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Curtoviruses are monopartite geminiviruses that are vectored by leafhoppers, often in a species-specific manner. In common with viruses in the genus Begomovirus, curtoviruses occur in both the New and Old Worlds. They infect a wide variety of vegetable crops and are thought to be widely prevalent in non-cultivated dicot species (Varsani et al., 2014). The genomes most often have seven genes, but isolates have been discovered that have only five or six genes. Curtovirus genomes may be associated with defective-interfering DNAs (DI particles) that function in some instances to reduce symptom severity. The species working demarcation in this genus has been established at <77% shared nt identity (Varsani et al., 2014).
See discussion under family description.
The genomes of curtoviruses consist of a single circular ssDNA component of 2.9-3.0 kb, encoding six to seven proteins (Figure 1.Curtovirus). The three ORFs encoded on the virion-sense strand are the coat protein (CP, ORF V1), which encapsidates the virion-sense ssDNA and is involved in virus movement and insect vector transmission, a movement protein (MP, ORF V2), and V3, which is involved in the regulation of the relative levels of ssDNA and dsDNA. The complementary-sense strand encodes the replication-associated protein (Rep, ORF C1), which is required for the initiation of viral DNA replication, the C2 protein (ORF C2), which acts as a pathogenicity factor in some hosts, a replication enhancer protein (REn, ORF C3), and the C4 protein (ORF C4), which is an important symptom determinant implicated in cell-cycle control (Hanley-Bowdoin et al., 2013). Nucleotide sequence comparisons suggest that curtoviruses and begomoviruses diverged after a recombination event altered insect vector specificity (Rybicki 1994).
Figure 1.Curtovirus. Genomic organization of curtoviruses. ORFs are denoted as being encoded on the virion-sense (V) or complementary-sense (C) strand, and corresponding protein products are indicated where these are known. ORF C3 is not present in horseradish curly top virus (HrCTV). The position of the stem-loop containing the conserved TAATATTAC sequence located in the intergenic region (IR) is shown. CP, coat protein; MP, movement protein; Rep, replication-associated protein; REn, replication enhancer protein.
Serological tests show beet curly top virus (BCTV), tomato leaf roll virus (TLRV) and tomato pseudo-curly top virus (TPCTV, genus Topocuvirus) to be relatively closely related. Distant relationships between curtoviruses and begomoviruses have been shown in serological tests (Harrison and Robinson 1999).
Isolates of the type species, Beet curly top virus, have a very wide host range within dicot plants, including over 300 species in 44 plant families (Strausbaugh et al., 2008).
Curtoviruses are transmitted in nature by specific leafhoppers (order Hemiptera, family Cicadellidae) in a persistent (circulative, non-propagative) manner (Soto and Gilbertson 2003). BCTV may be transmitted with difficulty by mechanical inoculation. Most members are transmitted experimentally to plants by Agrobacterium-mediated transfer (agroinoculation) from partially or tandemly repeated cloned genomic DNA.
The following criteria should be used as a guideline to establish taxonomic status:
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