Circular ssRNA genomes between 294 and 342 nt depending on species and sequence variants. The most stable secondary structure is a rod-like conformation with five domains, a central conserved region (CCR) similar to that of members of the genera Pospiviroid and Cocadviroid, and the terminal conserved hairpin (TCH) (Figure 2. Pospiviroidae, Figure 3. Pospiviroidae). Besides the type species Hop stunt viroid, the genus Hostuviroid also includes the species Dahlia latent viroid, which only partially fulfils these criteria. Dahlia latent viroid (DLVd) originated from recombination events (Verhoeven et al., 2013) and has the terminal conserved region (TCR) instead of the TCH. By analogy with potato spindle tuber viroid (PSTVd), replication most likely occurs through an asymmetric rolling-circle model (Figure 1.Viroids).
Members of the genus Hostuviroids (hostuviroids) may have either a wide or restricted host range. Hop stunt viroid (HSVd) infects a very broad range of natural hosts and has been reported to cause several different diseases worldwide (Hataya et al., 2017). Conversely, the host range of DLVd is restricted to dahlia, in which no symptoms have been observed (Verhoeven et al., 2013). Hostuviroids are mainly transmitted by vegetative propagation of infected plants, by cutting tools and by mechanical inoculation. In grapevine HSVd is also seed-transmitted at a low rate (Wan Chow Wah and Symons 1999). HSVd has been detected in the pollen grains of cucumber plants although its involvement in the viroid transmission has not been proven (Castellano et al., 2016).
Species demarcation criteria
Viroids with molecular features (rod-like conformations, CCR, TCH) identical to that of members of the type species of the genus (or with TCR instead of TCH), less than 90% sequence identity and distinct biological properties with respect to the other members of the genus, are classified in different species (Di Serio et al., 2014).