Résumé
The sequences of the coat protein gene of a representative sample of 40 isolates of Rice yellow mottle virus (RYMV) from 11 African countries were analysed. The overall level of nucleotide diversity was high (approximately 14%). Great geographical distances between the sites where isolates were collected were consistently associated with high genetic distances. In contrast, a wide range of genetic distances occurred among isolates spread over short geographical distances. There was no evidence of long-range dispersal. RYMV diversity in relation to land area was eight times greater in East Africa than in West/Central Africa. West/Central African isolates with up to 9 % divergence belonged to a monophyletic group, whereas the East African isolates with up to 13 % divergence fell into distantly related groups. In East Africa, each Tanzanian strain had a specific and restricted geographical range, whereas West/Central African strains had large and partially overlapping geographical distributions. Overall, our results suggest an earlier RYMV diversification in East Africa and a later radiation in West/Central Africa. The West African situation was consistent with virus adaptation to savanna, forest and other ecological conditions. In contrast East Africa, as exemplified by the Tanzanian situation, with numerous physical barriers (mountain chains, sea channel, lakes), suggested that RYMV strains resulted from divergence under isolated conditions. For RYMV and for two other viruses, phylogenetic relationships were established between isolates from Madagascar and isolates from the Lake Victoria region.