Genus: Betapartitivirus

Distinguishing features

Members of the genus Betapartitivirus infect plants and fungi. Betapartitiviruses have two dsRNA segments that are believed to be individually encapsidated in separate particles, and genomes typically ranging from 4.3 - 4.8 kbp in total.

Virion

Morphology

Based on negative-contrast electron microscopy, the virions of white clover cryptic virus 2 are isometric and approximately 38 nm in diameter [{Boccardo et al., 1985:18640558RJOHTXBoccardo et al., 1985, Three seedborne cryptic viruses containing double-stranded RNA isolated from white clover, Virology, 147, 1, 29-40}]. The particles appear rounded and are not penetrated by stain (Figure 1.Betapartitivirus). The structure of Fusarium poae virus 1 has been determined by electron cryomicroscopy and 3D reconstruction [{Tang et al., 2010:20643089RJOHTXTang et al., 2010, Backbone trace of partitivirus capsid protein from electron cryomicroscopy and homology modeling, Biophys J, 99, 2, 685-94}]. The outer diameter of the capsid is about 40 nm, and each particle contains 120 CP molecules arranged with icosahedral symmetry [{Tang et al., 2010:20643089RJOHTXTang et al., 2010, Backbone trace of partitivirus capsid protein from electron cryomicroscopy and homology modeling, Biophys J, 99, 2, 685-94}]. Other reported betapartitivirus particle sizes range from 25 to 35 nm in diameter [{Strauss et al., 2000:10644855RJOHTXStrauss et al., 2000, Molecular characterization of the genome of a partitivirus from the basidiomycete Rhizoctonia solani, Journal of General Virology, 81, Pt 2, 549-55RJOMREFLim et al., 2005:15681061RJOHTXLim et al., 2005, Complete nucleotide sequence and genome organization of a dsRNA partitivirus infecting Pleurotus ostreatus, Virus Research, 108, 1-2, 111-9RJOMREFSasaki et al., 2005:15750865RJOHTXSasaki et al., 2005, Molecular characterization of a partitivirus from the plant pathogenic ascomycete Rosellinia necatrix, Archives of Virology, 150, 6, 1069-83RJOMREFLi et al., 2009:19267226RJOHTXLi et al., 2009, A novel double-stranded RNA virus detected in Primula malacoides is a plant-isolated partitivirus closely related to partitivirus infecting fungal species, Archives of Virology, 154, 4, 565-72RJOMREFZiegler et al., 2012:22075921RJOHTXZiegler et al., 2012, Complete sequence of a cryptic virus from hemp (Cannabis sativa), Archives of Virology, 157, 2, 383-5}].

Figure 1.Betapartitivirus. Negative-contrast electron micrograph of particles of an isolate of White clover cryptic virus 2. The bar represents 50 nm. [{Ghabrial et al., 2000:RJOHTXGhabrial et al., 2000, Partitiviridae, Virus Taxonomy: Seventh Report of the International Committee on Taxonomy of Viruses, 503-13}].

Physicochemical and physical properties

Virion buoyant density of white clover cryptic virus 2 in CsCl is 1.375 g cm⁻³ [{Boccardo et al., 1985:18640558RJOHTXBoccardo et al., 1985, Three seedborne cryptic viruses containing double-stranded RNA isolated from white clover, Virology, 147, 1, 29-40}].

Nucleic acid

Viral genome comprises two dsRNA segments, which are each about 2.1-2.4 kbp. 

Proteins

There is a single major CP with predicted Mr of 71-77 kDa. The predicted Mr of the RdRP, as deduced from nucleotide sequence analysis, ranges from 77 to 87  kDa. Virion-associated RNA polymerase activity is present.

Genome organization and replication

Each genome segment is monocistronic: the larger genome segment typically encodes the RdRP and the smaller one encodes the putative CP. Atkinsonella hypoxylon virus has a third dsRNA segment appearing not to encode any proteins (see GenBank record L39127) [{Oh and Hillman 1995:7782774RJOHTXOh and Hillman 1995, Genome organization of a partitivirus from the filamentous ascomycete Atkinsonella hypoxylon, Journal of General Virology, 76 6, 1461-70}]. Members of genus Betapartitivirus commonly have poly(A) tracts (≥ 10 nt) near the coding-strand 3´-terminus of one or both genome segments.

Biology

Members of the genus Betapartitivirus infect ascomycetous and basidiomycetous fungi (ten species) and plants (seven species). They are transmitted intracellularly during cell division (plants, fungi) or cell fusion and sporogenesis (fungi). Plant betapartitiviruses include former members of the defunct genus Betacryptovirus, which are associated with persistent infections [{Lesker et al., 2013:23584419RJOHTXLesker et al., 2013, Molecular characterization of five betacryptoviruses infecting four clover species and dill, Archives of Virology, 158, 9, 1943-52}]. Various host effects have been described for fungal betapartitiviruses: Rosellinia necatrix virus 1 mediates changes in host colony morphology and decreased dispersal via conidiospores [{Kanematsu et al., 2010:20701490RJOHTXKanematsu et al., 2010, Extending the fungal host range of a partitivirus and a mycoreovirus from Rosellinia necatrix by inoculation of protoplasts with virus particles, Phytopathology, 100, 9, 922-30}], and the related, unclassified Sclerotinia sclerotiorum partitivirus 1 reduces the virulence of its host [{Xiao et al., 2014:24965462RJOHTXXiao et al., 2014, A novel partitivirus that confers hypovirulence on plant pathogenic fungi, Journal of Virology, 88, 17, 10120-33}].

Species demarcation criteria

The species demarcation criteria within genus Betapartitivirus are:

• ≤ 90% aa sequence identity in the RdRP, and
• ≤ 80% aa sequence identity in the CP

Member Species

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.
Download GenBank/EMBL query for sequences listed in the table here.

Related, Unclassified Viruses

* Representative isolate of a virus with a complete published genome sequence
^‡ Defective-interfering RNAs derived from dsRNA1 deposited under GenBank accessions LC062720-27