Revealing the History of Sheep Domestication Using Retrovirus Integrations
Scientists from all over the world (literally, Portugal to Pakistan, Italy to Iceland, Tanzania to Texas) have used the random insertion of endogenous retroviruses, specifically, Jaagsiekte sheep retrovirus, to determine the evolutionary history of sheepies:
enJSRVs can be used as highly informative genetic markers because the presence of each endogenous retrovirus in the host genome is the result of a single integration event in a single animal and is irreversible, so populations sharing the same provirus in the same genomic location are de facto phylogenetically related.
This crew focused on finding/analyzing six ERVs in 1362 animals from 133 kinds of sheep and sheepy-like relatives. They combined the PCR tricks I wrote about earlier (except they drew their figures in MS Paint instead of Poweroint, LOL!) with sequence analysis of the ERV LTRs. The LTRs on either end of the ERV should be identical– differences between them indicate age.
Proximal and distal LTRs of an endogenous retrovirus must be identical upon integration, but can diverge over time at the same rate as noncoding sequences (~2.3 x 10-9 to 5 x 10-9 substitutions per site per year). enJSRV-7 appears to be the oldest provirus in our samples because it displays five nucleotide (nt) substitutions between 5′ and 3′ LTRs (445 nt long), whereas all the other insertionally polymorphic proviruses (including enJSRV-18) have identical LTRs.
What they found matched archeological evidence, and provided support for outside hypotheses about the evolution and domestication of sheepies:
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. This may provide genetic support to the theory that specialized wool production arose in Southwest Asia and then spread throughout Europe (11). 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. Most, if not all, of the breeds we identified as of ancient origin were already considered primitive on the basis of morphological traits such as a darker and coarser hair (instead of a whiter woolly fleece), a moulting coat, and the frequent presence of horns in females as well as males (Fig. 4).
Ancestral sheepies founded a first wave of domestication for meat– yummy-sheep. When people started breeding them for wool as well as meat in SW Asia, the woolly yummy-sheep bounced out the yummy-only sheep in Europe. Domesticated yummy-only sheep in Europe were ‘de-domesticated’ back into their ‘wild’ state.
Also, Bible stories:
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. The origins of the Jacob are unknown. This breed owes its name to the Biblical story of Jacob who took “every speckled and spotted sheep” as a wage from his father-in-law Laban (Genesis 30:25-43; probably the first recorded use of selective breeding in livestock). Our retrotype analysis supports a direct link between the Jacob sheep and breeds in Southwest Asia or Africa rather than other British breeds. Our study also firmly links the Soay sheep with the Mediterranean and Asiatic Mouflon.