The Study of Racialism

Northwest African mtDNA Landscape

The main difference, found through the mtDNA analysis, between the populations of the two geographical areas studied is the presence of sub-Saharan L lineages in NW Africa compared to SW Europe, up to the point that, if L sequences were removed from the analyses, most NW African populations were genetically very close to SW Europeans. Since L sequences make up almost all mtDNA lineages in sub-Saharan Africa, and particularly in the areas just to the south of NW Africa, the frequency of L haplogroups in NW Africa can be read directly as a measure of gene flow. Thus, it can be estimated that 25.9±2.1% of the NW African mtDNA pool has a sub-Saharan origin, under the assumption of negligible back flow from NW to sub-Saharan Africa. A similar estimation can be performed for Y-chromosome lineages, since E1* and E3a* haplogroups (according to the nomenclature of the Y Chromosome Consortium, 2002) found in NW Africa at a frequency of 8.0%±2.0% (Bosch et al. 2001), are of sub-Saharan origin. The female- and male-mediated estimates of sub-Saharan gene flow into NW Africa are clearly different, which could be a local consequence of a global trend to higher female than male migration (Salem et al. 1996; Seielstad et al. 1998; Pérez-Lezaun et al. 1999). Autosomal markers such as Alu insertion polymorphisms also show frequency patterns compatible with gene flow from sub-Saharan Africa into NW Africa (Comas et al. 2000), although the absence of a clear phylogeographic structure in that case prevents the estimation of gene flow without specifying a parental, non-admixed population for NW Africa.

Within NW Africa, L sequences are most frequent in Mauritanians and Saharawi, whereas their frequency is lowest in northern populations. Alu insertion polymorphism analysis in NW Africa (Comas et al. 2000) has also shown that gene flow from sub-Saharan Africa in the southern part of this geographical area was more pronounced. A similar genetic gradient was also observed in NE Africa along the Nile valley from analysing Egyptian and Nubian mtDNA sequences (Krings et al. 1999), where south-north migration (and vice versa) could be facilitated by the Nile.

Sequence frequency and diversity, and nucleotide diversity, point to NW Africa as the cradle of U6, with an estimated age of 47,000 ± 18,000 years. Such an ancient age contrasts with the limited spread of U6, which is found in N Africa, the Canaries and Iberia, and at very low frequencies in Italy, the Middle East, and the Sahel. This could be explained because, with the exception of the Moslem invasions of Iberia and Sicily, no large population expansion has been known to originate in NW Africa, and the gene tree structure for U6 does not seem compatible with a strong population expansion. U6 represents, thus, a local background in NW Africa. Its relatively low frequency (10% overall, although ranging from absence in Algeria to 28.2% in the Mozabites) is in stark contrast with the high frequency of Y-chromosome haplogroup E3b2* (64%; Bosch et al. 2001), which may also have originated (or expanded to such high frequency) locally in NW Africa. This discrepancy may be the result of ancient, random, locus-specific drift, and/or of a male-biased bottleneck or migration. A locus-specific effect may be evidenced by the fact that AMOVA between Iberian and NW African populations is much higher for Y chromosome haplogroups than for multiple autosomal Alu insertion polymorphisms or mtDNA. Since men contribute their autosomes as well, the fact that population differentiation as demonstrated by autosomal loci is much closer to that for mtDNA than to that for the Y chromosome may be taken as evidence for ancient, random, locus-specific drift affecting the Y chromosome.

NW African populations are relatively heterogeneous in their mtDNA sequence pools. The eastern populations (Algeria and Tunisia) may have received more gene flow from the east, as evidenced by the frequencies of M1. This haplogroup originated in East Africa (Quintana-Murci et al. 1999) with a frequency 20% in Ethiopians (Passarino et al. 1998), and declines north-westwards (Nubians 10% and Egyptians 8%; Krings et al. 1999), whereas its frequency in the Middle East is lower (3% in Jordanians from Amman, Richards et al. 2000; 2% Israeli Palestinians, Richards et al. 2000; 2% in Israeli Druze, Macaulay et al. 1999).

The major outlier within NW Africa are the Mozabites, a well-known Berber isolated group in Algeria, where drift may have altered haplogroup frequencies.