Following the flock

Scientists have used traces of retrovirus DNA to map ancient sheep migration across Asia, Europe, and Africa, a paper in this week's Science reports. The results may help settle a debate about where humans first bred sheep for their white, fleecy coats, the researchers say.

Soay sheep on St. Kilda, Outer Hebrides
Image: Arpat Ozgul

"What's neat about [the study] is that you're learning something about humans indirectly by studying animals that they brought along with them," said Welkin Johnson, a virologist at Harvard Medical School and the Institute for Primate Research in Southborough, Mass., who was not involved in the study. Ancient plant and animal domestication fueled human population growth, spurred the rise of cities, and "changed the course of our whole species," said Melinda Zeder, director of the archaeobiology program at the Smithsonian Institution in Washington, DC, who studies the domestication of sheep and goats but was not involved in the study. Therefore, understanding how, where, and when sheep breeding came to be can provide insight into our own history, she said. A team of scientists led by Massimo Palmarini, a virologist at the University of Glasgow in Scotland, collected genetic samples from 1362 animals from more than 133 breeds across Europe, southwest Asia, and Africa. The samples included some from a wild sheep called the Urial, thought to be the one of the closest relatives of the domestic sheep, and the Asiatic and Mediterranean mouflon, ancestors of domestic sheep. The researchers searched for six variants of a common sheep retrovirus called Jaagsiekte, which is highly contagious and causes shortness of breath, at specific locations in the sheep's genomes. Most of the retroviruses had inserted themselves into the sheep's DNA at different times during the last 10,000 years.

Urial Sheep
Image: Wikimedia Commons

The researchers could use this marker because a virion occasionally splices its code into germ line cells such as eggs or sperm, said Johnson. When this happens, the viral code is passed on from a sheep to its offspring like any other gene. Since the virus splices itself into a random location, it's "highly unlikely" that sheep that have the same retrovirus sequence in the same spot got it from two different insertion events, Johnson said. Therefore, sheep (or other organisms) that share the same retrovirus sequence and position likely shared a common ancestor in their past. By analyzing the proportion of sheep with each retrovirus marker, Palmarini's team concluded that sheep were initially domesticated (likely bred for their meat) in the Near East, and radiated out to the Mediterranean, Europe, Asia, and Africa. That "largely confirms" her own findings of a Near Eastern domestication about 10,500 years ago, drawn from animal skeletal remains, Zeder said. "I think it is very important supportive evidence coming from a whole new avenue."

Mouflon in zoo
Image: Wikimedia Commons

However, the retrovirus marker frequencies also suggested that domestic sheep spread from the Near East in a second wave, Zeder said. Because the modern-day sheep with darker, less wooly coats shared retrovirus markers with older, more primitive breeds like the Urial or Mouflon, the authors deduced that second-wave animals were domesticated specifically for their fleecy coats, Palmarini said. The researchers don't have direct evidence for their hypothesis, Zeder said. "It's an inference but it has some legs." Animals in Scandinavia and far-flung islands such as Iceland and the Hebrides in Scotland largely came from the first, more primitive wave of domesticated sheep, the study concluded. Sheep from these regions have traits commonly thought to be older: scratchier, darker coats, more horned females, and seasonal molting. The woollier animals were an improvement over earlier domestic breeds, and supplanted their predecessors in most of the world, the authors suggest. "But the [more primitive breeds] that remain are in places where this replacement was made difficult" because they were so remote, Palmarini said. One limitation of the current method is that it can't date migration events, so future archaeological work should look for material--such as ancient animal remains with DNA--that can be dated, Zeder said.
**__Related stories:__***linkurl: Retrovirus invading koalas;http://www.the-scientist.com/news/display/23868/
[6th July 2006]*linkurl:Of sheep and grapes: DNA fingerprinting tracks ancestry;http://www.the-scientist.com/article/display/18706/
[27th September 1999]