Evolution of domestication

AgResearch's McEwen says two major centres of domestication have long been recognised, in the Middle East and Europe, and there is evidence of introgression between them in Iran and the Transcaucasus region.

Very recently geneticists have found evidence for a third centre of domestication in Asia, possibly in China.

"The new SNP chip should throw a lot of light on [the history of domestication]," McEwen says.

"For many years geneticists have used mitochondrial and Y-chromosome DNA to track how humans migrated around the world, but very recently SNP chips with variants of autosomal genes have been providing a more detailed history."

He says the contrasting selection pressures for sheep and cattle should make for interesting findings. Where cattle were originally domesticated as draught animals, to pull ploughs, and as a source of meat, sheep were initially bred as a food source, for meat and milk.

"People have been collecting wool from sheep for millennia, but strong selection pressure for traits like fleece quality began much more recently - perhaps only a few thousand years ago," he says.

That ancient division of interest is perpetuated on opposite sides of the Tasman Sea today. Most of Australia's wealth from sheep comes from wool; New Zealand's wealth rests on meat from fat lambs.

"We're very interested in speeding up the rate of genetic progress using whole-genome selection. We can genetically link elite animals at a younger age, which allows us to reduce the generation interval, and increase the rate of progress.

"We can also do whole-genome association studies to derive molecular breeding values, which is an improvement on doing measurements on individuals and using progeny testing."

McEwan says a number of traits the team is interested in are either sex-limited, or are not expressed until later in life.

"One is obviously reproductive ability, including the maternal component of lamb survival. We're interested in the ability to grow rapidly early in life, but without an equivalent increase in the mature size of the animal, so we finish up with a medium-sized animal that grows rapidly to maturity.

"A lot of New Zealand sheep have high mature weights, which is a problem for shearers.

"The other important traits are disease resistance - people are reluctant to challenge animals experimentally to assess disease resistance. Helminth parasites are the biggest problem, followed by facial eczema, viral pneumonia, footrot and flystrike.

"We also have a consortium in New Zealand studying susceptibility to Johne's disease in cattle, sheep and deer, and the sheep component of that hopes to make use of our SNP chip."