Diversity of Polymyxa graminis associated with cereals in West Africa.
The diversity of Polymyxa graminis, the vector of Peanut clump virus on cereals and groundnut in West Africa, was investigated. Isolates from Burkina Faso, Mali, Niger and Senegal were obtained in roots of cereal bait-plants grown on soils collected during the 2000 to 2003 growing seasons in fields thought to be infested by the virus. A direct molecular characterization of total DNA extracted from infected roots was performed by sequencing nuclear 18S, 5.8S, internal transcribed spacers (ITS1 and 2) ribosomal DNA after amplification of the DNA regions by polymerase chain reaction (PCR). Different sequences were identified among the 24 P. graminis isolates obtained on maize, pearl millet, sorghum and wheat. Originating from the four countries, most of them (18/22 West African tested isolates) were identified as P. graminis f. sp. tropicalis and one Senegalese isolate as P. graminis f. sp. subtropicalis. Two sequences, differing from the sequences described for the five formae speciales of P. graminis, were obtained for one isolate from Senegal and for two isolates from Burkina Faso. In the phylogenetic tree, the first new sequence appeared to be linked to P. graminis f. sp. tropicalis and P. graminis f. sp. temperata and the second to P. graminis f. sp. colombiana. The P. graminis diversity found in West Africa is far more important than the one observed in the Indian subcontinent. This could be related to the fact that Africa is recognized as the centre of origin and diversity for pearl millet and sorghum, two known preferred hosts for P. graminis in tropical areas. The development of two P. graminis f. sp. tropicalis isolates from Niger on several graminaceous species was quantified by real-time quantitative PCR. Their multiplication rate was high on sorghum, pearl millet and maize, and significantly lower on barley and wheat. These characteristics are common with the Indian isolates belonging to P. graminis f. sp. tropicalis, confirming the host specificity observed among the formae speciales of P. graminis.