Family: Avsunviroidae

Genus: Pelamoviroid


Distinguishing features

Three species, Peach latent mosaic viroid, Chrysanthemum chlorotic mottle viroid,and Apple hammerhead viroid are included in this genus, whose members have circular genomic RNAs between 337 and 434 nt depending on the isolates and sequence variants. The most stable secondary structure is a branched conformation (Ambrós et al., 1998), stabilized by a kissing-loop interaction in the (+) strand (Bussière et al., 2000, Gago et al., 2005, Serra et al., 2018) (Figure 1. Avsunviroidae). Similar structures have been proposed to occur both in vitro and in vivo (Gago et al., 2005, Giguère et al., 2014, Serra et al., 2018). An additional element of tertiary structure has been reported in the (+) strand of Peach latent mosaic viroid (PLMVd) (Hernández et al., 2006). RNAs are insoluble in 2 M LiCl. Thermodynamically stable single-hammerhead structures can be formed in both strands, which self-cleave in vitro and in vivo as predicted by these structures (Figure 2. Avsunviroidae). Replication occurs by a symmetric rolling-circle mechanism since the monomeric circular (-) RNA has been identified in infected tissue. PLMVd and, most likely, chrysanthemum chlorotic mottle viroid (CChMVd) and apple hammerhead viroid (AHVd) replicate in plastids, mostly chloroplasts. 

Genome organization and replication

See discussion under family description


PLMVd, AHVd and CChMVd appear to be restricted to their natural hosts, but some reports indicate that PLMVd naturally infects additional species within and outside the genus Prunus (Kyriakopoulou et al., 2017). PLMVd and CChMVd  have been recorded in many peach- and chrysanthemum-growing areas, respectively. Most isolates of PLMVd are non-symptomatic or incite symptoms of leaf chlorosis in peach trees (Flores et al., 2006), but others containing a specific pathogenic determinant incite an extreme albinism (peach calico) (Malfitano et al., 2003, Rodio et al., 2007) by silencing a host mRNA via two 21-nt viroid-derived small RNAs resembling functionally host microRNAs (Navarro et al., 2012). Symptomatic and non-symptomatic isolates of CChMVd differ in the sequence of a hairpin tetraloop, wherein the pathogenic determinant has been mapped (de la Peña et al., 1999). Association with symptoms of AHVd is unclear {Serra, 2018 #3810}. 

Derivation of names

The name of the genus derives from the type species, Peach latent mosaic viroid

Species demarcation criteria

Peach latent mosaic viroidChrysanthemum chlorotic mottle viroid and Apple hammerhead viroid are three species of the genus Pelamoviroid. Viroids with similar molecular features (multi-branched conformations stabilized by a kissing-loop interaction in the (+) strands, and thermodynamically stable hammerhead ribozymes) but with less than 90% sequence identity and differential biological properties should be classified as different species.