Melon chlorotic spot virus (MeCSV) was isolated from a melon plant [Cucumis melo (L., 1753)] showing chlorotic spots and yellowing of the older leaves, and is assigned to Mechlorovirus cucumeris, the only species in the genus. The mechlorovirus genome has thirteen genes, these encoding two structural proteins, a large protein (L) and a nucleocapsid protein (N), and eleven non-structural proteins of unknown function. With eight genome segments, MeCSV has the most complex genome structure described so far for a member of the family Phenuiviridae. The apparent lack of a membrane-bound virus particle distinguishes mechloroviruses from some other viruses in the family Phenuiviridae. Based on well-supported Maximum Likelihood or Maximum Clade Credibility trees inferred from complete L protein sequences, viruses classified in the genus Mechlorovirus form a monophyletic cluster clearly distinguished from other phenuivirids (Gaafar et al., 2019, Lecoq et al., 2019).
Virions are thin filaments 2–2.5 nm in diameter with lengths (200–3,000 nm) proportional to the size of the encapsidated segmented genomic RNA. The filamentous particles may appear to be spiral-shaped, branched or panhandled. No envelope has been observed (Gaafar et al., 2019).
Nucleic acid and Protein
The mechlorovirus genome encompasses eight segments of negative-sense or ambisense RNA (Figure 1.Mechlorovirus). The terminal nucleotides of each segment occur in a canonical, conserved sequence (in coding sense) 5′-ACACAAAGUC…GACUUUGUGU-3′ and may form the panhandle structures typical of other members of the order Bunyavirales (Table 2.Phenuiviridae). RNA1 (9.1 kb) is a negative-sense RNA that encodes a protein with a predicted molecular mass of 341 kDa that includes a region homologous with the bunyaviral RNA-directed RNA polymerase (RdRP) domain. RNA2 (1.8 kb), RNA3 (1.6 kb), RNA5 (1.5 kb), RNA7 (1.5 kb), and RNA8 (1.4 kb) are all ambisense RNAs, each encoding two non-structural proteins. RNA4 (1.6 kb) and RNA6 (1.5 kb) are negative-sense RNAs and encode N (33 kDa, homologous with the tenuivirus/phlebovirus N domain) and a non-structural protein (46 kDa), respectively (Table 3.Phenuiviridae) (Gaafar et al., 2019, Lecoq et al., 2019).
Genome organization and replication
The mechlorovirus genome comprises eight RNA segments, three of which are negative-sense and five ambisense, collectively encoding two structural proteins, an RNA-directed RNA polymerase (RdRP) and N, and eleven non-structural proteins of unknown function (Figure 1.Mechlorovirus). Most plant viruses encode a viral movement protein (MP) to enable cell-to-cell movement in plant hosts and the viral suppressors of RNA silencing (VSR) that counteract plant antiviral defences. One or more of the mechlorovirus proteins may enable cell-to-cell movement in plant hosts and possibly interfere with RNA silencing. Details of virus replication are unknown (Lin et al., 2019, Kormelink et al., 2021).
|Figure 1.Mechlorovirus. Genome organization of a mechlorovirus. Coloured boxes depict ORFs that encode N, nucleocapsid protein and L, large protein. White boxes depict ORFs that encode non-structural proteins of unknown function.|
Mechloroviruses infects dicotyledonous plants of the families Cucurbitaceae, Fabaceae, and Solanaceae. Mechanical transmission using sap extracts is possible, but the vectors of the mechlorovirus are unknown (Gaafar et al., 2019, Lecoq et al., 2019).
Species demarcation criteria
Not defined as the genus currently includes only a single species.
Related, unclassified viruses
|Virus name||Accession number||Virus abbreviation|
|Ramu stunt virus||RNA1: KR094115; RNA2: KR094116; RNA3: KR698381; RNA4: KR094117; RNA5: KR094118; RNA6: KR094119||RmSV|
* Sequences do not comprise the complete genome segment.
Virus names and virus abbreviations are not official ICTV designations.