Family: Chrysoviridae

Genus: Betachrysovirus

Distinguishing features

Members of the genus Betachrysovirus infect ascomycetous or basidiomycetous fungi. 

Virion

Nucleic acid

Betachrysoviruses have four, five and in one case seven (Zhai et al., 2018) dsRNA genome segments ranging from 0.8 to 3.7 kbp, as exemplified by Botryosphaeria dothidea chrysovirus 1 (BdCV1) (Figure 2. .Chrysoviridae), with segments of 2.9–3.7 kbp (Table 1. Betachrysovirus). The genome comprises 10.9–16.0 kbp in total. The dsRNA segments are individually encapsidated in separate particles. 

Proteins

Virion proteins include the RNA-directed RNA polymerase (RdRP; P1), and the virus capsid protein (CP, often P2 but in some case P3 or P4).  The capsids of the cinquechrysoviruses (those with five dsRNA segments) appear to contain more than one type of structural protein. 

Genome organization and replication

Although the RNA-directed RNA polymerase (RdRP) encoded by betachrysoviruses has the P7/ P-loop NTPase domain at the N-terminus as well as the RdRP conserved motifs, the other virally encoded proteins lack the motifs identified in alphachrysoviruses (e.g. motifs of predicted cysteine proteases). Furthermore, the capsids of the cinquechrysoviruses (those with five dsRNA segments) appear to contain more than one structural protein. The functions of betachryso-P3 and betachryso-P4 are unknown and there is no similarity between the betachryso-P3 and the N-terminus of the RdRP. The proteins encoded by dsRNA5 are non-homologous between members of different species and their function is also unknown. It is noteworthy that dsRNA5 is dispensable for replication of Magnaporthe oryzae chrysovirus 1-B as stable derivatives with 4 dsRNA segments can be readily obtained via sub-culturing (Urayama et al., 2014). 

Table 1. Betachrysovirus.  Genome content and encoded proteins of Botryosphaeria dothidea chrysovirus 1 (BdCV1)

^a Deduced from sequence of full-length cDNA clones
^b Predicted from aa sequence encoded by full-length cDNA; the numbers of aa residues is given in parentheses. 

Species demarcation criteria

Species demarcation criteria:

• Host of isolation
• Nucleotide and deduced amino acid sequence data (≤ 70% and ≤ 53% aa sequence identity in the RdRP and CP, respectively)
• length of dsRNA genome segments
• length of 5′-UTR

Species demarcation considers a combination of each of the criteria listed above. While nucleotide and amino acid sequence relatedness are important criteria for species demarcation, the other listed criteria may be useful in demarcation of genetically closely related viruses that nevertheless belong to different species.