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DNA Sequencing Traces Dromedary Domestication Origins

NEW YORK (GenomeWeb) – Using ancient and modern DNA, an international team of researchers examining the dromedary's domestication has traced its origins in the Middle East and Africa.

The researchers, led by the Research Institute of Wildlife Ecology in Vienna's Pamela Burger, analyzed more than 1,000 modern dromedaries from 21 countries, as well as seven early-domesticated and eight wild dromedaries. As they reported today in the Proceedings of the National Academy of Sciences, the investigators found a high level of genetic diversity in the animals, suggesting they likely experienced high levels of gene flow. The team also found evidence that after their initial domestication, the domestic dromedary population was restocked from now-extinct wild populations.

"Our results show that the domestication process and the current diversity of the species were shaped by early introgression from the wild as well as by human-mediated factors," Burger and her colleagues wrote in their paper.

The team combed through a dataset of 758 mitochondrial and 970 microsatellite genotypes representing dromedaries from five geographical regions: Eastern Africa, Western and Northern Africa, North Arabian Peninsula, South Arabian Peninsula, and Southern Asia and Australia. Rather than the genetic variation in this dataset pointing to any one locale as the likely origin of dromedaries, the researchers uncovered similar levels of heterozygosity and allelic richness throughout the various populations.

They attributed this "blurring [of] genetic signals" to the dromedary's role in cross-continental caravan networks.

Still, analysis of the mitochondrial samples split the dromedaries' 76 haplotypes into two haplogroups, HA and HB, though the researchers did not note any phylogeographic pattern among these groups. At the same time, a nuclear DNA structural analysis split the cohort into two groups, separating the dromedaries from Eastern Africa from all the other populations.

The Eastern African dromedaries had the lowest nuclear — though highest mitochondrial — diversity of all the populations, which the researchers said could be explained by a large amount of ancestral mtDNA diversity or by cryptic population structure. These dromedaries also exhibited a more balanced ratio of haplotype groups than the other populations, suggesting a random founder effect followed by gene flow with a limited number of sires.

Burger and her colleagues also noted a lack of genetic structure between the Western and Northern African and the North Arabian Peninsula dromedary populations. This could indicate an exchange of dromedaries introduced into northeastern Africa from the Arabian Peninsula by way of the Sinai, they said.

At the same time, the researchers found that the two African dromedary populations — the Eastern Africa and the Western and Northern Africa-North Arabian Peninsula groups — were the most genetically distant despite their geographical closeness. Since the pairwise genetic distances for Eastern African dromedaries traced to the South Arabian Peninsula population, the researchers said this suggested two possibly entry routes for dromedaries into Africa: from the Arabian Peninsula across the Red Sea to Egypt, or across the Gulf of Aden and south toward modern-day Sudan, Eritrea, and Ethiopia.

To further examine this gene flow, the researchers turned to ancient dromedary DNA — some 7,000 years old — from eight different sites around Africa and the Middle East.

As these wild dromedaries all carried known mitochondrial haplogroups, this indicated to the researchers that both haplogroups, HA and HB, were present in the Levantine herds of the fourth to seventh centuries CE. Estimates of the most recent common ancestor of the HA and HB haplogroups predate domestication, further suggesting that at least two — but possibly six — wild maternal lineages were captured during domestication.

The researchers' mDNA analyses also indicated past demographic expansion, and one analysis implied a rise in the domesticated dromedary population some 600 years ago at about the time of Arab expansion, the rise of the Ottoman Empire, and the conquest of Constantinople.

The researchers noted four possible scenarios that could account for the patterns of genetic diversity they observed in domesticated dromedaries. When they simulated them, one showed the highest probability: a small initial population of domesticated dromedaries was re-stocked with wild dromedaries. As wild and domestic dromedaries only co-existed for a short while, the researchers said that this period of gene flow was rather short, and less than that of domesticated cattle and their wild forbears.

Burger and her colleagues also found that the modern dromedary populations have largely maintained their ancestral diversity, which other livestock have typically lost. This, the team added, highlights the animals' "potential to adapt sustainably to future challenges of desertification and climate change."