The particles have limited stability between pH 6 and 8. Infectivity does not survive in crude sap held at 50°C for 10 minutes. Particle structure survives limited treatment with organic solvents and nonionic or zwitterionic detergents.
The genome consists of ssRNA, though after deproteinization of purified virions both ds- and ssRNA are detected. The genome is 11-12 kb in size, divided into three components of 7.5-9.0, 1.6-1.8, and 1.5 kb. The virions are circular, suggesting the presence of a panhandle structure of complementary sequences at the 5 and 3-ends of each RNA, but this is not established.
There is one capsid protein (CP) varying in size from Mr 43 to 50 103, according to species.
None.
None.
Genome Organization and Replication
All ORFs detected in the genome are in the negative strand. The capsid protein is encoded by RNA3. Where known, the coat protein antigen accumulates in the cytoplasm of parenchyma cells.
The CP is a poor antigen. There are no cross-reactions between species except in one weak and doubtful case.
The viruses are mechanically transmissible to a limited range of test plants, inducing local lesions and systemic mottle. The natural hosts are dicotyledonous for two recognized species, and tulip (Liliaceae) for the third tentative species, but plants in the family Gramineae have not been infected. Transmission is commonly by vegetative propagation of the host; no natural vectors have been identified.
List of Species Demarcation Criteria in the Genus
The list of species demarcating criteria is:
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Differing size of CP, | |
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No or very distant serological relationship between CPs of different species, | |
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Natural host ranges non-overlapping (citrus, ranunculus, tulip). |
Given the lack of detailed molecular data, species differences are presently based on three criteria: differing size of CP, no or very distant serological relationships between CPs, and differing natural host ranges. For example the Mr of the CP is 48-50 103 in the type species, CPsV, 43 103 in the second species, Ranunculus white mottle virus (RWMV), and 47 103 in the third species, Tulip mild mottle mosaic virus (TMMMV). In Western blots, CPsV and RWMV do not cross-react, whereas there is a very faint one-way reaction between purified RWMV and antiserum to TMMMV. The natural hosts so far identified are very different: citrus, Ranunculus, and tulip, respectively.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, host names [ ], and assigned abbreviations ( ) are:
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Citrus psorosis virus |
[citrus, Rutaceae] |
(CPsV) |
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Ranunculus white mottle virus |
[ranunculus, Ranunculaceae] |
(RWMV) |
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Tulip mild mottle mosaic virus |
[tulip, Liliaceae] |
(TMMMV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
None reported.
The virion morphology resembles that of the tenuiviruses and the internal nucleocapsid component of members of the family Bunyaviridae. However, members of the genus Ophiovirus do not, like the tenuiviruses, infect Graminae, and do not, like members of the family Bunyaviridae, possess an envelope. The capsid protein (Mr 43-50 103) is significantly larger than that of the tenuiviruses or members of the family Bunyaviridae. No nucleotide or amino acid sequence homologies with members of the family Bunyaviridae or tenuiviruses have been detected, nor is there any serological relationship with them. Unlike these viruses, RNAs 2 and 3 each contain only one ORF expressed in the negative sense. RNA1 contains a large negative-sense ORF with a region of significant amino acid sequence homology to the genus Nucleorhabdovirus in the putative RNA polymerase. RNA1 also contains a second smaller negative-sense ORF. Thus the genome appears to be tripartite but entirely negative-stranded, so far a unique combination.
The genus may have affinities with tenuiviruses, and members of the families Bunyaviridae and Rhabdoviridae.
Ophio is derived from the Greek “ophis”, a serpent, and refers to the snaky appearance of the virions.
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