Family: Chrysoviridae

Genus: Alphachrysovirus

Distinguishing features

Members of the genus Alphachrysovirus infect ascomycetous fungi and have been found associated with plants and insects. 

Virion

Nucleic acid

Members have three or four dsRNA genome segments ranging from 2.5 to 3.7 kbp, as exemplified by Penicillium chrysogenum virus (PcV) (Figure 2. .Chrysoviridae), whose segments are 2.9–3.6 kbp (Table 1. Alphachrysovirus) (Jiang and Ghabrial 2004). The genome comprises 8.9–13.1 kbp in total. The dsRNA segments are individually encapsidated in separate particles. 

Proteins

Genome organization and replication

Protein database searches reveal that PcV-P3 sequence shares a “phytoreo S7 domain” with a protein family consisting of several phytoreovirus P7 proteins that are known to be virus core proteins (Jiang and Ghabrial 2004, Liu et al., 2012). Interestingly, the N-terminal regions of PcV-P3 (and the corresponding alphachryso-P3 proteins of other alphachrysoviruses) share significant sequence similarity with comparable N-terminal regions of the RNA-directed RNA polymerase (RdRP) encoded by dsRNA1 of alphachrysoviruses (Jiang and Ghabrial 2004, Liu et al., 2012). Furthermore, HHPred (Soding et al., 2005) predicts a separate P-loop NTPase domain near the N terminus of alphachryso-P1, which overlaps the region of alphachryso-P1 that is represented by homologous sequences in alphachryso-P3. The P7/P-loop NTPase domain (Pathak et al., 2014) in alphachryso-P1 is independent (non-overlapping) of the RdRP domain in alphachryso-P1. Thus, alphachryso-P1 has at least two distinct enzymatic activities, RdRP and NTPase, mediated by at least two different domains. The PcV-P4 (and comparable proteins of other alphachrysoviruses) contains the motifs that form the conserved core of the ovarian tumour gene-like superfamily of predicted cysteine proteases (Covelli et al., 2004). HHPred also shows that alphachryso-P4 has a region of homology with alphachryso-P2/capsid protein. All of these motifs/homologies are shared by all alphachrysoviruses with quadripartite genomes (quadrichrysoviruses). Trichrysoviruses, which mostly infect plants, lack a PcV-P3 homolog; otherwise the virus-encoded proteins share the same motifs in the same relative positions as in the homologous proteins of fungal chrysoviruses with quadripartite genomes. 

Table 1. Alphachrysovirus.  Genome content and encoded proteins of Penicillium chrysogenum virus (PcV)

^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. 

Biology

Trichrysoviruses are mostly plant viruses (Figure 3. Chrysoviridae) (Zhang et al., 2017, Li et al., 2013, Zhong et al., 2016, Peyambari et al., 2019). More specifically, five out of six known trichrysoviruses infect plants. Only Colletotrichum gloeosporioides chrysovirus 1 (CgCV1) infects a plant pathogenic fungus (Zhang et al., 2017). Interestingly, CgCV1clusters with the fungal quadrichrysoviruses, but not with the plant trichrysoviruses. 

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 the demarcation of genetically closely related viruses that nevertheless belong to different species.