Genus Curtovirus

Type Species	Beet curly top virus	(BCTV)

Distinguishing Features

Nucleic Acid

Curtovirus genomes consist of a single circular ssDNA component (2.9-3.0  kb).

Proteins

Curtoviruses produce six to seven proteins in infected plants. Virion-sense ORFs V2 and V3 produce proteins involved in regulation of genomic ss/dsDNA levels and movement respectively; ORF V1 protein (CP) is involved in encapsidation and movement. Complementary sense ORF C1 encodes the Rep protein; C2 encodes a protein of unknown function; C3 encodes a protein analogous to the replication enhancer protein (REn) of begomoviruses; ORF C4 produces a protein which can initiate cell division, and is an important symptom determinant.

Genome Organization and Replication

The complete nucleotide sequences of infectious genomic clones of BCTV (3), Horseradish curly top virus (HrCTV) and Tomato pseudo-curly top virus (TPCTV) have been determined. The genomes of BCTV and TPCTV encode seven genes, three on the virion strand and four on the complementary strand. Replication is initiated by the Rep protein in the stem-loop sequence (Fig. 3).

Antigenic Properties

Serological tests show BCTV, TPCTV and Tomato leafroll virus (TLRV) to be relatively closely related. Distant relationships between curtoviruses and begomoviruses have been shown in serological tests.

Biological Properties

Host Range

Type species BCTV has a very wide host range, over 300 species in 44 plant families. TPCTV has a narrower host range.

Transmission

Transmitted in nature by leafhoppers (Homoptera: Cicadellidae), with the exception of TPCTV, which is transmitted by a treehopper (Homoptera: Membracidae, Micrutalis malleifera Fowler). Mechanism of transmission is persistent (circulative, non-propagative). BCTV may be transmitted with difficulty by mechanical inoculation. Experimentally some species have been transmitted by Agrobacterium-mediated transfer using recombinant DNA methods (BCTV, HrCTV, TPCTV).

List of Species Demarcation Criteria in the Genus

The criteria demarcating species in the genus are:

	No trans-replication of genomes by Rep proteins,
	Complete genome sequence identity less than 80%,
	Antigen differences,
	Different vector species,
	Differences in host range/pathogenicity.

List of Species in the Genus

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

Beet curly top virus		(BCTV)
California/Logan (210)	[X04144, M24597]	(BCTV-Cal)
Beet curly top virus Iran/CFH	[U02311]	(BCTV-CFH)
Beet curly top virus Worland	[U56975]	(BCTV-Wor)
Horseradish curly top virus	[U49907]	(HrCTV)

Tentative Species in the Genus

Tomato leafroll virus		(TLRV)
Tomato pseudo-curly top virus	[X84735]	(TPCTV)