Family: Yadokariviridae

Genus: Betayadokarivirus


Distinguishing features

Members of the genus Betayadokarivirus form a distinct lineage to that of alphayadokariviruses in phylogenetic analysis of the RdRP protein.



The known capsid donors for members in the genus Betayadokarivirus include Rosellinia necatrix megabirnavirus 3 (RnMBV3, family Megabirnaviridae), the unclassified virus Rosellinia necatrix megatotivirus 1 (RnMTV1), and Sclerotinia sclerotiorum botybirnavirus 3 (SsBV3, genus Botybirnavirus), which partner with yado-kari virus 3 (YkV3), yado-kari virus 4a and yado-kari virus 4b (YkV4a and YkV4b) or Sclerotinia sclerotiorum yadokarivirus 1 (SsYkV1), respectively (Jia et al., 2022, Sato et al., 2022). The heterocapsids show a non-enveloped spherical form of about 50 nm in diameter.

Physicochemical and physical properties

Unlike the alphayadokarivirus yado-kari virus 1 (YkV1), trans-encapsidated betayadokariviruses (YkV3, YkV4a, and SsYkV1) are always co-fractionated with their respective capsid donors by CsCl or sucrose density gradient centrifugation. No fraction exclusively containing only betayadokarivirus dsRNA is observed (Jia et al., 2022, Sato et al., 2022). Instead, fractions containing virions with dsRNA of only the partner virus or both dsRNAs of the betayadokarivirus and its partner virus are obtained.

Nucleic acid

Heterocapsids of the betayadokariviruses YkV3, YkV4a, and SsYkV1 encase their replicative form dsRNA, like the alphayadokariviruses (YkV1 and Aspergillus foetidus slow virus (AfSV2). In contrast to the alphayadokariviruses, however, the ratio of dsRNA of the betayadokariviruses relative to that of the capsid donors appears less than one in the total virus particles fraction, based on the estimation of band intensity by agarose gel electrophoresis (Jia et al., 2022, Sato et al., 2022).


YkV3 and YkV4a dsRNAs are hetero-encapsidated in the capsid protein (CP) encoded by RnMBV3 and RnMTV1, respectively (Sato et al., 2022). The CP of RnMTV1 is produced by cleavage of a polyprotein encoded by the 5′-proximal ORF. The capsid of SsYkV1 is composed of two structural proteins separately encoded by the two genomic segments with two CP units each derived from different genomic segments of the bipartite botybirnavirus SsBV3 (Sato et al., 2022). Heterocapsids of SsYkV1 contain its encoded RdRP, as also observed for the alphayadokarivirus YkV1 (Jia et al., 2022).

Genome organization and replication

Like alphayadokariviruses, most betayadokariviruses have a monocistronic genome (Osaki et al., 2016, Arjona-Lopez et al., 2018, Velasco et al., 2019, Jia et al., 2022). However, YkV4a (species Betayadokarivirus yobani) appears to have a bicistronic genome that encodes two proteins lacking the 2A-like cleavage site (Arjona-Lopez et al., 2018). The 5′-proximal ORF encodes an RdRP, while the 3′-proximal ORF encodes a hypothetical protein with unknown function which is homologous to the proteins of other yadokarivirids cleaved by the 2A-like peptide from the polyprotein. It remains unknown whether and how the 3′-proximal ORF is expressed or whether it is required for replication.
The majority of the betayadokariviruses have a poly(A) tail at the 3′-termini of their genome (Arjona-Lopez et al., 2018, Velasco et al., 2019, Jia et al., 2022). It is unknown whether Fusarium poae mycovirus 2 (FpMyV2) has a poly(A) tail (Osaki et al., 2016). Heterogeneity at the 5′-termius has been reported for yado-kari virus 2 (YkV2), YkV3, YkV4a and YkV4b (Arjona-Lopez et al., 2018). For YkV2, YkV3 and YkV4a, the 5′-terminal nucleotide is C or U, while for YkV4b it is C or A.
The replication mechanism of betayadokariviruses is assumed to be the same as that of alphayadokariviruses (see the descriptions for the family Yadokariviridae), though there is less experimental support. For several betayadokariviruses, replication does not occur in the absence of their respective capsid donors (Sato et al., 2022). In addition to the linear genomic RNA, a circular form has been reported for the unclassified virus SsYkV1 (Jia et al., 2022).


Betayadokariviruses and a closely related unassigned virus have been found in four ascomycetous fungal species, namely Rosellinia necatrix, Fusarium poae, Entoleuca sp., and Sclerotinia sclerotiorum, collected from Spain, Japan or China (Osaki et al., 2016, Arjona-Lopez et al., 2018, Velasco et al., 2019, Jia et al., 2022). The biological roles of have been investigated for YkV3, YkV4a, YkV4b, and the unassigned virus SsYkV1. As mentioned in the Biology section for the family Yadokariviridae, YkV4a has a harmful or beneficial relationship with its partner dsRNA virus or a host fungus, respectively (Jia et al., 2022). No effects of YkV3 and SsYkV1 on their respective partner viruses and host fungi have been observed (Jia et al., 2022, Sato et al., 2022).

Species demarcation criteria

· Nucleotide and deduced amino acid sequence (≤ 70% RdRP amino acid sequence identity between members of distinct species)
· Genome organization (e.g., coding strategy)
The genus Betayadokarivirus includes four species, encompassing six viruses.

Related, unclassified viruses

Virus name Accession number Virus abbreviation
Sclerotinia sclerotiorum yadokarivirus 1 MZ867703 SsYkV1

Virus names and abbreviations are not official ICTV designations