A group of geneticists and archeologists from Ireland, France, Iran, Germany, and Austria sequenced the DNA of 1,600-year-old sheep mummies discovered in an ancient Iranian salt mine, Chehrābād.
This unique specimen has revealed ancient Near Eastern sheep husbandry techniques and demonstrates how natural mummification might impact DNA degradation. The fantastic discoveries were just published in the peer-reviewed international journal Biology Letters.
Chehrābād Salt Mine
The Chehrābād salt mine is notable for preserving biological material. Indeed, the famous "Salt Men's" human bones were discovered in this mine, desiccated by the salty atmosphere. The new study reveals that the natural mummification process involves removing water from a body to preserve soft tissues that would otherwise decay and preserve animal remains.
The researchers took advantage of this by extracting DNA from a tiny slice of mummified flesh from a leg found in the mine, headed by geneticists from Trinity College Dublin.
While most old DNA is damaged and fragmented, the researchers discovered that the DNA from the sheep mummies was exceptionally well preserved, with longer fragment lengths and less damage than one would expect from such an ancient period. The mummification process is credited with this, with the salt mine providing perfect conditions for preserving animal tissues and DNA.
The researchers also took advantage of the sheep's DNA preservation to look at genes linked to a fluffy fleece and a big tail, two significant economic characteristics in sheep. A "hairy" coat distinguishes certain wild sheep, such as the Asian mouflon, from the woolly coats observed in many farmed sheep today. Fat-tailed sheep are also prevalent in Asia and Africa, where they are prized for their meat and may be well-adapted to dry environments.
The researchers created a genetic imprint of the sheep and determined that the mummy lacked the gene variation linked with a woolly coat. Still, fiber examination using SEM revealed tiny hair fibers compatible with hairy or mixed coat breeds.
Surprisingly, the mummies had genetic variations linked with fat-tailed breeds, suggesting that the sheep was comparable to the hairy-coated, fat-tailed sheep observed in Iran.
"Because mummified remains are uncommon, limited empirical data on the survival of ancient DNA in these tissues existed before our work," says Conor Rossi, a Ph.D. student in Trinity's School of Genetics and Microbiology, and the paper's primary author.
Remarkable DNA Integrity
"The DNA's remarkable integrity was unlike anything we'd seen before in ancient bones and teeth." This DNA retention, along with the metagenomic profile's uniqueness, demonstrates how important the environment is to tissue and DNA degradation dynamics.
The work was overseen by Dr. Kevin G Daly, also of Trinity's School of Genetics and Microbiology. "Our team managed to construct a genetic picture of what sheep breeds in Iran may have looked like and how they may have been utilized 1,600 years ago using a mix of genomic and microscopic methods," he continued.
"We may learn about what ancient societies valued in animals by using cross-disciplinary techniques, and our study informs us that the inhabitants of Sasanian-era Iran may have maintained flocks of sheep specialized for meat consumption, implying well-developed husbandry practices."
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