A poor mix can harm the long-term survival of wild birds and the ability to raise better chickens.
Today’s red junglefowl—the wild ancestors of domesticated chickens—are becoming increasingly chicken-like. New research shows that a significant proportion of the DNA of wild birds was inherited from chickens, and relatively recently.
Continued interbreeding between the two birds could threaten the future of wild bird populations in the jungle and even weaken people’s ability to breed better chickens the researchers reported on January 19 PLoS Genetics .
Red jungle bird ( Gallus gallus ) are forest birds native to Southeast Asia and parts of South Asia. Thousands of years ago, people domesticated birds, perhaps in the rice fields of the region.
“Chickens are probably the most important domestic animal on Earth,” says Frank Reindt, an evolutionary biologist at the National University of Singapore. He points to their global ubiquity and abundance. Chicken is also one of the cheapest sources of animal protein available to humans.
It is known that domestic chickens ( G. gallus domesticus ) interbreed with jungle birds near human settlements in Southeast Asia. Given the unknown impact on jungle birds and the importance of chickens to humanity, Reindt and his team wanted to gather more details. Wild jungle birds contain a store of genetic diversity that can be an important resource for breeding chickens resistant to disease or other threats.
The researchers analyzed and compared the genomes—the complete set of an organism’s DNA—of 63 junglefowl and 51 chickens from across Southeast Asia. Some of the jungle bird samples were taken from museum specimens collected between 1874 and 1939, allowing the team to see how the genetic makeup of jungle birds has changed over time.
Over the past century or so, the genomes of wild junglefowl have become increasingly similar to those of chickens. The team found that about 20 to 50 percent of the genomes of modern jungle birds come from chickens. In contrast, many of the approximately 100-year-old junglefowl had chicken ancestry in the range of a few percent.
The rapid changes likely stem from human communities expanding in the region’s desert, Reindt says. Most modern junglefowl live in close proximity to free-ranging chickens, with which they often interbreed.
Reindt says that such interbreeding has become “almost the norm” for any globally domesticated species, such as dogs interbreeding with wolves and domestic cats interbreeding with wild cats . Meanwhile, pigs mingle with wild boars and ferrets — with ferrets .
Wild populations that interbreed with their domesticated counterparts can acquire physical or behavioral traits that alter the functioning of the hybrids in their ecosystem, says Claudio Quilodran, a conservation geneticist at the University of Geneva who was not involved in the study.
The effect is likely to be negative, Quilodran says, because some of the traits that entered the wild population were honed for human use rather than survival in the native environment.
Wild jungle birds lost their genetic diversity as they also interbred. Avian heterozygosity—a measure of the genetic diversity of a population—is now only one-tenth of what it was a century ago.
“This result is counterintuitive at first,” says Reindt. “If you mix one population with another, you tend to expect more genetic diversity.”
But domesticated chickens have such low genetic diversity that certain versions of jungle chicken genes were washed out of the population by a tsunami of genetic homogeneity. A reduction in the genetic toolkit of these animals could make them vulnerable to conservation threats.
“Having a lot of genetic diversity within a species makes it more likely that certain individuals contain the genetic background to adapt to a variety of environmental changes and diseases,” says Graham Etherington, a computational biologist at the Earlham Institute in Norwich, England, who did not take participation in this study.
A smaller junglefowl gene pool may also mean fewer resources for breeding better chickens. The genetics of wild relatives are sometimes used to increase the resistance of domesticated crops to diseases or pests. Jungle bird genomes may be just as valuable for this reason.
“If this trend continues, future generations of humans will only be able to access the full genetic diversity of chicken ancestors in the form of museum specimens,” says Reindt, which could prevent the breeding of chickens using wild bird genes.
Some countries, such as Singapore, Reindt says, have begun managing jungle bird populations to reduce interbreeding with chickens.
