Sub-Saharan DNA admixture in Europe

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Sub-Saharan DNA admixture in Europe refers to the way in which Sub-Saharan African DNA is lightly scattered throughout the European continent.

Not every population has been studied yet, but enough have so that a picture is starting to emerge. The amount of Sub-Saharan African admixture in Europe today ranges from a few percent in the Iberian Peninsula to almost none around the Baltic. It seems to show a decreasing cline from the Southwest to the Northeast, which corresponds with the areas most affected by the Moorish (North Africa) expansion and the African slave trade.

Between 1500 and up to 1900, about four million African slaves were transported to island plantations in the Indian Ocean; eleven million were taken by the Atlantic slave trade to the Caribbean, North America, Central America, and, above all, South America—mainly to Brazil; an estimated eight million were transported north across the Sahara to North Africa by the Arab slave trade.[1] Of the vast majority shipped by the Atlantic trade, most were sent directly to the Americas as part of an Atlantic triangular trade, and so never saw Europe. Most of the trans-Saharan trade ended at markets in North Africa and the Middle East.

In the same period about 200,000 Africans were sold into Europe via the Atlantic slave trade[2], and these seem to have "vanished" without a trace. However, they can account for much of the presence of Sub-Saharan African DNA markers in the modern European gene pool, although it is not clear how much (in opposition to traces from pre-historic and medieval migrations). It also must be noted that levels of African DNA from these relatively recent arrivals are too low to have had an appreciable effect on Phenotypes.

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[edit] Approaches to detecting admixture

There are three main approaches to detecting continent-of-ancestry admixture: gender-specific markers (Mitochondrial DNA and Y-chromosome DNA), neutral autosomal markers, and adaptive autosomal markers.

Each approach has strengths and weaknesses in distinguishing ancient sub-Saharan markers (from our species' common origin in Africa) from more recent ones. Some approaches are more quantifiable than others.

It should be noted that differences among the major population groups of the world constitute only 3% to 5% of genetic variation, while within-population differences among individuals account for 93% to 95% of such variation. [3]

[edit] Gender-specific markers

Mitochondrial DNA (mtDNA) and Y-chromosome DNA trace individual lineages, matrilineal and patrilineal, respectively.

They do not mix or recombine at each generation. Hence, they can identify different population migrations. The descendants of the sub-Saharan Africans who first began the Great Diaspora about 70 millennia ago can be distinguished from the sub-Saharan groups who helped to re-colonize Europe after the glaciers melted 16 millennia ago, and from sub-Saharan people who crossed or went around the Mediterranean in Ancient Egyptian or Roman times or thereafter as slaves, soldiers, settlers, or traders.

Although mtDNA and Y-DNA can quantifiably estimate a modern population's overall admixture, the approach cannot measure an individual's genealogy. A person born around the year 2000 would have had about slightly more than half a million ancestors alive in the year 1500 alone[4], if there were no pedigree collapse, but only two of them would have carried that person's mtDNA and Y-DNA. Differences between the patterns of mtDNA and Y-DNA can suggest why populations migrated: military conquest tends to propagate Y lineages but leave mtDNA lineages in place (men tend to be mobile, women stay in place), mass migrations in search of a new homeland tend to propagate mtDNA and Y lineages equally, and a slave trade tends to propagate mtDNA lineages but leave Y lineages in place (female slaves are encouraged to propagate, males are not).

[edit] mtDNA

A study by Gonzalez et al. 2003[5] found L haplogroups at rates of 0.1% in Scotland, 0.4% in England, 0.7% in North Germany, 1.4% in France, 2.9% in Galicia, 2.2% in Northern Portugal, 4.3% in Central Portugal, and 8.6% in Southern Portugal (Alentejo and Algarve) (note that these figures do not count the L3 lineage, which may be of ancient introduction and so remains ambiguous). For comparison, sub-Saharan mtDNA runs 21.8% in North Africa.

According to another study by Pereira et al. 2005,[6] sub-Saharan mtDNA L haplogroups were found at rates of 0.62% in a German-Danish sample, 0.94% in Sicilians, 1% in the British/Irish, 2.38% in Albanians, 2.86% in Sardinians. This paper which provides a deeper and more global insight into the African female influence in Iberia shows that the mean frequency reaches 3.83% in Iberians. The frequency is clearly higher in Portugal (32 sequences in 549 individuals; 5.83% with a high frequency of 11% in southern Portugal) than in Spain (8 out of 496; 1.61% with a higher frequency of 3.26% in Galicia) and without parallel in the rest of Europe.

[edit] Y-DNA

For the reasons outlined above, sub-Saharan markers are much less common in Europe. The small presence of the Haplogroups E(xE3b) (i.e. clades of E other than E3b) and Haplogroup A in Europe is almost exclusively attributable to the slave trade, as these haplogroups are characteristic of western, central and southern Africans and are barely observed elsewhere.[7] The haplotypes have been detected in Portugal (3%), France (2.5% - in a very small sample), Germany (2%), Sardinia (1.6%), Austria (0.78%), Italy (0.45%), Spain (0.42%) and Greece (0.27%). By contrast, North Africans have about 5% paternal black admixture.[8]

[edit] Neutral autosomal markers

Neutral autosomal markers are odd fragments of DNA that do not affect a person's physical traits.

Because they are autosomal (within the Nuclear DNA that is subject to Meiosis), such markers reflect the recombination of paternal and maternal DNA with each generation. Hence, they are less useful than mtDNA or Y-DNA in tracking migrations and they are less precise as to time.

This makes it hard to tell if any particular marker dates from the 1500-1800 slave trade, or from the post-glacial re-colonization of Europe, or from some time in between. On the other hand, neutral autosomal markers are useful for individual genealogies since they reflect just how much of an individual's genome came from which population group. Two studies by Rosenberg et al. 2002[9] and Wilson et al. 2001[10] failed to detect any sub-Saharan admixture in Scots, Russians, Basques, Frenchmen or Italians, while 1% was observed in Norwegians.

[edit] Adaptive autosomal markers

Adaptive autosomal markers are those that evolved and spread because they enhance survivability.

The best-known example is HbS, which produces the sickle-cell trait. This Allele emerged in Arabian Peninsula shortly after the invention of Agriculture[citation needed] and spread to Europe because it confers near immunity to the most lethal form of Malaria.

There are many other such traits and they have two main advantages for population studies: First, they have been well-studied for centuries, so different strains are easily identified and tracked. Second, because their adaptive advantages are known, their dates of origin and spread are also known to reasonable precision. The main disadvantage of adaptive autosomal markers is that they cannot tell what fraction of a population came from which ancestry. That HbS is found in, say, 10 percent of some European population does not mean that ten percent have sub-Saharan ancestry; it may simply be that many of those lacking the trait in the past died without progeny due to malaria.

[edit] The Arnaiz-Villena controversy

An often-cited study from 2001 by Antonio Arnaiz-Villena et al.[11] which maps 28 world population based on the HLA DRB1 locus, concluded that "the reason why Greeks did not show a close relatedness with all the other Mediterraneans analyzed was their genetic relationship with sub-Saharan ethnic groups now residing in Ethiopia, Sudan, and West Africa (Burkina Faso)." Later that year, the same data was used in another study by the same author published in a different journal.[12] This second paper dealt specifically with the relatedness of Palestinians and Israelis and was subsequently "deleted from the scientific literature" because, according to the editor-in-chief Nicole Suciu-Foca, it "confounded the elegant analysis of the historic basis of the people of the Mediterranean Basin with a political viewpoint representing only one side of a complex political and historical issue".[13]

Erica Klarreich's report on the controversy further quotes Suciu-Foca as saying that the reaction against the paper was so severe that "We would have had mass resignations and the journal would have been destroyed if this paper were allowed to remain."[14] The controversy was further reported on in numerous locations including The Observer.[15]

Shortly after this, three respected geneticists, Luca Cavalli-Sforza, Alberto Piazza and Neil Risch, argued that the scientific limitations of Arnaiz-Villena's methodology.[16] They stated that "Using results from the analysis of a single marker, particularly one likely to have undergone selection, for the purpose of reconstructing genealogies is unreliable and unacceptable practice in population genetics.", making specific allusion to the findings on Greeks (among others) as "anomalous results, which contradict history, geography, anthropology and all prior population-genetic studies of these groups."

No multiple-marker analysis has ever duplicated Arnaiz-Villena's results. In The History and Geography of Human Genes (Princeton, 1994), Cavalli-Sforza, Menozzi and Piazza grouped Greeks with other European and Mediterranean populations based on 120 loci (view MDS plot[17]). Then, Ayub et al. 2003[18] did the same thing using 182 loci (view dendrogram[19]).

Another study was conducted in 2004 at Skopje's University of Ss. Kiril and Metodij, using high-resolution typing of HLA-DRB1 according to Arnaiz-Villena's methodology. Contrary to his earlier conclusion, however, no sub-Saharan admixture was detected in the Greek sample.[20]

A 2006 study by Tunisian scientists again asserted the relatedness of the Greeks to sub-Saharans by calculating genetic distances at the DRB1 locus,[21] the same marker used in the controversial Arnaiz-Villena paper. Both papers interpreted those results as suggesting an admixture occurred due to the displacement of Egyptian-Ethiopic people during the Pharaonic period. However, the Tunisian scientists failed to analyze any new Greek genetic material, relying solely on the data contained in the earlier Arnaiz-Villena paper, and no Greek laboratory contributed to their research.[22]

The credibility of Arnaiz-Villena was seriously damaged after he was suspended without pay from the Hospital Doce de Octubre in Madrid, where he heads the department of immunology and molecular biology, after being charged with embezzlement of funds.[23] In addition to this charge, Dr Arnaiz-Villena is facing allegations of "moral harassment" at the Universidad Complutense de Madrid, where he chairs a research and teaching immunology unit.

[edit] Footnotes

  1. ^ Pier M. Larson, Reconsidering Trauma, Identity, and the African Diaspora: Enslavement and Historical Memory in Nineteenth-Century Highland Madagascar, William and Mary Quarterly 56, no. 2 (1999): 335-62.
  2. ^ Hugh Thomas, The Slave Trade: The Story of the Atlantic Slave Trade: 1440-1870 (New York: Simon and Schuster, 1997), 804
  3. ^ Genetic structure of human populations. Rosenberg NA, et al, Dec 2002
  4. ^ If one calculates 20 generations in 500 years, that would make a total of 524,288 individual genealogical positions (without pedigree collapse) in the 20th generation alone; plus an exactly equal number, minus 1, in all the preceding generations (1 to 19th generations). Notice that the number doubles with each generation, thus, in the 21st generation one has 1,048,576 individual genealogical positions in that generation alone.
  5. ^ Gonzalez et al. 2003
  6. ^ Pereira et al. 2005 African female heritage in Iberia: a reassessment of mtDNA lineage distribution in present times
  7. ^ Sanchez et al. (2005). "High frequencies of Y chromosome lineages characterized by E3b1, DYS19-11, DYS392-12 in Somali males". European Journal of Human Genetics; 13:856–866
  8. ^ Cruciani et al. 2004,
    Flores et al. 2004,
    Brion et al. 2005,
    Brion et al. 2004,
    Rosser et al. 2000,
    Semino et al. 2004,
    DiGiacomo et al. 2003.
  9. ^ Rosenberg et al. 2002
  10. ^ Wilson et al. 2001
  11. ^ Arnaiz-Villena et al.
  12. ^ Abstract
  13. ^ Human Immunology, Vol: 62, Issue: 10, October, 2001, pp1063
  14. ^ Nature
  15. ^ The Observer
  16. ^ Nature
  17. ^ MDS plot
  18. ^ Ayub et al. 2003
  19. ^ dendrogram
  20. ^ http://www.blackwell-synergy.com/links/doi/10.1111%2Fj.1399-0039.2004.00273.x High-resolution typing of HLA-DRB1 locus in the Macedonian population]
  21. ^ HLA genes in Southern Tunisians (Ghannouch area) and their relationship with other Mediterraneans.
  22. ^ HLA genes in Southern Tunisians (Ghannouch area) and their relationship with other Mediterraneans.
  23. ^ [ http://www.bmj.com/cgi/content/full/324/7339/695 Controversial immunologist faces court case]

[edit] See also

[edit] External links