Wednesday, December 29, 2010

Assyrians and the ADMIXTURE Southwest Asian Component Revisited

Last month, I wrote a series of articles in which I tried to date the expansion of the ADMIXTURE Southwest Asian component.

Assyrians,
Demic Diffusion Model:  Bounded and Unbounded,
Estimating a First Arrival Date for Southwest Asian peoples in the Fertile Crescent

In writing these articles, I was under the assumption that the Southwest Asian component emerged in the Arabian peninsula and moved northward.  I had used Assyrians as a timestamp of a Northern Fertile Crescent population that had not changed in the last 3500 years, but had absorbed the Southwest Asian component during the Bronze Age. From this assumption, I then tried to date the introduction of the Southwest Asian component into the northern Fertile Crescent.

As discussed in yesterday's post, it is clear that the J1e y-chromosome haplogroup, which is spatially correlated with the ADMIXTURE Southwest Asian component, originated in the Zagros Mountains with an expansion time of 16.2 kya (SD 6.4 kya).  Clearly, Assyrians, as a Zagros descended people, have not acquired the J1e haplogroup during the Bronze Age!

The distribution of the J1 haplogroup is deceptive in that its area of greatest frequency on the Arabian peninsula is not distributed about its region of origin.   That's certainly a warning about making assumptions about origin from spatial frequency distributions.

My original curiosity about the Southwest Asian component stemmed from looking at ADMIXTURE results for European populations.  The Southwest Asian component does not appear in Northern European countries while Mediterranean populations possess it in small proportions (See Dodecad K10 ADMIXTURE result.)  Even more curiously, all European populations, with the exception of French Basques and Sardinians, have a proportion of the West Asian component. 

One question I asked myself is "Why does the West Asian component, but not the Southwest Asian component, appear in Northern European populations?"

In these November articles, I assumed that the Southwest Asian component had been confined on the Arabian peninsula at the time of the early expansions into Europe.   As it turns out, the Southwest Asian component and the J1e y-haplogroup have been distributed, along with the West Asian component, throughout the Fertile Crescent and the Arabian peninsula for at least 9,000 years (Chiaroni, et al).

The El-Sibai et al paper

lends a possible answer to the absence of the J1 haplogroup in Europe as well as to why ADMIXTURE detects two separate components in West Asia: "We sought, through STR network analysis, to assess whether or not the observed geographic distribution of each haplogroup was reflected in geographic variations of STR haplotype distributions. The J1 and J2 (Fig. 5A) sister clades depicted a clear non-uniform geographic distribution of STR haplotypes and few instances of haplotype sharing across geographic regions. Consistent with previous analyses, coastal Levantine regions were well represented in the J2 network. The J1 network was dominated by inland Levantine samples (mainly Jordan and inland Lebanon and Syria)." This suggests populations correlated with the J2 y-chromosome haplogroup as a source of the West Asian component in Europe. J1 y-chromosome correlated populations remained inland and separate from their coastal J2 cousins.

This separation in the J1 and J2 correlated populations may have existed over millenia. Early Holocene J2 (and other coastal agriculturalist populations) could have travelled from West Asia to Europe, bringing with them the West Asian component, but not the Southwest Asian component. Eventually, the Neolithic wave of advance across Europe carried the West Asian component as far north as Scandinavia. Meanwhile, back in the Bronze Age Levant, the J1 and J2 populations once again came into contact with each other. From the Bronze Age onward, these J1/J2 populations established themselves throughout the Mediterranean. This would account for the Southwest Asian component that appears in Mediterranean populations such as Tuscans and Greeks.

For simplicity, I've ignored the contribution of other West Asian haplogroups to Europeans such as y-chromosome haplogroup G.

My apologies for this error. I hope that the above is a more plausible explanation for the distribution of the J1 haplogroup and the Southwest Asian component.

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