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Viruses assigned to the genus Dichorhavirus form a monophyletic group based on well-supported Maximum Likelihood trees inferred from complete L sequences. Dichorhaviruses produce short bacilliform virions that lack a clearly distinguishable lipid envelope and have bi-segmented genomes. They share several characteristics with nucleorhabdoviruses, such as nuclear viroplasm, cytopathological effects in the nucleus, composition of structural proteins and gene order, sequence similarity and transcription mechanism. They are transmitted by false spider mites (genus Brevipalpus) in which they appear to replicate.
Dichorhaviruses produce short, bacilliform virions, 40 × 100–110 nm in size. Virions appear not to be enveloped but may be found in association with host membranes (Kitajima et al., 2003, Kondo et al., 2009).
The negative-sense single-stranded RNA genome is bi-segmented. RNA 1 (6.4–6.6 kb) contains five genes and RNA 2 (6.0 kb) contains a gene coding for L (Kondo et al., 2006, Ramalho et al., 2014). The termini of each segment are complementary and all genes are separated by conserved intergenic regions. Anti-genomic (positive-sense) RNAs are also present as a minor component (10–20 %) in purified orchid fleck virus (OFV) virions (Kondo et al., 2009).
Dichorhaviruses express the five canonical rhabdovirus structural proteins, nucleocapsid protein (N), phosphoprotein (P), matrix protein (M) and glycoprotein (G) and RNA polymerase (L). N, G and L have moderate to high sequence similarity with the cognate proteins of nucleorhabdoviruses. The P3 protein is a putative movement protein (Kondo et al., 2006, Ramalho et al., 2014). In OFV, the masses of the proteins are 49 kDa (N), 26 kDa (P), 38 kDa (P3), 20 kDa (M), 61 kDa (G) and 212 kDa (L).
The genome organisation of viruses in the genus Dichorhavirus resembles that of plant-infecting rhabdoviruses in the genera Cytorhabdovirus and Nucleorhabdovirus, except that the RNA genome occurs as two segments: RNA 1 contains genes encoding five proteins in the order 3′-N-P-P3-M-G-5′ and RNA 2 codes for L (Figure 1.Dichorhavirus). Replication and transcription mechanisms closely resemble those reported for nucleorhabdoviruses.
Figure 1.Dichorhavirus. Schematic representation of dichorhavirus genome segments (RNA1 and RNA2). N, P, M, G and L represent ORFs encoding the structural proteins. ORF 3, encoding the putative viral cell-to-cell movement protein gene, is highlighted (blue).
Dichorhaviruses are transmitted by false spider mites of the genus Brevipalpus (Arthropoda: Arachnida: Acari: Tenuipalpidae) in which they also probably replicate (Kitajima et al., 2003, Chagas et al., 2003, Kitajima and Alberti 2014, Kondo et al., 2003, Roy et al., 2015). Dichorhavirus infections lead to local chlorotic and/or chlorotic spots on leaves and fruits (and/or systemic symptoms in some cases). OFV infection is characterised by typical cytopathic effects in the nuclei of infected cells (Kitajima et al., 2001, Chang et al., 1976). OFV has a global distribution and infects orchids and other ornamentals (such as lilyturf) (Peng et al., 2013, Mei et al., 2016, Dietzgen et al., 2018). Strains of OFV infecting citrus with >90% nucleotide sequence identity have been reported and their occurrence may be geographically constrained. In addition, strains of two novel unclassified dichorhaviruses, citrus leprosis virus N (CiLV-N) and citrus chlorotic spot virus (CiCSV), were recently identified infecting citrus species in Brazil (Ramos-Gonzalez et al., 2017, Chabi-Jesus et al., 2018)). Whereas CiLV-N seems to infect only sweet oranges, CiCSV was also reported infecting beach hibiscus and Agave desmettiana (Chabi-Jesus et al., 2019), suggesting a broader host range. A third new dichorhavirus was also characterised in Brazil infecting the ornamental Clerodendrum sp. (Ramos-González et al., 2018). Coffee ringspot virus (CoRSV) has been reported to infect coffee in South America and its genetic variability and epidemiology has been studied (Ramalho et al., 2016).
Dichorhavirus: from the bi-segmented characteristic of the viral genomes (Dicho: from Greek, meaning “in two, apart or asunder”).
Viruses assigned to different species within the genus Dichorhavirus have a minimum nucleotide sequence divergence of 25% in the L gene (Dietzgen et al., 2014)}, although the threshold of 20% nucleotide sequence divergence in the L gene has been proposed due to the availability of new dichorhavirus sequences (Dietzgen et al., 2018a).
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