The trans-Saharan slave trade - clues from interpolation analyses and high-resolution characterization of mitochondrial DNA lineages

Abstract

Background: A proportion of 1/4 to 1/2 of North African female pool is made of typical sub-Saharan lineages, in higher frequencies as geographic proximity to sub-Saharan Africa increases. The Sahara was a strong geographical barrier against gene flow, at least since 5,000 years ago, when desertification affected a larger region, but the Arab trans-Saharan slave trade could have facilitate enormously this migration of lineages. Till now, the genetic consequences of these forced trans-Saharan movements of people have not been ascertained.

Results: The distribution of the main L haplogroups in North Africa clearly reflects the known trans-Saharan slave routes: West is dominated by L1b, L2b, L2c, L2d, L3b and L3d; the Center by L3e and some L3f and L3w; the East by L0a, L3h, L3i, L3x and, in common with the Center, L3f and L3w; while, L2a is almost everywhere. Ages for the haplogroups observed in both sides of the Saharan desert testify the recent origin (holocenic) of these haplogroups in sub-Saharan Africa, claiming a recent introduction in North Africa, further strengthened by the no detection of local expansions.

Conclusions: The interpolation analyses and complete sequencing of present mtDNA sub-Saharan lineages observed in North Africa support the genetic impact of recent trans-Saharan migrations, namely the slave trade initiated by the Arab conquest of North Africa in the seventh century. Sub-Saharan people did not leave traces in the North African maternal gene pool for the time of its settlement, some 40,000 years ago.

Figure 1

Routes for trans-Saharan slave trade. Adapted from Segal (2002) and Lovejoy (1983).

Figure 2

Map showing location of the population samples (A) used in this work and interpolation map for the L lineages in those samples (B).

Figure 3

Interpolation maps for L0 haplogroup in the sub-Saharan pool observed in each sample.

Figure 4

Interpolation maps for L1 haplogroup in the sub-Saharan pool observed in each sample.

Figure 5

Interpolation maps for L2 haplogroup in the sub-Saharan pool observed in each sample.

Figure 6

Interpolation maps for L3 total, L3b, L3d, L3e and L3f haplogroups in the sub-Saharan pool observed in each sample.

Figure 7

Interpolation maps for L3h, L3i, L3x and L3w haplogroups in the sub-Saharan pool observed in each sample.

Figure 8

Spatial correlograms of Moran's I indeces for the total L frequency in the populations, and for the L0, L1, L2 and L3 proportions of the sub-Saharan pools in the samples. Geographic distances separating samples are distributed into 14 classes. Full dots represent significant p-values (p < 0.05); empty dots are non-significant p-values.

Figure 9

Phylogeny of the complete L3 sequences from El Jadida. Integers represent transitions when the suffixes "A", "G", "C" or "T" are appended and transversions when the suffixes "a", "g", "c" or "t" are appended. Deletions are indicated by a "d" following the deleted nucleotide position. Underlined nucleotide positions appear more than once in the tree.