Family: Avsunviroidae

Genus: Elaviroid

Distinguishing features

This genus includes only the species Eggplant latent viroid whose members have a circular RNA between 332 and 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 (López-Carrasco et al., 2016, Giguère et al., 2014). 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., 2003), ELVd replication is thought to proceed through a symmetric rolling-circle mechanism presumably occurring in plastids. 

Genome organization and replication

See discussion under family description. 

Biology

Found naturally in eggplant growing areas in eastern Spain. No symptoms 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. Seed-transmitted at a relatively high rate (16–26%). 

Derivation of names

The name of the genus derives from the type species, Eggplant latent viroid. 

Species demarcation criteria

Eggplant latent viroid is the only species of the genus Elaviroid reported so far. Viroids with similar molecular features (quasi-rod-like conformation and thermodynamically stable hammerhead ribozymes) but with less than 90% sequence identity and differential biological properties with respect to ELVd should be classified as different species.