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A genome for ewe

04 Nov, 2014

Mapping the whole genome of the sheep (Ovis aries) was completed earlier this year. The information is contributing to a myriad of new research projects to improve the health and meat and wool yields from New Zealand’s most populous farm animal.

The genome mapping was carried out by the International Sheep Genomics Consortium (ISGC), which includes two New Zealanders from AgResearch and the University of Otago.

The sheep breed used in the genome-mapping project was the Texel – originally from the island of Texel in The Netherlands but now cross-bred and farmed widely throughout Europe, the US, Australia and New Zealand. The breed was introduced here in 1990 because of its lean meat, wool yield and disease resistance.

Around the world, sheep are an important agricultural species for meat, milk and wool. They have a specialised digestive system, the rumen, which carries out the initial digestion of plant material, and a unique fat metabolism process, which helps maintain its thick woolly coat.

Mapping out the DNA code that is the blueprint for making a sheep and then comparing this to other mammals allowed the researchers to identify genes that explain the sheep’s unique qualities, including those that help support secretion of the grease (lanolin) needed to maintain wool. They also uncovered genes that, compared to their equivalents (homologs) in other animals, were expressed differently, and they identified genes that apparently gained new function during rumen evolution.

“We identified highly expressed genes encoding keratin cross-linking proteins associated with rumen evolution. We also identified genes involved in lipid [fat] metabolism that had been amplified and/or had altered tissue expression patterns. This may be in response to changes in the barrier lipids of the skin, an interaction between lipid metabolism and wool synthesis and an increased role of volatile fatty acids in ruminants compared with non-ruminant animals,” the researchers write in their published paper.

The researchers created a phylogenetic tree showing when the sheep diverged away from other mammals and then later other ruminants such as camels, cattle and goats. According to their tree, the modern-day sheep separated with goats some 4.2 million years ago. Earlier divergences away from other ruminants happened much earlier during the late Cretaceous and Palaeogene as grass species emerged.

The research team writes that they have identified “major genomic signatures associated with interactions between diet, the digestive system and metabolism in ruminants”.

“These include two extensions of their biochemical capabilities that have been extensively exploited by humans: the production of wool by sheep and the evolution of an organ that houses a diverse community of microorganisms that enable efficient digestion of plants,” the researchers conclude.

Already, AgResearch have used information from the 8-year genome project to develop the sheep SNP (Single Nucleotide Polymorphism) chip. Using a blood test, a farmer can use the small SNP (pronounced ‘snip’) chip with thousands of microscopic reaction sites to identify thousands of individual genes for a single animal to determine if the sheep they wish to breed has particular desirable traits they can pass on to their offspring. These traits might include meat yield, meat quality, adult live weight, facial eczema resistance, parasite resistance, longevity, fertility, various aspects of wool quality and number of lambs born.

The International Sheep Genomics Consortium involved 73 researchers in 26 institutions across eight countries.

The full genome project results, focusing on the two gene families that drive the sheep’s unique fat metabolism process and ruminant digestion system, were published in June 2014 in the journal Science.

References

Yu Jiang et al. (2014). The sheep genome illuminates biology of the rumen and lipid metabolism. Science 6,  344 (6188), 1168–1173. doi: 10.1126/science.1252806.

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