Scientists complete genome sequence of sheep

An international team of scientists said Thursday they have successfully sequenced the complete genome of sheep, revealing new information about the species' unique and specialised digestive and metabolic systems, Xinhua reported.

Sheep, a major source of meat, milk, and fiber in the form of wool, is one of the represent animals for the ruminants, which are the dominant land herbivores. These animals have a unique digestive organ called rumen that turns plant material into a source of protein.

In the new study, the team, led by researchers from China's Northwest A&F University, Kunming Institute of Zoology, Chinese Academy of Sciences and BGI, reported two kinds of proteins with ruminant specific structures, and also specifically and very highly expressed in rumen wall.

"They are Trichohyalin-like 2 protein and PRD-SPRRII family proteins, which play the role of rumen substrate surface, crosslinking themselves and keratins via transglutaminase, to form the tough cornification layer of rumen called sheep tripe, commonly known as Maodu in Chinese," said lead author Jiang Yu, associate professor from Northwest A&F University.

They also screened all of the lipid metabolic pathway genes and were surprised to find that two genes known as MOGAT2 and MOGAT3 are highly expressed in sheep skin, but not in liver or duodenum.

In humans, MOGAT3 is an essential enzyme for the absorption of dietary fat in the small intestine and is an important liver enzyme, but never express in human skin, they said.

The findings suggested the loss of MOGAT2 and MOGAT3 in liver may reduce the importance of liver in long chain fatty acid metabolism in ruminants compared to non-ruminants.

The collaborative study, also involving Utah State University and Baylor College of Medicine in the US, the Roslin Institute in the UK, Commonwealth Scientific and Industrial Research Organisation in Australia and other institutes, was published in the US journal Science.

"The release and genomic comparative analysis of sheep genome enhances our understanding to the ruminant biology," Jiang said. 

"It will also promote the sheep genome re-sequencing projects and reveal more economics traits related genes. Researchers will have better tools to facilitate molecular breeding for specialised mutton sheep, to meet the increasing demand of mutton consumption."

According to Jiang, China raised about 300 million sheep and goats, but still imported 259,000 metric tonnes of lamb and mutton last year, accounting for about 20 percent of the world trade volume of sheep meat.

"The grazing ban has been implemented in China to restore the environment since 1999, we need a better stabling mutton sheep breed," he said.