Subfamily: Betarhabdovirinae

Genus: Varicosavirus

Distinguishing features

Viruses assigned to the genus Varicosavirus form a monophyletic group based on well-supported Maximum Likelihood or Maximum Clade Credibility trees inferred from complete L sequences. Varicosaviruses infect plants and they have bi-segmented genomes. Lettuce big vein-associated virus (LBVaV; species Varicosavirus lactucae) has non-enveloped, flexuous, rod-shaped virions that, in electron micrographs, resemble nucleocapsids of other rhabdoviruses.

Virion

Morphology

LBVaV vrions 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).

Physicochemical and physical properties

Virus particles have a density in Cs₂SO₄ of about 1.27 g cm^-3 (Kuwata et al., 1983).

Nucleic acid

LBVaV, Alopecurus myosuroides varicosavirus 1 (AMVV1; species Varicosavirus alopecuri) and red clover associated varicosavirus (RCaVV; species Varicosavirus trifolii) genomes consist 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 orchid fleck virus (OFV; species Dichorhavirus orchidaceae), 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.

Proteins

LBVaV 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 (Sasaya et al., 2004, Kormelink et al., 2011). LBVaV 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.

Genome organisation and replication

LBVaV RNA1 contains two genes in the order 3′-6-L-5′, while AMVV1 and RCaVV RNA1 contain one gene in the order 3′-L-5′; LBVaV RNA2 contains five genes in the order 3′-N-2-3-4-5-5′, whereas AMVV1 and RCaVV RNA2 contain three gene in the order 3′-N-2-4-5′ (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.

Biology

LBVaV and tobacco stunt virus (TStV; currently unclassified) are transmitted in soil and in hydroponic systems by zoospores of the chytrid fungus Olpidium virulentus (a noncrucifer strain of Olpidium brassicae) (Campbell 1962, Hiruki 1967, 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; species Ophiovirus mirafioriense, 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, Saudi Arabia and Australia (Navarro et al., 2005, Maccarone et al., 2010a, Alemzadeh and Izadpanah 2012, Umar et al., 2017).

Antigenicity

N does not induce a strong antibody response in laboratory animals. LBVaV is closely related serologically to TStV, which is currently unclassified but may be considered to be a strain of LBVaV (Kuwata and Kubo 1986, Sasaya et al., 2005).

Species demarcation criteria

Viruses assigned to different species within the genus Alphanucleorhabdovirus have several of the following characteristics: A) the nucleotide sequence identity of complete genomes is lower than 75%; B) they occupy different ecological niches as evidenced by differences in hosts and/or chytrid vectors; and C) they can be clearly distinguished in serological tests or by nucleic acid hybridisation.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.

* Coding region sequence incomplete