Complete genome sequencing of two causative viruses of cassava mosaic disease in Ghana.

Cassava mosaic disease (CMV), caused by one or a combination of cassava mosaic geminiviruses, is ranked among the most important constraints to profitable and efficient production of cassava. Effective control measures require in-depth knowledge of the viral causative agent. Using rolling-circle amplification and unique enzymes, the full genome of two species of cassava mosaic geminivirus isolated from infected cassava plants in Ghana were cloned into pCambia 1300 and pET-28b. The sequences of the genome were determined on an ABI sequencer and a pairwise comparison was performed with other cassava-infecting geminiviruses from different countries. It was revealed that cassava grown in Ghana is attacked by two species of geminivirus in either single or mixed infections. These are the African cassava mosaic virus (ACMV) and the East African cassava mosaic virus (EACMV)-like, with high sequence similarity of 94% and 80%, respectively, between the DNA-A and DNA-B components of each virus, and 66% and 41% similarity of the common region (CR) (for A and B accordingly). The DNA-A of ACMV and EACMV-like contained 2781 and 2800 nucleotides, respectively, while their DNA-B components had 2725 and 2734 nucleotides, respectively. ACMV DNA-A was over 97% similar to those of other ACMVs from the continent. In contrast, EACMV-like DNA-A was over 98% similar to the isolates from Cameroon and other West African countries, and less than 88% similar to other EACMV species. Thus ACMV and EACMV-like were named African cassava mosaic virus-Ghana and East African cassava mosaic Cameroon virus-Ghana. Computer analysis revealed that their genome arrangement follows the typical old world bipartite begomovirus genome. The association of these two species and their interaction might account for the severe symptoms observed on infected plants in the field and in the greenhouse.