Family: Pospiviroidae

Genus: Cocadviroid

Distinguishing features

Genomes are circular ssRNA genomes ranging from 248–364 nt depending on species and sequence variant. The most stable secondary structure is a rod-like or quasi-rod-like conformation with five domains, a central conserved region (CCR) similar to that of members of the genera Pospiviroid and Hostuviroid, and the terminal conserved hairpin (TCH) (Figure 3 Pospiviroidae). Single or double cytosine residues occur at a specific locus in the genomic RNA, whose T_R domain in members of the species Cocadviroid cadangi is variable in length as a result of 41-, 50- or 55-nt duplications that generate larger variants of 287–301 nt (Haseloff et al., 1982). As disease progresses from the early to the mid stages, the small genome forms are replaced by the large forms. Dimeric forms of coconut cadang-cadang viroid (CCCVd) coexist with their corresponding monomeric forms. Replication most probably occurs through an asymmetric rolling-circle model by analogy with potato spindle tuber viroid (PSTVd) (Figure 1 Viroids). At the subnuclear level, CCCVd distributes throughout the nucleoplasm but it mainly is concentrated in the nucleolus (Bonfiglioli et al., 1994).

Biology

Members of the genus Cocadviroid infect monocots (CCCVd; coconut tinangaja viroid, CTiVd) or dicots (hop latent viroid, HLVd; citrus bark cracking viroid, CBCVd). CCCVd and CTiVd only infect members of the family Arecaceae (Vadamalai et al., 2017). The geographic distribution of CCCVd includes the Philippines, Malaysia and Sri Lanka, and CTiVd appears restricted to Guam. HLVd and CBCVd have broader geographic distributions (Lavagi et al., 2017, Vadamalai et al., 2017). The natural host range of HLVd is less restricted than previously thought. It includes commercial hop, Japanese hop (Humulus japonicus) and Urtica dioica (Lavagi et al., 2017) but also industrial and medicinal cannabis (Bektaş et al., 2019, Warren et al., 2019). CBCVd has a broader natural host range that includes citrus species and hops (Lavagi et al., 2017) and appears to be a natural chimera containing sequence regions related to HSVd and CEVd (Puchta et al., 1991). Low rates of CCCVd transmission by seed and pollen transmission have been reported (Pacumbaba et al., 1994), while HLVd is neither seed nor pollen transmitted (Matoušek and Patzak 2000, Matoušek et al., 2008). CCCVd and CTiVd infections may be lethal to coconut palm (Wall and Randles 2003, Rodriguez et al., 2017). CBCVd causes bark cracking in citrus plants, and more recently it has been found to induce a severe disease in hop (Jakse et al., 2015).

Species demarcation criteria

Viroids with rod-like conformation, with the TCH, with the CCR identical to that of members of the species in the genus and with less than 79% sequence identity are classified in different species. For viroids with pairwise identity scores close to 79%, evidence of distinct biological properties should be provided (Chiumenti et al., 2021).