Genus Closterovirus

Type Species	Beet yellows virus	(BYV)

Distinguishing Features

Virions are above 1000  nm in length (1250-2200  nm) and contain a single molecule of linear, positive sense, ssRNA from 15.5 to 19.3  kb in size. Viral species have particles of one size. CTV has also smaller than full-length particles that may encapsidate subgenomic or multiple species of defective RNAs (D-RNA) containing all of the cis-acting sequences required for replication. sgRNAs are involved in the construction of recombinant D-RNAs. The presence of D-RNA makes the dsRNA pattern of CTV isolates more complex than that of other members of the genus. Mealybug-transmitted species, and a few others with unknown vector, have CP subunits with unusually high molecular weight (35-46 10³). There are three types of genome organization among the sequenced species of the genus. BYV genome contains nine ORFs flanked by 5- and 3-untranslated regions (UTR) of 107 and 181  nts, respectively (Fig. 2). CTV has twelve ORFs and UTRs of 107  nts at the 5-end and 275  nts at the 3-end (Fig. 3). Differences with BYV consist in the presence of two papain-like protease domains in ORF1a, of an extra 5-proximal ORF (ORF2) encoding a 33 10³ product with no similarity to any other protein in databases, and of two extra 3-proximal ORFs (ORF9 and ORF11). The BYSV genome has ten ORFs and a 3 UTR of 241  nts, a length intermediate between that of the BYSV and CTV UTRs. A further difference with the BYV genome rests in the presence of an extra ORF (ORF2) encoding a 30 10³ polypetide with no similarity to any other protein in the databases. This ORF is located downstream of ORF1b, i.e., in the same position as the unrelated CTV ORF2. Thus, the organization of BYSV genomes is intermediate between that of BYV and CTV, suggesting that these three viruses might represent three distinct stages in closterovirus evolution. The LChV genome (16.9  kb) is the second largest closterovirus sequenced to date, containing nine ORFs and UTRs of 76  nts at the 5-end and 210  nts at the 3-end, respectively. LChV is the member of the genus with the largest HSP70 analogue (70 10³) and larger CP (46 10³) and CP analogue (76 10³) than BYV and CTV. Some of the members are transmissible by sap inoculation with difficulty. Natural vectors are aphids, mealybugs, or whiteflies (Trialeurodes), which transmit in a semi-persistent manner.

List of Species Demarcation Criteria in the Genus

The criteria demarcating species in the genus are:

	Particle size,
	Size of CP, as determined by deduced amino acid sequence data,
	Serological specificity using discriminatory monoclonal or polyclonal antibodies,
	Vector type and specificity,
	Magnitude and specificity of natural and experimental host range,
	Cytopathological features (i.e., aspect of inclusion bodies and origin of cytoplasmic vesicles),
	Genome structure and organization (number and relative location of the ORFs),
	Amino acid sequences of CP differing by more than 10%.

List of Species in the Genus

Molecular investigations have disclosed wide differences in genome structure and organization among sequenced viral species, as exemplified by BYV, CTV, and BYSV. Moreover, the arrangement of the genes of the whitefly-transmitted CCSV, is similar to that of the whitefly-transmitted criniviruses LIYV and SPCSV, and a close relationship was found between the HSP70 homologue of whitefly-transmitted BPYV closterovirus and CYSDV crinivirus. This may call for a re-classification of some species now included in the genus.

Virus species in the Genus are grouped according to the type of vector.

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:

Species in the Genus

1-Aphid-transmitted
Beet yellows virus (13)	[X73476]	(BYV)
Beet yellow stunt virus (207)	[U51931]	(BYSV)
Burdock yellows virus		(BuYV)
Carnation necrotic fleck virus (136)		(CNFV)
Carrot yellow leaf virus		(CYLV)
Citrus tristeza virus (33,353)	[U56902; U16304]	(CTV)
Wheat yellow leaf virus (157)		(WYLV)
2. Whitefly-transmitted
Beet pseudoyellows virus		(BPYV)
3-Mealybug-transmitted
Grapevine leafroll-associated virus 3	[U82937]	(GLRaV-3)
Little cherry virus	[Y10237]	(LChV)
4-Vector unknown
Grapevine leafroll-associated virus 2	[Y14131]	(GLRaV-2)

Tentative Species in the Genus

1-Aphid-transmitted:
Clover yellows virus	(CYV)
Dendrobium vein necrosis virus	(DVNV)
Heracleum virus 6	(HV-6)
2-Mealybug-transmitted:
Pineapple mealybug wilt-associated virus 1	(PMWaV-1)
Pineapple mealybug wilt-associated virus 2	(PMWaV-2)
Sugarcane mild mosaic virus	(SMMV)
3-Whitefly-transmitted:
Cucumber chlorotic spot virus	(CCSV)
Diodea vein chlorosis virus	(DVCV)
4-Vector unknown:
Alligatorweed stunting virus	(AWSV)
Festuca necrosis virus	(FNV)
Grapevine leafroll-associated virus 1	(GLRaV-1)
Grapevine leafroll-associated virus 4	(GLRaV-4)
Grapevine leafroll-associated virus 5	(GLRaV-5)
Grapevine leafroll-associated virus 6	(GLRaV-6)
Grapevine leafroll-associated virus 7	(GLRaV-7)
Megakepasma mosaic virus	(MegMV)