Reevaluating a Model of Gender-Biased Gene Flow among Sub-Saharan Hunter-Gatherers and Farmers

In a previous study, we proposed a model for genetic admixture between African hunter gatherers and food producers, in which we integrated demographic and genetic aspects together with ethnographic knowledge (Destro Bisol et al. 2004b). Due to the scarcity of data, it was possible to test the model only using genetic information from widely dispersed and genetically heterogeneous populations. In this paper we re-evaluate the congruence between the model and patterns of genetic variation using an anthropologically and geographically more homogeneous dataset including Pygmies and farmers from Central Western Africa. As implied by the model, the ratios of mtDNA to Y chromosome Nm estimates (0.154 in Pygmies and 6.759 in farmers) support an asymmetric gene flow, with a higher Pygmy-to-Pygmy gene flow for paternal than maternal lineages and vice versa for farmers. Analyses of intra- and inter-population genetic variation further support the above observation, showing a prevailing effect of genetic drift on maternal lineages and gene flow on paternal lineages among Pygmies, and an opposite pattern among farmers. We also detected differences between patterns for classical and molecular measures of Y chromosome intra-population variation, which likely represent signatures of the introgression of Bantu lineages into the gene pool of Pygmy populations. On the whole, our results seem to reflect differences in the demographic history and the degree of patrilocality and polyginy between the two population groups, thus providing further support to our micro-evolutionary model in an anthropologically coherent framework.