Abstract:
Geminivirus isolates associated with the epidemic of
severe cassava mosaic disease in Uganda were
studied and compared with virus isolates from the
part of Uganda outside the epidemic area, and with
African cassavamosaic virus (ACMV) andEast African
cassava mosaic virus (EACMV). Isolates of a novel
type [the Uganda variant (UgV)] were detected in
severely affected plants from the epidemic area,
whereas those from plants outside the epidemic
area were typical of ACMV. The complete nucleotide
sequences of DNA-A of UgV (2799 nt) and of a
Tanzanian isolate of EACMV (2801 nt) were determined and are extremely similar, except for the
coat protein (CP) gene. The CP gene of UgV has
three distinct regions: the 5« 219 nt are 99%
identical to EACMV (only 79% to ACMV); the
following 459 nt are 99% identical to ACMV (75% to EACMV); and the 3« 93 nt are 98% identical to
EACMV (76% to ACMV). UgV DNA-A therefore is
considered to have arisen by interspecific recombination of EACMV and ACMV. Despite the hybrid
nature of their CP, UgV isolates were indistinguishable from ACMV in tests with 20 monoclonal antibodies (MAbs), including seven which reacted with
ACMV but not EACMV. The discontinuous epitopes
detected by these seven MAbs must involve amino
acids which lie in the central part of the CP (residues
74–226) and which differ in ACMV and EACMV. UgV
isolates were detected in severely mosaic-affected
plants from all 11 widely separated locations
sampled. The probable role of recombination in
geminivirus evolution in the short to medium term is
discussed.