Summary auto-generated
Researchers investigated a severe cassava mosaic disease epidemic in Uganda that emerged around 1988 and devastated crops. They identified a novel geminivirus variant (UgV) consistently present in severely affected plants from the epidemic region, while plants outside the epidemic area contained African cassava mosaic virus (ACMV). Complete nucleotide sequencing of the DNA-A component revealed that UgV is most closely related to East African cassava mosaic virus (EACMV) overall (92% identity), but its coat protein gene shows a mosaic structure: the 5' region (219 nt) is 99% identical to EACMV, the central region (459 nt) is 99% identical to ACMV, and the 3' region (93 nt) is 98% identical to EACMV. This pattern indicates that UgV arose through interspecific recombination between EACMV and ACMV. Despite its hybrid genetic structure, UgV isolates were serologically indistinguishable from ACMV in tests using 20 monoclonal antibodies. The researchers detected UgV in severely affected plants from 11 widely separated locations across Uganda. This study provides the first clear-cut evidence of recombination occurring between two recognized geminivirus species and suggests recombination may be an important factor in geminivirus evolution.
Key findings
- A novel geminivirus variant (UgV) associated with severe cassava mosaic disease in Uganda was identified, with DNA-A structure arising from interspecific recombination between EACMV and ACMV
- The UgV coat protein gene contains three distinct segments with alternating sequence identity to either EACMV or ACMV, providing strong evidence of recombination between the two parent virus species
- Despite carrying a hybrid coat protein, UgV isolates were serologically identical to ACMV in all monoclonal antibody tests, indicating that critical epitopes are located in ACMV-like regions
- UgV was consistently detected in severely affected plants from 11 geographically dispersed locations across Uganda, while ACMV-type viruses were found in less severely affected plants outside the epidemic region
- This study demonstrates the first clear-cut example of interspecific recombination in geminiviruses, with implications for understanding viral evolution and emergence of new geminivirus forms
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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 cassava mosaic virus (ACMV) and East 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.