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Viruses assigned to the genus Varicosavirus form a monophyletic group based on well-supported Maximum Likelihood trees inferred from complete L sequences. Varicosaviruses infect plants. They have bi-segmented genomes and non-enveloped, flexuous, rod-shaped virions that, in electron micrographs, resemble nucleocapsids of other rhabdoviruses.
Virions are fragile, non-enveloped rods mostly measuring 324–152 nm x approximately18 nm; each has a central canal approximately 3 nm in diameter and an obvious helix with a pitch of about 5 nm (Figure 1.Varicosavirus). The helix of particles, especially those in purified preparations, tends to loosen and particles are then seen as partially uncoiled filaments (Kuwata and Kubo 1981, Kuwata et al., 1983, Vetten et al., 1987).
Figure 1.Varicosavirus. Negative-contrast electron micrograph of virions of an isolate of lettuce big vein-associated virus. The bar represents 100 nm. (Courtesy J.A. Walsh and C.M. Clay.)
Virus particles have a density in Cs2SO4 of about 1.27 g cm-3 (Kuwata et al., 1983).
The lettuce big vein-associated virus (LBVaV) genome consists of negative-sense, single-stranded RNA which is encapsidated by N (previously termed coat protein, CP). The genome (12.9 kb) occurs as two segments of 6.8 kb (RNA1) and 6.1 kb (RNA2) (Sasaya et al., 2002, Sasaya et al., 2004). The 3′- and 5′-terminal sequences of the two genome segments are similar but do not exhibit inverse complementarities. Compared with OFV (see genus Dichorhavirus), purified LBVaV virions contain relatively large amounts of anti-genomic RNAs (Sasaya et al., 2001). Conserved motifs in the transcription initiation and termination signals resemble those of viruses in the genus Cytorhabdovirus.
RNA2 encodes the 397-aa N with a predicted mass of 44.5 kDa (Mr approximately 48 kDa), as well as protein 2 (36 kDa), protein 3 (32 kDa), protein 4 (19 kDa) and protein 5 (41 kDa) which are of unknown functions but thought to be equivalent to P, P3, M and G of cytorhabdoviruses (Kormelink et al., 2011, Sasaya et al., 2004). RNA1 encodes protein 6 (5 kDa), also of unknown function, and the 232 kDa L (Sasaya et al., 2002). N and L share a moderate amount of amino acid sequence identity with their counterparts in other rhabdoviruses.
RNA1 contains two genes in the order 3′-6-L-5′; RNA2 contains five genes in the order 3′-N-2-3-4-5-5′ (Figure 2.Varicosavirus). Although the genome organisation of LBVaV is similar to that of other rhabdoviruses, LBVaV does not have a 3′-non-coding leader sequence. Capped and polyadenylated monocistronic mRNAs are transcribed from individual genes.
Figure 2.Varicosavirus. Schematic representation of the lettuce big vein-associated virus genome (RNA1 and RNA2). N and L represent ORFs encoding the structural proteins. ORFs numbered 2 to 6 encode putative proteins of unknown function.
Varicosaviruses occur naturally in members of two families of plants (Compositae and Solanaceae). LBVaV and TStV are transmitted in soil and in hydroponic systems by zoospores of the chytrid fungus Olpidium virulentus (a noncrucifer strain of Olpidium brassicae) (Hiruki 1967, Campbell 1962, Sasaya and Koganezawa 2006, Maccarone et al., 2010). LBVaV is frequently associated with lettuce big vein disease that is caused by Mirafiori lettuce big-vein virus (MLBVV; genus Ophiovirus, family Aspiviridae) (Roggero et al., 2000, Lot et al., 2002). Due to the longevity of infectious LBVaV from stored O. virulentus resting spores, it is assumed that the virus is carried internally by the fungus. The viruses are also transmitted experimentally, sometimes with difficulty, by mechanical inoculation (Huijberts et al., 1990). Neither of the viruses is reported to be seed-transmitted. LBVaV has been reported from Europe, United States, Japan, Saudia Arabia and Australia (Umar et al., 2017, Navarro et al., 2005, Maccarone et al., 2010a, Alemzadeh and Izadpanah 2012).
N does not induce a strong antibody response in laboratory animals. LBVaV is closely related serologically to tobacco stunt virus (TStV), currently unclassified (Kuwata and Kubo 1986).
Varicosavirus: from the symptom previously thought to be induced by LBVaV (Vari: from Latin varix, meaning abnormal dilation or enlargement of a vein or artery). However, lettuce big-vein disease, although long thought to be induced by a virus previously designated lettuce big-vein virus, is now considered to be caused by isolates of Mirafiori lettuce big-vein ophiovirus (genus Ophiovirus). Viruses of this species are soil-borne and often occur in lettuce together with isolates of LBVaV.
LBVaV is assigned to the only species currently approved in the genus.
Alopecurus myosuroides varicosavirus 1
tobacco stunt virus
red clover associated varicosavirus
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