Genetic structure of pastoral and farmer populations in the African Sahel.

Traditional pastoralists survive in few places in the world. They can still be encountered in the African Sahel, where annual alternations of dry and wet seasons force them to continual mobility. Little is known about the genetic structure of these populations. We present here the population distribution of 312 hypervariable segment I mitochondrial DNA (mtDNA) and 364 Y-short tandem repeat haplotypes in both farmer and pastoralist groups from the Lake Chad Basin and the West African Sahel. We show that the majority of pastoral populations (represented in the African Sahel by the Fulani nomads) fail to show significant departure from neutrality for mtDNA as evidenced by Fu's Fs statistics and exhibit lower levels of intrapopulation diversity measures for mtDNA when contrasted with farmers. These differences were not observed for the Y chromosome. Furthermore, analyses of molecular variance and population distributions of the mtDNA haplotypes show more heterogeneity in the sedentary groups than in the pastoralists. On the other hand, pastoralists retain a signature of a wide phylogenetic distance contributing to their male gene pool, whereas in at least some of the farmer populations, a founder effect and/or drift might have led to the presence of a single major lineage. Interestingly, these observations are in contrast with those recorded in Central Asia, where similar comparisons of farmer and pastoral groups have recently been carried out. We can conclude that in Africa, there have been no substantial mating exchanges between the Fulani pastoralists coming to the Lake Chad Basin from the West African Sahel and their farmer neighbors. At the same time, we suggest that the emergence of pastoralism might be an earlier and/or a demographically more important event than the introduction of sedentary agriculture, at least in this part of Africa.