Lactase persistence and pastoralism in Africa

AJHG doi:10.1016/j.ajhg.2014.02.009

Genetic Origins of Lactase Persistence and the Spread of Pastoralism in Africa

Alessia Ranciaro et al.

In humans, the ability to digest lactose, the sugar in milk, declines after weaning because of decreasing levels of the enzyme lactase-phlorizin hydrolase, encoded by LCT. However, some individuals maintain high enzyme amounts and are able to digest lactose into adulthood (i.e., they have the lactase-persistence [LP] trait). It is thought that selection has played a major role in maintaining this genetically determined phenotypic trait in different human populations that practice pastoralism. To identify variants associated with the LP trait and to study its evolutionary history in Africa, we sequenced MCM6 introns 9 and 13 and ∼2 kb of the LCT promoter region in 819 individuals from 63 African populations and in 154 non-Africans from nine populations. We also genotyped four microsatellites in an ∼198 kb region in a subset of 252 individuals to reconstruct the origin and spread of LP-associated variants in Africa. Additionally, we examined the association between LP and genetic variability at candidate regulatory regions in 513 individuals from eastern Africa. Our analyses confirmed the association between the LP trait and three common variants in intron 13 (C-14010, G-13907, and G-13915). Furthermore, we identified two additional LP-associated SNPs in intron 13 and the promoter region (G-12962 and T-956, respectively). Using neutrality tests based on the allele frequency spectrum and long-range linkage disequilibrium, we detected strong signatures of recent positive selection in eastern African populations and the Fulani from central Africa. In addition, haplotype analysis supported an eastern African origin of the C-14010 LP-associated mutation in southern Africa.

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Y chromosomes of pastoralists and farmers from the Sahel

Am J Phys Anthropol DOI: 10.1002/ajpa.22236

Multiple and differentiated contributions to the male gene pool of pastoral and farmer populations of the African Sahel

Jana Bučková et al.

The African Sahel is conducive to studies of divergence/admixture genetic events as a result of its population history being so closely related with past climatic changes. Today, it is a place of the co-existence of two differing food-producing subsistence systems, i.e., that of sedentary farmers and nomadic pastoralists, whose populations have likely been formed from several dispersed indigenous hunter-gatherer groups. Using new methodology, we show here that the male gene pool of the extant populations of the African Sahel harbors signatures of multiple and differentiated contributions from different genetic sources. We also show that even if the Fulani pastoralists and their neighboring farmers share high frequencies of four Y chromosome subhaplogroups of E, they have drawn on molecularly differentiated subgroups at different times. These findings, based on combinations of SNP and STR polymorphisms, add to our previous knowledge and highlight the role of differences in the demographic history and displacements of the Sahelian populations as a major factor in the segregation of the Y chromosome lineages in Africa. Interestingly, within the Fulani pastoralist population as a whole, a differentiation of the groups from Niger is characterized by their high presence of R1b-M343 and E1b1b1-M35. Moreover, the R1b-M343 is represented in our dataset exclusively in the Fulani group and our analyses infer a north-to-south African migration route during a recent past.

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Pastoral and farmer populations from the Sahel

In Central Asia, the expansion of the Mongol nomads led to the predominance of a few Y-chromosome lineages of relatively recent vintage. The opposite seems to be true for the African pastoralists examined here: they see to be more diverse in their Y-chromosomes than farmers are, and less so in their mtDNA. This suggests that they were a fairly old group who did not particularly marry farmers' daughters.

Mol Biol Evol (2011) doi: 10.1093/molbev/msr067

Genetic structure of pastoral and farmer populations in the African Sahel

Viktor Černý et al.

Traditional pastoralists survive in but 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 HVS-I mtDNA and 364 Y-STR 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 intra-population diversity measures for mtDNA when contrasted with farmers. These differences were not observed for the Y chromosome. Furthermore, AMOVA analyses 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.

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