Chapter Version: ICTV Ninth Report; 2009 Taxonomy Release
Since only one genus is currently recognized, the family description corresponds to the genus description.
Type species Botrytis virus F
Virions are flexuous filaments of 720 nm modal length and about 13 nm in diameter.
Physicochemical and physical properties
Virions contain a single molecule of linear ssRNA, 6827 nt in length, excluding the 3’-poly(A) tail.
The only structural protein is the coat protein composed of 302 aa (32 kDa).
Genome organization and replication
The genomic RNA comprises two major ORFs on the positive strand, a 5’-UTR of 63 nt and a 3’-UTR of 71 nt, followed by a poly(A) tail (Figure 1). ORF1 contains conserved methyltransferase and helicase motifs, terminating with an opal stop codon (UGA) and yielding a protein of 153 kDa. Readthrough of this codon is expected to extend the protein to 212 kDa that then also includes an RNA-dependent RNA polymerase (RdRp) domain. ORF2 encodes the 32 kDa coat protein.
The virus was discovered infecting an isolate of the plant pathogenic fungus Botrytis cinerea. Its mode of transmission is unknown. The same fungal isolate was also infected with a virus now classified as Botrytis virus X (genus Botrexvirus, family Alphaflexiviridae).
Species demarcation criteria in the genus
List of species in the genus Mycoflexivirus
Botrytis virus F
Botrytis virus F-New Zealand:Auckland
Species names are in italic script; names of isolates are in roman script. Sequence accession numbers [ ] and assigned abbreviations ( ) are also listed.
List of other related viruses which may be members of the genus Mycoflexivirus but have not been approved as species
List of unassigned species in the family Gammaflexiviridae
Similarity with other taxa
There are clear phylogenetic relationships in both ORFs to other viruses in the family Tymovirales. Nearly all these are plant-infecting viruses and many of them also have flexuous virions (which is unusual for a mycovirus). This may suggest that the virus was originally acquired by a filamentous fungus from its plant host but has subsequently lost the cell-to-cell movement protein(s) characteristic of plant viruses. A separate family for this genus is justified by phylogenetic analyses of the order and also because of the readthrough translation strategy of the replication protein, a strategy that has not been found elsewhere in the order.
Derivation of names
Flexi: from flexus, Latin for “bent”.
Mycoflexi: from mycovirus with flexuous virions.
Howitt et al., 2001 R.L.J. Howitt, R.E. Beever, M.N. Pearson, R.L.S. Forster, Genome characterization of Botrytis virus F, a flexuous rod-shaped mycovirus resembling plant “potex-like” viruses. J. Gen. Virol. 82 (2001) 67–78.
Martelli et al., 2007 G. Martelli, M.J. Adams, J.F. Kreuze, V.V. Dolja, Family Flexiviridae: a case study in virion and genome plasticity. Annu. Rev. Phytopathol. 45 (2007) 73–100.
Adams, M.J., Kreuze, J.F. and Pearson, M.N.
Figure 1 Genome organization of Botrytis virus F showing the relative positions of the ORFs and their expression products. Mtr, methyltransferase; Hel, helicase; RdRp, RNA-dependent RNA polymerase; CP, capsid protein. The arrow marks the leaky opal stop codon in ORF1.