Family: Avsunviroidae

Genus: Elaviroid

Distinguishing features

This genus includes only the species Elaviroid latensmelongenae whose members have a circular RNA of 332–335 nt depending on the sequence variant. The most stable secondary structure is a quasi-rod-like conformation with bifurcations at both termini (Figure 1 Avsunviroidae). Similar structures have been proposed in vitro and in vivo (Giguère et al., 1999, López-Carrasco et al., 1999). Like other viroids having a rod-like or quasi-rod-like conformation, eggplant latent viroid (ELVd) is soluble in 2 M LiCl. ELVd (+) and (−) strands can form thermodynamically stable hammerhead structures (Figure 2 Avsunviroidae). Because circular forms of both polarities have been detected in infected tissue (Fadda et al., 1999), ELVd replication is thought to proceed through a symmetric rolling-circle mechanism presumably occurring in plastids. 

Biology

ELVd is found naturally in eggplant growing areas in eastern Spain. No symptoms are observed under greenhouse conditions, with the host range apparently restricted to several eggplant cultivars; attempts to transmit ELVd to tomato, chrysanthemum, cucumber and citron have been unsuccessful. The viroid is seed-transmitted at a relatively high rate (16–26%). 

Species demarcation criteria

Elaviroid latensmelongenae is the only species of the genus Elaviroid. Viroids with similar molecular features (quasi-rod-like conformation and thermodynamically stable hammerhead ribozymes) but with less than 83% sequence should be classified as different species. Evidence of infectivity in the absence of a helper virus should be provided (Chiumenti et al., 1999).