April 26, 2009

Retroviruses and the Origin of domesticated sheep

This paper suggests an "early" dispersal of sheep from the Near East, relics of which exist in peripheral areas, and a secondary major dispersal, also from the Near East, which is responsible for most modern sheep breeds.

Remnants of the earlier sheep breeds tend to have lower-quality wool, and this suggests that the secondary dispersal of sheep was associated with the beginning of the exploitation of the sheep's fur, rather than only its meat, resulting in the popularity of the new breeds.

It will be extremely exciting to track this dispersal archaeologically. We ought to have a few diagnostic SNPs of the various breeds of all the major domesticated animals very soon, which would be relatively easy to amplify in archaeological bone samples.

From the paper:
Collectively, the data we obtained indicate that relicts of the first migrations are still present in the Mouflon of Sardinia, Corsica, and Cyprus and in breeds in peripheral north European areas.


The homogeneous retrotypes (R2 only, or both R2 and R4) that we observed in the sheep of modern-day Turkey, Iran, Saudi Arabia, Syria, Israel, and Egypt, combined with available archaeological evidence, suggest that selection of domestic sheep with the desired secondary characteristics common to the modern breeds occurred first in Southwest Asia and then spread successfully into Europe and Africa, and the rest of Asia. [...] The primitive breeds survived the second migrations of improved breeds from Southwest Asia by returning to a feral or semiferal state in islands without predators or by occupying inaccessible areas less prone to commercial exchanges and associated introgression.


Our study also provides genetic evidence supporting the anecdotal origin of some less common sheep breeds. For example, one of the 10 populations analyzed from the British Isles, the Jacob sheep, displayed a homogeneous R2 retrotype very different from that of the other British populations and more similar to that of the southwestern Asiatic and African breeds [...] Our study also firmly links the Soay sheep with the Mediterranean and Asiatic Mouflon.

Science doi:10.1126/science.1170587

Revealing the History of Sheep Domestication Using Retrovirus Integrations

Bernardo Chessa et al.


The domestication of livestock represented a crucial step in human history. By using endogenous retroviruses as genetic markers, we found that sheep differentiated on the basis of their "retrotype" and morphological traits dispersed across Eurasia and Africa via separate migratory episodes. Relicts of the first migrations include the Mouflon, as well as breeds previously recognized as "primitive" on the basis of their morphology, such as the Orkney, Soay, and the Nordic short-tailed sheep now confined to the periphery of northwest Europe. A later migratory episode, involving sheep with improved production traits, shaped the great majority of present-day breeds. The ability to differentiate genetically primitive sheep from more modern breeds provides valuable insights into the history of sheep domestication.



Maju said...

It will be extremely exciting to track this dispersal archaeologically.That may be difficult. At least not so many years ago what manuals said about those kind of remains was "ovicaprids" (i.e. sheep or goat) without any specifics. Picking apart a boar from a pig seemed impossible, not to mention wild olive from cultivated olive.

Maybe the technology and the sistematics have improved but guess that in such aspects genetics has much more to say.

Wonder why did they study retroviruses instead of directly the sheep genome. It may be an innovative approach but left me somewhat surprised.

I find somewhat odd anyhow that most of the diversity appears to be in Europe. Wonder if sheep are less used in Africa and Asia or if they arrived there at a later date for some reason.

Shayan said...

Actually the diversity in Europe is due to the humidity more than anything. If you're familiar with sheep breeds, the fat-tailed variety common in Asia from Syria to Mongolia are extremely hardy, parasite resistant, easy-birthing, low upkeep, and intelligent. Far more so in almost all ways than European breeds, such that American farmers in my area have started breeding them.

And the article clearly states the sheep breeds ORIGINATED in Asia, not Europe.

Europe's sheep breed variety is due to the fact that many breeds can survive in Europe due to its more temperate climate, whereas the same breeds have astounding morbidity in harsher, more arid steppe or desert climes.