Genomic Data Reveal a Complex Making of Humans

Figure 5 illustrates the network of the control-region mtDNA haplotypes associated with each haplogroup found in this survey [Additional file 2]. The majority of haplogroups were present in both population samples although with scarce sub-haplogroup and haplotype overlapping. In addition, differently from the control sample, a number of Marsh Arab haplotypes were shared between two or more subjects.



Figure 5
Network of 233 mtDNA control-region haplotypes observed among 319 Iraqi samples. These haplotypes [Additional file 2] refer to the variation observed between np 16024 and np 200. Circles are proportional to the number of subjects, with the smallest circle equal to 1. Connecting lines are proportional to the number of mutations including haplogroup diagnostic markers. Haplogroups and sub-haplogroups are labelled according to Table 1.

Discussion
Two hypotheses have been proposed for the origin of Marsh Arabs: (i) they could be aboriginal inhabitants of Mesopotamia, correlated to the old Sumerians; (ii) they could be foreign people of unknown origin. Although the origin of Sumerians has yet to be clarified [5], the two main scenarios, autochthonous vs foreign ancestry, may have produced different genetic outcomes with Marsh Arabs being genetically closer to Middle Eastern groups or other populations, for instance those of the Indian sub-continent. Thus, in order to shed some light on this question Marsh Arab population was investigated for mtDNA and Y chromosome markers. Due to their characteristics (uniparental transmission and absence of recombination) and their wide datasets, they are, at present, among the best genetic systems for detecting signs of ancient migration events and to evaluate socio-cultural behaviours [35, 36].

Evidence of a Middle Eastern origin of the Marsh Iraqi Arabs comes mainly from the Y chromosome
Although different Western European mtDNA haplogroups were present in the Middle East in Palaeolithic times, they cannot always be interpreted as markers of Middle Eastern origin. For example, even if the mtDNA haplogroup H evolved in the Middle East ~18,000-15,000 years ago [34], different H sub-groups observed in this region, albeit at a rather low frequency, such as H1, arose outside and are most likely the result of gene flow from Europe [34, 37].

Y-chromosome variation, like that of mtDNA, is highly geographically structured [34, 38, 39]. However, Middle Eastern haplogroup J, which accounts for the great majority of paternal lineages of this region and marks different migration events toward Europe, Africa and Asia, does not display, at present, evidence of back migrations.

A common ancestral origin of Marsh Arabs and Southern Arabian peoples
Haplogroup J, with its two branches J1-M267 and J2-M172, is a Y-chromosome lineage dating to about 30,000 years ago. Its place of origin is still under discussion, but it is considered a landmark geographically linked to the Near Eastern region where the agricultural revolution and animal domestication appeared for the first time [34]. Accordingly, the frequency distribution of Hg J [13, 40] shows radial decreasing clines toward the Levant area, Central Asia, the Caucasus, North Africa, and Europe from focal points of high frequency in the Near East [12, 40, 41]. Although both clades (J1-M267 and J2-M172) evolved in situ and participated in the Neolithic revolution, their different geographic distributions suggest two distinct histories. While J2-M172 has been linked to the development and expansion of agriculture in the wetter northern zone and is also considered the Y-chromosome marker for the spread of farming into South East Europe, J1-M267 has been associated with pastoralism in the semi-arid area of the Arabian Peninsula [42, 43]. Despite this purported initial association, no evidence of pastoralism has been reported in the marsh area where one of the J1-M267 highest values (81.1%) has been observed (Additional file 6, Figure 6).



Figure 6
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 details are reported in Additional files 6, 7 and 8.

Recent expansions shape the present Marsh Arab Y-chromosome landscape
When the two J1-M267 sub-clades, J1-M267* and J1-Page08 are considered (Figure 6), differential frequency trends emerge. The less represented J1-M267* primarily diffuses towards North East Mesopotamia and shows its highest incidence in the Assyrians of northern Iraq, and Turkey. By contrast, J1-Page08 accounts for the great majority of the J1 distribution in South Western Mesopotamia, reaching its highest value (74.1%) in the marsh area. By considering the STR haplotypes associated with the two branches, the highest values of variance are localized in northern Mesopotamia (North Iraq/South East Turkey) (Figure 6, Additional files 7, 8 and 9). For the J1-Page08 lineage, high variance values were also observed in Ethiopia, Oman and South Eastern Italy (Table 2). Although present data are not adequate to define the homeland of the J1-Page08 sub-clade, some useful information can be obtained from the haplotype network analysis (Figure 4). Thus, the pheripheric position of the Ethiopian and South Eastern Italian (European) haplotypes suggests that the high values of variance registered in these regions likely reflect the stratification of different migratory events, some of which occurred before the expansion and diffusion of the lineage outside the Middle Eastern area. As previously reported [31, 41], also the value of variance in the Omani is affected by the concomitant presence of both pheripheric and centrally expanded haplotypes. In this context, the low variance (0.118) observed in the Marsh Arabs underlines a recent expansion involving few haplotypes, all of which occupying a central position in the J1-Page08 network (Figure 4). In the less frequent J1-M267* clade, only marginally affected by events of expansion, Marsh Arabs shared haplotypes with other Iraqi and Assyrian samples, supporting a common local background (Figure 4).

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 (Table 2). 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.

BMC Evolutionary Biology 2011 11:288