ISOLATES OF CITRUS-EXOCORTIS VIROID RECOVERED BY HOST AND TISSUE SELECTION

Isolates of citrus exocortis viroid (CEV) from a single sweet orange c itrus source have been selected by sequential passage through the alte rnative hosts citron, Gynura aurantiaca, a hybrid tomato Lycopersicon esculentum x L. peruvianum, and from disorganized callus culture of th e hybrid tomato. The distinctions in symptom expression, titre and ele ctrophoretic mobility among the CEV isolates, operationally termed CEV c (citron), CEVg (Gynura), CEVt (tomato) and CEVcls (callus) are suppo rted by characteristically different nucleotide sequences. The nucleot ide sequence of full-length cDNA clones of CEVc purified from citron s hows exchanges not reported for any previously described CEV variant. An unusual number of exchanges have been localized in the terminal dom ains of all the isolates analysed here. A common pattern of nucleotide exchanges, described as a 'tomato signature', can be detected in all of the isolates derived from hybrid tomato tissues.