July 23, 2011
Shah, Anish M., R. Tamang, et al. (2011). “Indian Siddis:
African Descendants with Indian Admixture.” American Journal
of Human Genetics 89(1):154-161.
Compared to Tran-Atlantic slave trade, Indian Ocean Slave trade
is less known, maybe less understood, but has longer history.
Indigenous Africans were captured and traded by other Africans,
Arabs, and Europeans. Some of the slaves were sent to Middle
East and South Asia. Sub-Saharan African mtDNA haplogroups
have been found among the Middle Eastern populations and the
frequencies range from 9 to 34%. Sub-Saharan African Y
chromosome haplogroups are rare, but are also found among the
Middle Eastern Arab populations. Richards et al. (2003) and
Quintana-Murci et al. (2004) argue that these sub-Saharan African
mtDNA haplogroups were brought to the Middle East and reached South
Asian through the Arab slave trade. African females were
incorporated into Islamic societies, but African males did not have
much chance of reproduction.
In this article, Shah et al. (2011) demonstrate that Siddis, or
Habishis, from India, the descendants of slaves from Africa, have
genetic characteristics of sub-Saharan Africans. They
genotyped 850,000 autosomal SNPs, 32 Y chromosome biallelic
markers, and 17 Y chromosome STR and sequenced mtDNA hypervariable
region I.
Among Siddis, sub-Saharan genetic contribution estimated based
on autosomal SNPs is quite large ranging 62.3-74.4% and they are
plotted more closely to HapMap Yorubans than Indians, Europeans, or
Asians on the PC plot. Contrary to the previous studies, they
found more male sub-Saharan contribution to the Siddis than female
contribution. You could expect this from the Indians marriage
rule of endogamy, but gene flow between the Siddis and neighboring
ethnic groups or communities was unidirectional. They found
South Asians and Eurasian genetic
contribution to the Siddis from their neighboring communities, but
they did not find Sub-Saharan genetic contribution from the Siddis
to neighboring ethnic groups.
Genetic studies to understand slave trade are usually conducted
using uniparental markers (mtDNA and Y-chromosome). The
molecular genetic and analytical techniques to trace the origin are
relatively simple, but the problem is that you are tracing only two
lineages (maternal and paternal) out of thousands of possible
ancestors for a particular individual. By analyzing autosomal
markers, you are getting genetic information of all the
ancestors. Recent advancement in molecular genetics allows
researchers to genotypes many single nucleotide polymorphisms
(SNPs) per individual. Sometime in the future, it will be
possible to genotype over 1 million SNPs per individual without
huge cost. Down side of this is that it requires more
sophisticated statistical and analytical techniques.
References:
Quintana-Murci L, Chaix R, Wells S, Behar DM, Sayar H, Scozzari
R, Rengo C, Al-Zaheri N, Semino O, Santachiara-Benerecetti AS,
Coppa A, Ayub Q, Mohyuddin A, Tyler-Smith C, Mehdi SQ, Torroni A,
and McElreavey K (2004) Where west meets east: the complex mtDNA
landscape of southwest and Central Asian corridor. American Journal
of Human Genetics 74:827-845.
Richards M, Rengo C, Cruciani F, Gratrix F, Wilson JF, Scozzari
R, Macaulay V, and Torroni A (2003) Extensive female-mediated gene
flow from Sub-Saharan Africa into Near Eastern Arab Populaitons.
American Journal of Human Genetics 72:1058-1064.