Genomic and biological diversity of the African cassava geminiviruses

The virological situation of cassava in Africa is increasing in complexity due to the number and types of viruses isolated from different locations within the continent. Here, we report the complete nucleotide sequences of both A and B components of two geminivirus species infecting cassava in the Ivory Coast and review the current knowledge of the molecular and biological diversity of the African cassava geminiviruses. As a whole, newly obtained sequences are compared with those of the African cassava mosaic geminiviruses identified to date. Results indicate that all isolates of African cassava mosaic virus (ACMV), irrespective of their geographical origin are clustered together with little or no variation in their genomic sequence. On the contrary, the genomes of the East African cassava mosaic virus (EACMV) are more genetically diverse due to the frequent occurrence of recombinations within their two components. Indeed, the EACMV-like viruses vary so much that their classification is becoming problematic. In addition, there is also a large range of phenotypic symptom variation for each of these virus species, irrespective of the location of isolation. Furthermore, it has been shown that ACMV and EACMV can be synergistic in cassava, resulting in a greater DNA accumulation and consequently inducing severe symptoms. For all these reasons, this paper initiates a discussion concerning the species demarcation for cassava geminivirus.