Host Effect on the Genetic Variability of Citrus exocortis viroid (CEVd)

Natural infections of Citrus exocortis viroid (CEVd) have been found in both citrus and non-citrus hosts. Field isolates of CEVd are complex populations of sequence variants (haplotypes) that fit a quasispecies model. We have previously shown that a CEVd isolate initially recovered from citrus and maintained in Etrog citron contained two predominant haplotypes with low nucleotide diversity. This CEVd infected citron was used as the inoculum source for viroid transmission to trifoliate orange and sour orange seedlings that have been growing in the field since their inoculation in 1993. CEVd recovered from trifoliate orange showing the characteristic symptoms of stunting, bark scaling and stem blotching was also characterized as a population of variants with low nucleotide diversity and containing a clearly predominant haplotype distinct from those identified in the inoculum source. CEVd recovered from sour orange, which remained symptomless, was characterized as a population with a higher nucleotide diversity in which no dominant haplotype could be identified. The tolerant viroid-host combination appeared to act as permissive host and maintained a greater CEVd variability than sensitive hosts. Similarly, a CEVd isolate recovered from a symptomless broad bean plant was characterized as a diverse population that lacked a predominant haplotype. When this CEVd source was mechanically transmitted to tomato and from tomato back to broad bean, the plants displayed stunting symptoms, and the CEVd population exhibited a predominant haplotype and low nucleotide diversity