Figure 5.

(A) The central domain (containing the CCR, boxed by the broken red line) of PSTVd and by extension of all members of the family Pospiviroidae, can be considered as a replicative module (blue box), likely derived from a protoviroid generated in the RNA world, that may have originated the current viroids through discontinuous transcription mediated by a jumping RNA polymerase and by further adaptation to the respective hosts. The five domains reported in PSTVd (Keese and Symons 1985) are indicated: TL: terminal left; P, pathogenic; C, central; V, variable; TR, terminal right. (B) Conserved domains (TCR, TCH, and CCR) shared by members of the family Pospiviroidae are reported. Members of the genera Apscaviroid, Coleviroid and Pospiviroid generally contain the TCR, while those of the genera Cocadviroid and Hostuviroid generally contain the TCH.