Family: Partitiviridae

Genus: Alphapartitivirus

Distinguishing features

Members of the genus Alphapartitivirus infect plants, ascomycetous or basidiomycetous fungi. All members have two dsRNA segments of similar sizes that are individually encapsidated in separate particles. Alphapatitivirus genomes typically range from 3.6 to 3.9 kbp in total. 

Virion

Morphology

Based on negative-contrast electron microscopy, the virions of white clover cryptic virus 1 (WCCV1) are isometric and ~34 nm in diameter (Boccardo et al., 1985). The particles appear rounded and are usually penetrated by stain to give a ring-like appearance (Figure 1 Alphapartitivirus). Other reported alphapartitivirus particle sizes range from 25 to 40–50 nm in diameter (Chiba et al., 2013, Abou-Elnasr et al., 1985, Magae and Hayashi 1999, Osaki et al., 2002). 

Figure 1 Alphapartitivirus.  Negative-contrast electron micrograph of particles of an isolate of Alphapartitivirus trifolii. The bar represents 50 nm. (Ghabrial et al., 2000).

Physicochemical and physical properties

The buoyant density of WCCV1 virions in CsCl is 1.392 g cm⁻³ (Boccardo et al., 1985) and that of Vicia cryptic virus is 1.37 g cm⁻³ (Abou-Elnasr et al., 1985). 

Nucleic acid

The viral genome comprises two dsRNA segments which are individually 1.9–2.0 and 1.7–1.9 kbp. 

Proteins

There is a single major coat protein (CP) with predicted Mr of 51–57 kDa. The Mr of the RNA-dependent RNA polymerase (RdRP), as deduced from nucleotide sequence analysis, ranges from 68 to 73 kDa. Virion-associated RNA polymerase activity is present. 

Genome organization and replication

Each genome segment is monocistronic: the larger dsRNA segment typically encodes the RdRP and the smaller one encodes the CP. The alphapartitivirus Rosellinia necatrix partitivirus 2 has a third genome segment, which is a truncated version of the RdRP that may function as defective-interfering RNA (see GenBank record AB569999) (Chiba et al., 2013). A second version of dsRNA2, encoding seemingly full-length capsid protein has been reported for the related, unclassified Raphanus sativus cryptic virus 1. Cherry chlorotic rusty spot associated partitivirus has dsRNA satellites appearing not to encode any proteins (GenBank records AM749118–20). Alphapartitivirus dsRNA typically possess a poly(A) stretch which may be interrupted by other nucleotides at the 3′-end of the coding-strand of one or both genome segments. 

Biology

Members of the genus Alphapartitivirus infect ascomycetous, basidiomycetous fungi (ten species) or plants (four species). They are transmitted intracellularly during cell division (plants, fungi) or cell fusion and sporogenesis (fungi). Plant alphapartitiviruses include former members of the defunct genus Alphacryptovirus, which cause persistent infections. Various host effects have been described for fungal alphapartitiviruses: the related, unclassified Rhizoctonia solani partitivirus 2 causes hypovirulence in Rhizoctonia solani, a soil-borne basidiomycete fungus (Zheng et al., 2014), and Heterobasidion partitivirus 3 seems to reduce the growth rate of the basidiomycete fungus Heterobasidion abietinum at low temperatures (Vainio et al., 2010). The presence of Flammulina velutipes browning virus correlates with brown discoloration of the host fungus (Magae and Sunagawa 2010). 

Species demarcation criteria

The criteria to differentiate species in genus Alphapartitivirus are:

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

Related, unclassified viruses 

Virus names and virus abbreviations are not official ICTV designations. 

* Representative isolate of a virus with a complete published genome sequence

^‡ Segment encoding the larger version of CP