After 6 years of work by more than 300 researchers, and $53 million in funding, the cow genome has arrived. Yes, this is just one of many animal genomes that have been sequenced since humans took the spotlight in the year 2000, but its adherents say the cow code tells a fascinating tale of evolution and domestication. It's been seized upon by a cattle industry keen to improve on the quality of cow milk and beef.
A comparison of the cow genome, with more than 22,000 genes, with other mammalian genomes is already turning up surprises. Although humans share a more recent common ancestor with rodents than they do with cows, it turns out that our genome more closely resembles those of cows and dogs. This is probably because mice and rats evolve so quickly thanks to rapid reproduction, much quicker than other species, says one of the team leaders, Kim Worley, a genomicist from Baylor College of Medicine in Houston, Texas, where the sequencing was done. The genome sequenced, belonging to a Hereford cow, also has more genes linked with immune functions than does the human genome and more duplication of such genes. One possible reason, according to the sequencers, is that cows are ruminants with multiple stomachs and are exposed to many more microorganisms. "There's a lot of questions with immunity" yet to be resolved, says Richard Gibbs, head of Baylor's sequencing center and a project leader.
Meanwhile, a $500,000 spinoff project is having an immediate impact. The Bovine Haplotype Map, as it's called, characterizes genetic diversity among breeds. Here, a separate but overlapping group of researchers looked for tens of thousands of single nucleotide-polymorphisms (SNPs)--variations in DNA molecules at a single DNA base-- identified in the Hereford cow and sequenced them in six additional breeds. From there, the team examined the most interesting SNPs, more than 37,000, in nearly 500 cows, some groups of which diverged evolutionarily more than 250,000 years ago. The researchers found surprisingly high genetic diversity within individual breeds--higher than what's seen in human and dog populations. They also saw different genetic patterns depending on whether the cow was bred to produce meat or milk.
The Haplotype Map is generating excitement because it offers the chance to select for features that cattle breeders want in their cows--in particular, high-quality milk. Until now, the only way to guarantee the best cow's milk was by taking a bull, inseminating cows with his semen, and then waiting for the female offspring to grow and produce calves and milk to feed them, at a cost of $25,000 to $50,000 per bull. (Most of the genetic improvements in the cattle industry come through males, because each male can produce tens of thousands of females.) Already, cattle breeders are eagerly mapping SNPs in most of their bulls, with an eye toward identifying which SNPs are linked with good milk and other promising qualities. The Haplotype Map was led by Curt Van Tassell, a geneticist at the U.S. Department of Agriculture; Jeremy Taylor of the University of Missouri, Columbia; and Gibbs; both it and the sequencing paper appear tomorrow in Science.
The papers offer "the most comprehensive view on an animal genome we ever had," wrote Arne Ludwig of the Leibniz Institute for Zoo and Wildlife Research in Berlin, in an e-mail to Science. The code should be a real boon to farmers, noted Ludwig, whose team is publishing a paper tomorrow in Science on horse domestication. "We are at a brink [of] a new age of animal breeding."