Genomic Diversity Among Populations of Two Citrus Viroids from Different Graft-Transmissible Dwarfing Complexes in Israel

The nucleotide sequences of citrus bent leaf viroid (CBLVd) (formerly designated CV-Ib) and citrus exocortis viroid (CEVd), both found among the citrus viroid (CVd) populations from five graft-transmissible dwarfing complexes (GTDCs) originating from different source plants and geographical locations in Israel, were determined. The sequence homology varied only slightly among the CBLVd sequence variants, i.e., 0-7 nucleotides (nts); originating from a single GTDC and 2-8 nts between CBLVds that were obtained from different GTDCs. The lowest level of homology between CBLVd variants obtained from citrus was 97.5%. Considerably larger variation (8-15 nucleotide changes) was observed between CBLVd variants derived from citrus and the type strain that was passed through avocado seedlings. The type strain differed from all variants in six positions, located at 38, 62, 138, 179, 264, and 268 nts. The CEVd sequence variants showed considerable heterogeneity. Five variants derived from four GTDCs differed only in 2-9 nts. Five other variants derived from two GTDCs showed 27-50 nucleotide changes compared with the first group of CEVd variants. The largest variation within a single GTDC of up to 41 nucleotide changes was observed between CEVd variants derived from GTDC-G. Using CEVd # 225 as a reference strain, most of the nucleotide changes occurred in the V, LT, and RT domains. Additional changes in the P domain were found only among CEVd isolates derived from GTDC-M and GTDC-G. The sequence homologies to CEVd # 225 ranged from 89-2 (CEVd-G 2 ) to 99-0% (CEVd-NG 1 )