Résumé
Cultivation of Pearl millet in West Africa is still mainly traditional. During the past decades, important human and climatic changes have occured in Sahelian countries. In Niger, the cultivated area and human population have doubled in 25 years. Moreover isohyete 400 mm has moved southwards by 200 km in the west and by 100 km in the east of the country. The impact of these changes on genetic diversity and adaptation of pearl millet landraces is still unknown. In this study we analyzed samples of pearl millet landraces collected in the same villages in 1976 (ex situ collections) and 2003 (on-farm collections) throughout the entire cultivated area of Niger. The diversity assessed with microsatellite loci did not display significant changes between the 1976 and 2003 collections. We tested the change in allele frequency at the flowering locus PHYC. The results suggested a positive selection for the earliness allele, which was consistent with the phenological trends towards earliness observed between the 1976 and 2003 collections by comparing them over three crop seasons in a common garden experiment. In the context of a changing climate, the shortening of the life cycle is an adaptive change in landraces that can cope with climatic change by flowering earlier in drier environments. This study shows how genomics-based diversity studies boost the usefulness of ex situ collections of crop genetic resources as baselines to monitor the dynamics of evolutionary changes in agroecosystems, and to identify traits and loci involved in adaptation to changing environmental conditions which can then be targeted by varietal improvement programs. (Texte intégral)