Sequence variability in the PCR amplified cDNAs from two citrus apscaviroid
isolates CVd-Ia and CVd-IIId from Spain, was analysed. CVd-IIId sequence w
as shown to be identical to previously described CVd-III sequences and no i
mportant variability was encountered within the viroid population. Converse
ly, CVd-Ia displayed population heterogeneity as shown by SSCP analysis, Sa
l I restriction site polymorphism and sequences of 27 CVd-Ia cloned DNAs. T
he CVd-Ia genomic heterogeneity is characterised by two major subpopulation
s with the most divergent sequences, and by the presence of individual vari
ants, making a sequence continuum between the two major groups. Most sequen
ce variations are clustered in the left part of the viroid molecule.