Sheep and goats such common fixtures of pastoral and farming life that few bother to ask where they came from or how they became such major hallmarks of human agriculture and animal husbandry. The typical narrative to explain how these now ubiquitous goats and sheep were engrained in the collective imagination of the rural countryside is that they accompanied humans along their migrations along the globe, or were domesticated as humans spread to new regions of the world. Essentially, goats and sheep, as well as chickens, cattle, and pigs became domesticated in circumstances where they could provide food resources necessary to sustain growing populations in the many new environments in which early human migrants found themselves.
Though these theories are quite convenient and provide an idyllic story, there are large gaps in explaining exactly why these particular animals were domesticated in the first place. Unfortunately there are no historical or archaeological records that can explain why early humans found it useful to domesticate certain animals, but thankfully genetics and biology can offer some provocative hints. Some work has been done on cattle, chickens and pigs, but relatively little is known about either goats or sheep. Several years ago, Wang Wen of the Kunming Institute of Zoology (KIZ), Chinese Academy of Sciences (CAS) noticed this oversight, and began an ambitious inter-institutional project to begin mapping out the goat and sheep genomes, both to gain some insights into their evolutionary history, but also to identify the key traits that may have underlie their initial selection for domestication.
In two recent studies published Nature Biotechnology and Science, Wang’s teams published their results of the sequenced genomes of both goats and sheep. Sheep and goats are thought to be the first domesticated livestock species, likely because of their hardiness and ability to digest plant material. Both animals are ruminants, capable of digesting plant material in a four-chambered stomach, effectively converting rough plant material—incidentally, that is quite low-value to human diets—into useable animal proteins. This feature alone would have made goats and sheep attractive targets for domestication, but they both animals are also highly valued for their hair, i.e., wool in sheep and cashmere in goats. Wool, for example, has a great deal of lanolin, a naturally secreted wax that today continues to be an important commodity in raising sheep. Lanolin primarily formed from wax esters, which may be linked to fatty acid metabolism. During their mapping of the sheep genome, the research team noted major genomic signatures associated with interactions between diet, digestion, and metabolism. Interestingly, the human-drive selection of sheep for foodstuffs and wool production might then hold some insights for exploring the genetic underpinnings of digestion and fatty acid metabolism, and in turn help clarify some of the history of their domestication.
Alongside their historical importance, both sheep and goats continue to be key livestock, especially in China. Recent estimates place China as the largest producer and consumer of sheep, with total numbers of goats and sheep in excess of 300 million. Meeting growing demand for meat, milk, cheese, cashmere, wool, or other animal byproducts could be greatly enhanced by more sophisticated breeding. These newly published genomes will likely serve as a foundation for researchers interested in isolating key traits or elite genes connected with superior features of particular breeds, which could potentially yield large boosts in productivity, profit, and efficiency.
The domesticated goat genome was recently published Nature Biotechnology (http://www.nature.com/nbt/journal/v31/n2/full/nbt .2478. html), and the study on rumen digestion and lipid metabolism in sheep was published in Science (http://www.sciencemag.org/content/344/6188/ 1168.full).
(By Andrew Willden)
