Demographic history was a formative mechanism of the genetic structure for the taste receptor TAS2R16 in human populations inhabiting Africa's Sahel/Savannah Belt.

OBJECTIVES: Mode of subsistence is an important factor influencing dietary habits and the genetic structure of various populations through differential intensity of gene flow and selection pressures. Previous studies suggest that in Africa Taste 2 Receptor Member 16 (TAS2R16), which encodes the 7-transmembrane receptor protein for bitterness, might also be under positive selection pressure. METHODS: However, since sampling coverage of populations was limited, we created a new TAS2R16 population dataset from across the African Sahel/Savannah belt representing various local populations of differing subsistence modes, linguistic affiliations, and geographic provenience. We sequenced the TAS2R16 exon gene and analyzed 2250 haplotypes among 19 populations. RESULTS: We found no evidence for selection as a driving force of genetic variation at this locus; instead, we discovered a highly significant correlation between TAS2R16 genetic and geographical distances based on provenience of the sampled populations, strongly suggesting that genetic drift most likely prevailed over positive selection at this specific locus. We also found significant correlations with other independent loci, mainly in sedentary farmers. DISCUSSION: Our results do not support the notion that the genetic diversity of TAS2R16 in Sahelian populations was shaped by selective pressures. This could result from several alternative and not mutually exclusive mechanisms, of which the possibility that, due to the pleiotropic nature of TAS2R16, selective pressures on other traits could counterbalance those acting on bitter taste perception, or that the change of diet in the Neolithic generally relaxed selective pressure on this gene.

Subsistence strategy was the main factor driving population differentiation in the bidirectional corridor of the African Sahel.

OBJECTIVES: The Sahel belt is occupied by populations who use two types of subsistence strategy, nomadic pastoralism and sedentary farming, and who belong to three linguistic families, Niger-Congo, Nilo-Saharan, and Afro-Asiatic. Little is known, however, about the origins of these two populations and their mutual genetic relationships. MATERIALS AND METHODS: We have built a large dataset of mitochondrial DNA sequences and Y chromosomal STR haplotypes of pastoralists and farmers belonging to all three linguistic phyla in the western, central, and eastern parts of the Sahel. We calculated pairwise genetic, geographic, and linguistic distances between populations and analyzed the effects of geography, language, and subsistence on population genetic structure. RESULTS: We found that subsistence mode significantly contributed to the generally low population structure in the Sahel and that language affiliation plays a more important role for pastoralists than for farmers. We also demonstrated that geographic isolation significantly influenced the population structure of sedentary farmers but not of nomadic pastoralists. Finally, we found haplotypes shared between the Fulani and Arabic-speaking Baggara, supporting the theory of Baggarization, which explains the recent adaptation of Arabic-speaking nomads in the Sahel region through contact with autochthonous sub-Saharan populations. CONCLUSIONS: Based on various genetic and archaeological evidence pertaining to the Sahel, we suggest that the idea of a bidirectional Sahelian corridor is valid, but that pastoralists made a more important contribution to its population structure. It is also possible that agropastoralists diverged into farmers and pastoralists in the early stages of formation of the Sahelian gene pool.

Alu insertion polymorphisms in the African Sahel and the origin of Fulani pastoralists.

BACKGROUND: The origin of Western African pastoralism, represented today by the Fulani nomads, has been a highly debated issue for the past decades, and has not yet been conclusively resolved. AIM: This study focused on Alu polymorphisms in sedentary and nomadic populations across the African Sahel to investigate patterns of diversity that can complement the existing results and contribute to resolving issues concerning the origin of West African pastoralism. SUBJECTS AND METHODS: A new dataset of 21 Alu biallelic markers covering a substantial part of the African Sahel has been analysed jointly with several published North African populations. RESULTS: Interestingly, with regard to Alu variation, the relationship of Fulani pastoralists to North Africans is not as evident as was earlier revealed by studies of uniparental loci such as mtDNA and NRY. Alu insertions point rather to an affinity of Fulani pastoralists to Eastern Africans also leading a pastoral lifestyle. CONCLUSIONS: It is suggested that contemporary Fulani pastoralists might be descendants of an ancestral Eastern African population that, while crossing the Sahara in the Holocene, admixed slightly with a population of Eurasian (as evidenced by uniparental polymorphisms) ancestry. It seems that, in the Fulani pastoralists, Alu elements reflect more ancient genetic relationships than do uniparental genetic systems.

The historical spread of Arabian Pastoralists to the eastern African Sahel evidenced by the lactase persistence -13,915*G allele and mitochondrial DNA.

OBJECTIVES: Thanks to the ability to digest lactose, Arabian nomads had become less dependent upon their sedentary neighbors and some of these populations spread to Africa. When and by which route they migrated to their current locations have previously been addressed only by historical and archaeological data. METHODS: To address the question of Arab expansion into Africa, we collected samples from several Arabic populations, especially the Baggara in Chad and Sudan. We analyzed mutations associated with lactase persistence and reconstructed the surrounding haplotypes defined by SNP polymorphisms. We also sequenced their mitochondrial DNA to investigate relative proportions of sub-Saharan and Eurasian origins. RESULTS: We estimated the expansion age of the -13,915*G mutation in four different Arabian datasets. The oldest age was identified in Yemen (1,356-1,799 ya) and the youngest in a Sudanese group of Rashaayda Arabs (219-312 ya). We also found a negative correlation between the frequency of the -13,915*G allele and the frequency of sub-Saharan mtDNA haplotypes. CONCLUSIONS: Even if the age of the most recent common ancestor of -13,915*G is ∼4 ka as shown in a previous study, our results suggest that its spread to Africa was more recent, which is consistent with the migrations of Arabic tribes. Because the incidence of sub-Saharan mtDNA haplotypes is negatively correlated with the occurrence of -13,915*G, we suggest that the decrease of its frequency in Africa has been caused by progressive admixture of the Arabian nomads with sub-Saharan populations.

Genetic Structure of the Western and Eastern African Sahel/Savannah Belt and the Role of Nomadic Pastoralists as Inferred from the Variation of D-Loop Mitochondrial DNA Sequences.

The objective of this study was to provide deeper knowledge of the maternal genetic structure and demographic history of the human populations of the Sahel/Savannah belt, the extensive region lying between the Sahara and tropical rainforests, spanning from the Atlantic Ocean to the Red Sea coast. The study aimed to confirm or disconfirm archaeological and linguistic data indicating that the region's populations underwent diversification as a result of the spread of agropastoral food-producing subsistence lifestyles, over time dividing the region into separate areas of nomadic pastoralism, on the one hand, and sedentary farming, on the other. To perform both descriptive and coalescence analyses from the Sahel/Savannah belt's entire region, including western and eastern rather than just central populations studied previously, we generated a new mitochondrial DNA (mtDNA) data set not only having almost 2,000 samples (875 of which were newly collected) but also encompassing whole mtDNA D-loop segment rather than only the previously studied hypervariable segment 1. While comparing our analyses with previous results from the Lake Chad Basin (central Sahel/Savannah Belt), we found similar intrapopulation diversity measures (i.e., lower values in pastoralists than in farmers). However, the new data set pointed to significant differences in mating strategies between western and eastern pastoralists: our results suggest higher gene flow between the Arabic pastoralists and neighboring farmers in the eastern part than between the Fulani pastoralists and their sedentary neighbors in the western part of the Sahel/Savannah belt. The findings are discussed in light of archaeological and linguistic data, allowing us to postulate that the genetic differentiation of Fulani pastoralists from the common western African agropastoral gene pool occurred at around the same time as the arrival of the Arabic pastoralists to eastern Africa. However, it seems that while the process of divergence of the Fulani pastoralists in the west was accompanied by a loss of Fulani females to other populations, the Arab pastoralists' immigration to the Sahel/Savannah belt conversely resulted in some gain of local females into this Arab population.