The first analysis of genetic diversity in a modern agricultural commodity has returned some disturbing news: Market-driven chicken farming has produced a race of genetically homogeneous fowl in dire need of new blood.

Industrial chickens, bred to grow big and fast, have lost about half of the genetic variation found in their wild counterparts. The precise role of each lost variant isn't known, but many likely affect resistance to disease.

Until now, the system has worked — but the evolutionary clock could be ticking.

"New diseases, or mutations of old ones, occur all the time. Nature overcomes those new challenges by creating new defenses from existing genetic variability," said Purdue University animal geneticist Bill Muir, lead author of a study published this week in the Proceedings of the National Academy of Sciences.

If commercial breeders don't introduce new stock, said Muir, "genetic variability will be exhausted."

Commercial chicken farming relies primarily on just three breeds — the

White Leghorn, Rhode Island Red and Indian Game. A single bird can produce 200 offspring; breeding populations are kept small and isolated, and a few chickens' genes can soon spread through millions of birds.

This never-ending cycle of inbreeding has made possible a $26-billion industry capable of producing one million birds an hour and 75 billion eggs each year in the United States alone, accounting for nearly half of all consumed meat. In financial terms the industry is healthy, but there are warning signs.

The avian lucosis virus, a fast-spreading and highly lethal hybrid of old and new chicken viruses first identified in 1988, has already put several poultry breeding companies out of business. Avian influenza has wreaked havoc on poultry in many parts of the world, and security experts see domestic food production as a prime target for bioterrorists.

What's needed, argue Muir and Hans Cheng, United States Department of Agriculture avian disease specialist and co-author of the study, is an infusion of genetic material from chickens outside the industry, especially those bred by small farmers in the developing world. The standards of commercial farming won't be easy to crack, but it's necessary.

"This will take much time and effort," said Muir, "but it's an insurance policy on the future."

The hardest part may be selling companies on a new approach to breeding. Even if fresh breeds aren't introduced directly but rather bred separately until ready to go commercial, they will almost certainly grow more slowly than standard chickens, whose lives have been compressed into a fraction of their natural span, and have physiques dictated by consumer preference.

"Using the same production systems, the food conversion and growth rate would be slower," said University of Bristol veterinary scientist Toby

Knowles, who was not involved in the study.

If demand for high-growth, high-speed chickens isn't tempered, the problem of genetic homogeneity might also be delayed rather than solved, with the latest birds entering the cycle of intensive inbreeding. But Knowles noted that growing numbers of consumers now want meat grown and produced in healthier ways.

"Highly intensive systems are likely to become less sustainable when there is consumer resistance," he said.

Once the chicken problem is fixed, scientists might move on to other menu items.

"The same concern exists for other commercially developed livestock species, such as dairy cattle and swine," said Muir. "They have had a similar domestication history but with perhaps even smaller breeding sizes."

Genome-wide assessment of worldwide chicken SNP genetic diversity indicates significant absence of rare alleles in commercial breeds [PNAS]

*Image: Laura Hadden

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