Résumé
Using microsatellite genetic markers, we analyzed 1,518 sorghum samples collected at different spatial scales in Burkina Faso, Mali, Niger, and in the village of Wante, from the landrace to the country scale. Genetic diversity and differentiation parameters were estimated to assess the effects of the main evolutionary processes on sorghum genetic diversity. The genetic variability found within a variety is mainly the result of the sorghum reproduction biology and the genetic drift process caused by the limited number of reproductive individuals at the time of variety introduction into a household or each year when farmers select their seeds. At the village scale, a low correlation is observed between the diversity of vernacular names and the genetic diversity assessed by microsatellites. No spatial genetic structuration is observed among villages more than 30 kilometers apart. This point highlightss that traditional seed systems operate at a very local scale. in Mali, a similar proportion of allelic richness is observed along a longitudinal transect and a latitudinal gradient that crosses a larger range of agroclimatic conditions. At the country scale, sorghum exhibits more genetic diversity in Niger than in Mali despite a lower agroclimatic range in Niger. These results demonstrate that the diversity of human groups acts together with the agro-ecological factors to shape the structure of sorghum genetic diversity. An important proportion of the overall genetic diversity present in the Cirad (Centre de cooperation internationale en recherche agronomique pour le developpement) "core collection" is found in Mali and Niger. The tremendous diversity cultivated by farmers in traditional agroecosystems of Western Africa supports the relevance of in situ approaches for sorghum conservation programs in this region. Both conservation and crop improvement requirements can be achieved through a better use of local germplasm in decentralized breeding programs.