Molecular Epidemiology of Begomoviruses That Infect Vegetable Crops in Southwestern Cameroon

Begomoviruses are plant-infecting viruses, which are transmitted by the whitefly vector Bemisia tabaci. They have a genome of single-stranded DNA that consists of either a single (monopartite) or two components (bipartite) with a component size of approximately 2.8 kb. Many monopartite begomoviruses in the Old World have been found to be associated with betasatellite and alphasatellite molecules, which are about half the size of their helper begomovirus genome. Betasatellites have been shown to be necessary for inducing severe disease symptoms. In Cameroon, B. tabaci has been associated with suspected begomovirus infections in many crop and weed species. Despite their growing importance, only begomoviruses infecting cassava have been studied in Cameroon in any detail. Thus, there was a need for additional information on diversity and distribution of begomoviruses and satellites in vegetable crops and dictyledonous weeds, which likely serve as virus reservoirs. In field studies carried out in this study, a high incidence of okra leaf curl disease was found in Cameroon. Sequencing of viral genomes showed that the okra plants were infected by viruses of two previously known begomovirus species (Cotton leaf curl Gezira virus and Okra yellow crinkle virus) as well as a new recombinant begomovirus species (Okra leaf curl Cameroon virus). In addition, a betasatellite (Cotton leaf curl Gezira betasatellite) and two alphasatellites (Okra leaf curl Mali alphasatellite and Okra yellow crinkle Cameroon alphasatellite) were identified. Tomato plants with leaf curling were shown to contain isolates of a new begomovirus, Tomato leaf curl Cameroon virus, and an alphasatellite, Tomato leaf curl Cameroon alphasatellite (ToLCCMA). To study the potential begomovirus complexes infecting weeds, begomoviruses and satellites in plants of the weed Ageratum conyzoides with leaf curl symptoms were characterized. Sequence analyses showed that they were infected by isolates of a new begomovirus (Ageratum leaf curl Cameroon virus), two new betasatellites (Ageratum leaf curl Cameroon betasatellite and Ageratum leaf curl Buea betasatellite), an alphasatellite (ToLCCMA) and two types of defective recombinants between a begomovirus and ToLCCMA. Putative recombinations were detected in begomovirus genomes for all three plant species studied, indicating that recombination is an important mechanism for their evolution. A close relationship between the begomoviruses infecting tomato and A. conyzoides, and the detection of the same alphasatellite in them support the idea that weeds are important reservoirs for begomoviruses and their satellites. This study has revealed a huge complexity of begomoviruses and DNA satellites previously largely unknown in West and Central Africa. With this high diversity, recombination potential and transmission by B. tabaci, begomoviruses and their associated DNA satellites pose a serious threat to crop production in the region.