Genetic variation and population structure of an isolate of Citrus exocortis viroid (CEVd) and of the progenies of two infectious sequence variants.

The population structure and diversity within a Citrus exocortis viroid (CEVd) isolate was estimated by single strand conformation polymorphism (SSCP) and sequence analysis. A predominant sequence variant (V1) representing 52.8% of the overall population was identified. V1 and other additional variants presented a composition of the P domain characteristic of severe strains of CEVd. The nucleotide diversity of this CEVd population was lower than expected according to a model of neutral evolution, suggesting a strong negative selection. Citron plants were inoculated with dimeric clones of nine sequence variants and two resulted infectious inducing the severe symptoms characteristic of the original isolate. De novo populations were generated from these infectious variants and like in the original CEVd isolate, both populations presented V1 as the predominant variant but they evolved to a higher nucleotide diversity.