Nucleotide sequence evidence for three distinct sugarcane streak mastreviruses

The complete sequences of four clones of sugarcane streak virus (SSV) isolates from Egypt and one SSV clone from Reunion island were determined. The four Egyptian genomes were highly similar to one another (97-99% nt identity) and were considered as variants of the same virus. The Egyptian SSV was genetically different from all other mastreviruses, the closest virus being SSV from South-Africa (60% nt identity), and defined as a new mastrevirus species named SSEV. The SSV clone from Reunion was highly related to the SSV from Mauritius and SSV from Nigeria, for which only partial sequences were available, indicating that the three sugarcane streak isolates from Mauritius, Reunion and Nigeria were strains of the same virus tentatively named SSMV. This work further confirms that SSMV is a distinct viral species compared to other mastreviruses, including the SSEV (59% nt identity) and SSV (66% nt identity). By comparing two clones from the Mascarene islands, we correlated substitutions in the C-terminal end of the coat protein with a different response to a monoclonal antibody, providing data on the mapping of a specific epitope. Agroinoculations experiments demonstrated that an SSEV clone induced more severe symptoms on maize than two clones from the Mascarene. Inside the African streak virus cluster, the sugarcane mastrevirus isolates were gathered in a sub-cluster of three viruses, SSEV, SSV and SSMV. The diversity of the SSVs is discussed in relation to its host, sugarcane, an imported crop in Africa.