Family: Avsunviroidae
Genus: Avsunviroid
Distinguishing features
This genus includes only the species Avocado sunblotch viroid, whose members have a genome of 246 to 251 nt that is unique in having a base composition low in G+C (38%) in contrast with that of the other viroids in the family that are rich in G+C (53–60%). The most stable secondary structure of avocado sunblotch viroid (ASBVd) in vivo is a rod-like conformation (López-Carrasco and Flores 2017). It is soluble in 2 M LiCl, resembling in this respect the members of the family Pospiviroidae that also adopt rod-like or quasi-rod-like secondary structures in vitro and in vivo (Giguère and Perreault 2017, López-Carrasco and Flores 2017). ASBVd (+) and (-) strands can form hammerhead structures. Because both single hammerhead structures of ASBVd are thermodynamically unstable, double hammerhead structures have been proposed to operate in the self-cleavage reactions, especially in that of the (+) polarity RNA (Hutchins et al., 1986, Forster et al., 1988) (Figure 2. Avsunviroidae). Replication occurs through a symmetric rolling-circle mechanism, since the (-) circular monomeric strand has been identified in infected tissue. ASBVd replicates and accumulates in plastids, mostly chloroplasts.
Genome organization and replication
See discussion under family description.
Biology
Found naturally only in avocado, ASBVd can be experimentally transmitted to several other members of the family Lauraceae (Da Graça and Van Vuuren 1981). ASBVd is transmitted by seed, grafting, and mechanical means. ASBVd has been reported to occur in USA, Mexico, Peru, Venezuela, Australia, Israel, Spain, Ghana and South Africa (Kuhn et al., 2017). Different ASBVd variants have been associated with different symptoms related to sunblotch syndrome (Semancik and Szychowski 1994).
Derivation of names
The name of the genus derives from the type species, Avocado sunblotch viroid.
Species demarcation criteria
Avocado sunblotch viroid is the only species of the genus Avsunviroid reported so far. Viroids with similar molecular features (low G+C content, a rod-like conformation and thermodynamically unstable hammerhead ribozymes,) but with less than 90% sequence identity and differential biological properties with respect to ASBVd should be classified as different species.