BACKGROUND:
For millennia, the southern part of the Mesopotamia has been a wetland region generated by the Tigris and Euphrates rivers before flowing into the Gulf. This area has been occupied by human communities since ancient times and the present-day inhabitants, the Marsh Arabs, are considered the population with the strongest link to ancient Sumerians. Popular tradition, however, considers the Marsh Arabs as a foreign group, of unknown origin, which arrived in the marshlands when the rearing of water buffalo was introduced to the region.
RESULTS:
To shed some light on the paternal and maternal origin of this population, Y chromosome and mitochondrial DNA (mtDNA) variation was surveyed in 143 Marsh Arabs and in a large sample of Iraqi controls. Analyses of the haplogroups and sub-haplogroups observed in the Marsh Arabs revealed a prevalent autochthonous Middle Eastern component for both male and female gene pools, with weak South-West Asian and African contributions, more evident in mtDNA. A higher male than female homogeneity is characteristic of the Marsh Arab gene pool, likely due to a strong male genetic drift determined by socio-cultural factors (patrilocality, polygamy, unequal male and
female migration rates).
Phylogeny of Y-chromosome haplogroups and their frequencies (%) in Marsh Arab and Iraqi populations. Haplogroups are labelled according to the Y Chromosome Consortium and the International Society of Genetic Genealogy . Differently from previously reported , the M365 mutation was observed in two J1-Page08 Y-chromosomes (Marsh Arabs). In these two subjects, M365 was observed in association with the new mutation L267.2 discovered while typing the M365 marker. It consists of an A to G transition at nucleotide position 159. The markers P37, M253, M223 of haplogroup I, M81 and M293 of haplogroups E, and M367, M368 and M369 of haplogroup J1 were typed but not observed. A star (*) indicates a paragroup: a group of Y chromosomes not defined by any reported phylogenetic downstream mutation. nt: not tested. (a) Heterogeneity.
CONCLUSIONS:
Evidence of genetic stratification ascribable to the Sumerian development was provided by the Y-chromosome data where the J1-Page08 branch reveals a local expansion, almost contemporary with the Sumerian City State period that characterized Southern Mesopotamia. On the other hand, a more ancient background shared with Northern Mesopotamia is revealed by the less represented Y-chromosome lineage J1-M267*. Overall our results indicate that the introduction of water buffalo breeding and rice farming, most likely from the Indian sub-continent, only marginally affected the gene pool of autochthonous people of the region. Furthermore, a prevalent Middle Eastern ancestry of the modern population of the marshes of southern Iraq implies that if the Marsh Arabs are descendants of the ancient Sumerians, also the Sumerians were most likely autochthonous and not of Indian or South Asian ancestry
Networks of the STR haplotypes associated with haplogroups J1-M267* and J1-Page08, respectively. The eight STR (YCAIIa, YCAIIb, DYS19, DYS389I, DYS389II, DYS390, DYS391 and DYS392) haplotypes observed in 54 and 377 samples, respectively, are listed in Additional file 5. Circles and coloured sectors are proportional to the number of subjects, with the smallest circle and sector equal to 1. Connecting lines are proportional to the number of mutations.
Principal component analyses of Y-chromosome and mtDNA haplogroup frequencies. The PCA analyses were carried out on haplogroups listed in Additional files 3 and 4. Haplogroups with frequencies lower than 5% in all the populations were not considered. On the whole, 28% of the total variance is represented for the Y-chromosome (16% by the first PC and 12% by the second PC) and 39% for the mtDNA (20% by the first PC and 19% by the second PC). Populations included are: IRM, Marsh Arabs; IRQ, Iraqi; Alb, Albania; Alg-A, Algeria-Arabs; Alg-B, Algeria-Berbers; Aze, Azerbaijan; Ben, Benin; Bos, Bosnia; Bul, Bulgaria; Cau, Caucasus; Crt, Crete; Cro, Croatia; Cze, Czech Republic; Dru, Druze; Egy, Egypt; Egy-A, Egypt-Arabs; S-Egy, South Egypt; N-Egy, North Egypt; Eth-A, Ethiopia-Amhara; Eto-O, Ethiopia-Oromo; Geo, Georgia; Gre, Greece; Hun, Hungary; Ind, India; Ind-AA, India-Austro-Asiatics; Ind-D, India-Dravidians; Ind-IN, India-Indo-Europeans; Ind-TB, India-Tibeto-Burmans; N-Eur, North Europe (Austria, Germany, Ireland, North Italy, Poland, Scotland); N-Irn, North Iran; S-Irn, South Iran; IRN, Iran; NeI, North East Italy; C-Ita, Central Italy; S-Ita, South Italy; Sar, Sardinia; Jor, Jordan; Kur, Kurds; Leb-C, Lebanon-Christians; Leb-D, Lebanon-Druze; Leb-M, Lebanon-Muslims; Mar, Morocco; Ber, Morocco-Berbers; Oma, Oman; Pak, Pakistan; Pak-D, Pakistan-Dravidians; Pak-B, Pakistan-Burushaski; Pak-IE, Pakistan-Indo-Europeans; Pal, Palestinian; Pol, Poland; Qat, Qatar; Rwa-H, Rwanda-Hutu; Rwa-T, Rwanda-Tutsi; Sau, Saudi Arabia; Slv, Slovenia; Som, Somalia; Spa, Spain; Sud-A, Sudan-Arabs; Sud-N, Sudan-Niloti; Taj, Tajikistan; Tun, Tunisia; Tur, Turkey; Tuk, Turkmenistan; Ukr, Ukraine; Uae, United Arab Emirates; Yem, Yemen
Frequency (left panels) and variance (right panels) distributions of Y-chromosome haplogroups J1-M267, J1-M267* and J1-Page08. Maps are based on 102 digit points [Additional file 6]. Variance data are relative to the microsatellite loci DYS19, DYS389I, DYS389II, DYS390, DYS391 and DYS392 typed in all the reported samples. Frequency and variance .
Gender differences in the Marsh Arab gene pool
In comparison with the control sample, representative of the general Iraqi population, Marsh Arabs are characterized by an important lower Y-chromosome heterogeneity (HY = 0.461 vs 0.887) whereas similar values of heterogeneity were observed for mtDNA (HmtDNA= 0.963 vs 0.957). This is due to the presence of one prevalent Y-chromosome haplogroup, the J1-M267, which alone characterizes more than 80% of the Marsh Y-chromosome gene pool. Although patterns of lower male than female heterogeneity have been reported in many populations and usually ascribed to patrilocal residence [62–64], such a scenario can explain only part of the large difference observed in the geographically isolated marsh population. Among the different factors (e.g. polygamy, unequal male and female migration rates and selective processes) that can differently affect the extent of mtDNA and Y-chromosome heterogeneity, nonrandom-mating practices, common in the area, in association with cultural beliefs that support polygamy, may have contributed to cause the difference observed in the Marsh Arabs.
Conclusions
The analyses carried out on the mtDNA and Y chromosome of the Iraqi Marsh Arabs, a population living in the Tigris-Euphrates marshlands, have shown: (i) a prevalent autochthonous Middle Eastern component both in male and female gene pools; (ii) weak South-West Asian and African heritages, more evident for mtDNA; (iii) a higher male than female homogeneity, mainly determined by the co-occurrence of socio-cultural and genetic factors; (iv) a genetic stratification not only ascribable to recent events. The last point is well illustrated by Y-chromosome data where the less represented J1-M267* lineage indicates Northern Mesopotamia contributions, whereas the most frequent J1-Page08 branch reveals a local recent expansion about 4,000 years ago . Although the Y-chromosome age estimates deserve caution, particularly when samples are small and standard errors large, it is interesting to note that these estimates overlap the City State period which characterised Southern Mesopotamia, and is testified to by numerous ancient Sumerian cities (Lagash, Ur, Uruk, Eridu and Larsa).
In conclusion, our data show that the modern Marsh Arabs of Iraq harbour mtDNAs and Y chromosomes that are predominantly of Middle Eastern origin. Therefore, certain cultural features of the area such as water buffalo breeding and rice farming, which were most likely introduced from the Indian sub-continent, only marginally affected the gene pool of the autochthonous people of the region. Moreover, a Middle Eastern ancestral origin of the modern population of the marshes of southern Iraq implies that, if the Marsh Arabs are descendants of the ancient Sumerians, also Sumerians were not of Indian or Southern Asian ancestry
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