A virtual map of the sheep genome will enable sheep breeders and farmers to make better decisions in managing their flocks.

Credit: USDA

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SYDNEY: The sheep genome has been 'virtually' mapped with the aid of data from cows, dogs, and humans, in a new international study.

Researchers created a 'virtual map' of 98 per cent of the sheep genome by comparing small, known portions to the well-mapped genomes of related mammals. The map could help breeders quickly identify animals with good wool quality, parasite resistance, or other favourable qualities.

"What this study has enabled us to do is to find DNA markers that can serve as diagnostic tests so that breeders and farmers can make better decisions about their flocks," said Rob Forage, program director at SheepGenomics, a partnership of industry groups that funded the research.

Billions of dollars have been spent in recent years to sequence the genomes of many animals, including bumble bees, humans and dogs. The team used the data and techniques pioneered in these studies to create a 'best-guess' map of the sheep genome for far less money.

"We have worked out how to leverage the investment in the other mammalian genomes to build a virtual genome around a small amount of existing data," said co-author Brian Dalrymple of Australia's national science agency, the CSIRO. The data generated by the study are due to be released on the CSIRO's website later this week.

"Most of the genes are the same in cattle, sheep, dogs and human beings, it's just that they're ordered differently," said Dalrymple. The team compared sequences from sheep with the already sequenced genomes of other animals to figure out those differences.

They used molecules called restriction enzymes - which cut DNA at specific places - to break up the sheep genome. This left them with about 200,000 DNA fragments of various lengths which they then inserted into bacteria to create what are known as 'bacterial artificial chromosomes', or BACs - a method for using living cells to replicate genes of interest.

The team allowed the bacteria to reproduce, removed the BACs, and sequenced their ends so that they could match them to similar sequences on the known genomes of other mammals. According to Dalrymple, "This technique required enormously less effort than sequencing the entire genome."

"It was like a giant puzzle. We mapped the BAC-end sequences onto the frameworks of the human, dog, and cow genomes. That honed the information down to 1,172 sections of sheep genome, which we had to put together in the right order with the right orientation," said Dalrymple.

According to Forage, this research is an important tool for breeders, especially for selecting for traits that can't be measured while the animal is alive. How efficiently an animal utilises its feed, for example, can currently only be ascertained after an animal's death. DNA markers will "enable us to find out about these traits while the animal is alive," said Forage.

"Some traits are deleterious, and some animals are carriers of the genes for these traits [without showing the traits themselves]. With proper markers we have the ability to breed for animals that don't [carry] these genes," he said.

According to Dalrymple, this technique is a first in genomics research, but is ultimately not a substitute for sequencing the full genome. "This is equivalent to an intermediate resolution digital photo, where sequencing the full genome would be a very high resolution photo," he said.

SheepGenomics, the organisation that funded this research, is a partnership of Meat and Livestock Australia and Australian Wool Innovation. The partnership is a part of the International Sheep Genomics Consortium.