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Type Species |
(ShVX) |
Virions are highly flexible filamentous particles, about 800 nm in length and 12 nm in diameter. They resemble potyviruses in their length but closteroviruses in their flexibility and cross-banded substructure (Fig. 1).
Physicochemical and Physical Properties
Shallot virus X (ShVX) virion S20w is about 170S in 0.1 M tris-HC1, pH 7.5 at 20°C. Buoyant density in CsCl is 1.33 g/cm3.
Virions contain a single molecule of linear ssRNA, about 9.0 kb in size, with a 3poly(A) tract. Shallot virus X (ShVX) RNA preparations, besides genomic ssRNA, contain molecules of dsRNA, 1.5 kb in length, whose genesis and function(s) are unknown. The complete nucleotide sequences of the genomic RNA of ShVX and Garlic virus X (GVX), and the partial sequences of the RNA of four allexiviruses have been determined.
Virions are composed of a single polypeptide with a Mr of 28-36 103.
None reported.
None reported.
Genome Organization and Replication
The genomic RNA of ShVX contains six large ORFs and non-coding sequences of 98 nts at the 5 terminus, and 112 nts followed by poly (A) tail at the 3 terminus (Fig. 2). The ORFs code for polypeptides of 195 103, 26 103, 11 103, 42 103, 28 103, and 15 103, respectively from 5-end to 3-end. The gene arrangement of other incompletely sequenced allexiviruses is similar. The 195 103 polypeptide is probably the viral RNA polymerase. In comparisons among the amino acid sequences of methyltransferase, helicase or polymerase domains, those of allexiviruses were most similar to those of potexviruses. The 26 103 and 11 103 proteins are similar to the first two proteins encoded by the triple gene block of potexviruses and carlaviruses are probably involved into cell-to-cell movement of the virus. There is a coding sequence for a small (7-8 103) triple gene block protein but it lacks the initiation AUG-codon. The 42 103 polypeptide has no significant homology with any proteins known but has been shown to be expressed in plants infected with ShVX in relatively large amounts. The 28 103 polypeptide is the CP. In PAGE it migrates as a 32-36 103 protein which could be due to the high hydrophilicity evident from its amino acid sequence. The 15 103 protein is similar to those of the 11-14 103 proteins encoded by the 3 ORFs of carlaviruses, has a zinc binding finger motif and an ability to bind nucleic acids. The function of this polypeptide is not known.
Allexivirus particles are good immunogens. Some members of the genus are serologically interrelated. Specific antisera and monoclonal antibodies against pure virus particles as well as antisera against recombinant CPs have been used for differentiation purposes.
Host range is extremely restricted. Some isolates from shallot, onion, garlic and sand leek have been experimentally transmitted to Chenopodium murale, in which they induced local lesions.
Allexiviruses are thought to be mite-borne. GarV-C and GarV-D have been shown to be transmitted by the eriophyd mite, Aceria tulipae. All are manually transmissible by sap inoculation of healthy host plants. None could be transmitted by aphids.
Allexiviruses have been identified in Russia, Japan, France, Germany, UK, The Netherlands, Korea, Taiwan, Thailand and Argentina.
Most induce no visible or only very mild symptoms in many species, although certain isolates can cause severe damage to crops. In infected tissue induce formation of granular inclusion bodies and small bundles of flexible particles.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Less than 90% amino acid sequence identity in the CPs, | |
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Less than 90% nucleotide sequence identity in the 3 non-coding region of genome RNA, | |
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Different reactions with antisera. |
The available information about the identity of the members in the genus Allexivirus is still fragmentary. There are nearly always mixed infections of vegetatively propagated species and it is difficult or impossible to isolate and/or separate the viruses.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
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Garlic mite-borne filamentous virus |
[L38892] |
(GarMbFV) |
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Garlic virus A |
[D11157] |
(GarV-A) |
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Garlic virus B |
[D11158; D49443] |
(GarV-B) |
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Garlic virus C |
[D11159] |
(GarV-C) |
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Garlic virus D |
[D11160] |
(GarV-D) |
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Garlic virus X |
[U89243] |
(GarV-X) |
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Shallot virus X |
[M97264; L76292] |
(ShV-X) |
Tentative Species in the Genus
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Garlic mite-borne latent virus |
(GarMbLV) |
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Onion mite-borne latent virus |
(OMbLV) |
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Shallot mite-borne latent virus |
(ShMbLV) |
Phylogenetic Relationships within the Genus
Not available.
Allexivirus genomes contain all the elements common to carlaviruses and/or potexviruses and are distinguished by containing an extra ORF (ORF4). Allexiviruses are thought to occupy an intermediate position in putative phylogeny between carlaviruses and potexviruses; there are no serological cross-reactions between either and allexiviruses.
Allexi: sigla from Allium (the genus name for the principal host, shallot) + X + vowels for liaison.
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