Ethnic and Geographic Differentiation of Helicobacter pylori within Iran:
The bacterium Helicobacter pylori colonizes the human stomach, with individual infections persisting for decades. The spread of the bacterium has been shown to reflect both ancient and recent human migrations. We have sequenced housekeeping genes from H. pylori isolated from 147 Iranians with well-characterized geographical and ethnic origins sampled throughout Iran and compared them with sequences from strains from other locations. H. pylori from Iran are similar to others isolated from Western Eurasia and can be placed in the previously described HpEurope population. Despite the location of Iran at the crossroads of Eurasia, we found no evidence that the region been a major source of ancestry for strains across the continent. On a smaller scale, we found genetic affinities between the H. pylori isolated from particular Iranian populations and strains from Turks, Uzbeks, Palestinians and Israelis, reflecting documented historical contacts over the past two thousand years.
Nothing that surprising, geography and ethnicity predict a lot of the H. pylori pattern of variation. Iranian Arabs are closer to the populations of the Levant than other Iranians. Those groups in Northwest and Northeast Iran seem similar to populations just over the border in Turkey and Uzbekistan. Finally, one of the local strains found was in Yazd, a notoriously isolated city (which explains the large Zoroastrian population, which persisted in out of the way locales). Though Yazd is near the geographic center of Iran, it is embedded within the vast agriculturally marginal lands of the center.
This geographic pattern has been used to explain particular patterns of genetic variation which make eastern and western Iran distinctive. Even if you imagine that Iranians descend from several pure ancient populations, Turks and Persians by and large, the fact that settlement is concentrated around the periphery will have long term effects on genetic variation due to gene flow. And yet while genes may change ineluctably, cultural variation seems to often be very sensitive to top-down dynamics. Even if the Persian-speakers of western Iran begin to resemble their Arab neighbors genetically because of a small, but consistent, flow of genes between the two groups, that does not entail the emergence of an Arabo-Persian hybrid language (though Arabic and Persian have influenced each other a great deal, though perhaps more from the former to the latter). That is because of the different natures of linguistic and genetic transmission.
Adding patterns of variation of genes of organisms which may be shaped by cultural habits or intercourse is another twist, which gives us a window into how humans interacted and behaved in the past. The concordance of genetic variation of the cattle and people of Tuscany in a surprising manner reinforced the plausibility of each via the common hypothesis which could explain the pattern. Or consider this paper from several years back: Genetic Analysis of Lice Supports Direct Contact between Modern and Archaic Humans.