Haywire gene causes chickens to pig out

Failure to send fullness signal | Selective breeding changes circuitry in the brain’s feeding centre controlling food intake

LINDELL BEACH, B.C. — British researchers may have figured out why some chickens don’t know when to stop eating.


Overeating in the chicken industry leads to excessive weight, which can interfere with their ability to reproduce and affect a farmer’s bottom line.


A study at the University of Edinburgh’s Roslin Institute has discovered a gene that does not respond to the signal of fullness. That signal would normally be sent from the stomach to the brain with a simple response to stop eating.


“What we have shown is that the animals that carry the high growth allele do not respond to the signal of fullness,” said Ian Dunn, a research scientist in avian biology whose findings were published in the American Journal of Physiology, Endocrinology and Metabolism. 


“The normal response when you inject the substance CCK is a reduction in food intake, but in these high growth individuals this does not happen.”


The gene in question is the protein cholecystokinin, or CCKAR, which is a receptor for CCK, a secretion in response to food reaching the gut. One of its functions is to send a nerve signal to the brain to indicate fullness, which should trigger the bird to stop eating. 


The research suggests that this genetic shift to not recognize fullness may have its origins centuries ago when chickens were domesticated and early breeders selected for larger sizes. 


“We use the term ‘during domestication,’ by which we mean not something critical to the original domestication event but something that has developed when farmers have decided to breed for a particular conformation,” said Dunn. 


“Perhaps chickens were bred larger for fighting. In general, the larger the breed the greater there is the occurrence of the marker for the high growth allele.”


Researchers cross-bred a fast-growing meat chicken with a relatively slow-growing one and then looked at the workings of the protein in both birds as well as the resultant cross-bred fowl. 


They found that some birds were better equipped than others with the target protein, making them more effective at sending the signal at the appropriate time.


“What we have shown is that there is no difference to the actual protein between the high growth and low growth allele,” he said.


“What is different is how much of it is present. There is less in the high growth animals, which we assume explains the difference in their response to (the secretion) CCK.”


He said that in the quest for largeness, the breeds were being unknowingly selected for the different response to CCD’s effect. 


Researchers were also able to show that the reduction in the level of protein also affected the chicken’s natural body weight. 


However, understanding all the implications of the protein is still a work in progress. 


“We imagine it should alter secretions of digestive enzymes, but we still don’t know yet and we have to prove that.”


Dunn is uncertain whether chicken breeders will shift their breed lines to work with the genetics and help keep levels of this protein at a point where it maximizes its signal strength. 


This should work in theory, but “I think the reality is no one is going to give up the growth potential of this locus,” he said. 


“It tells us more what has happened in the past regarding selection.”


Dunn said the protein is also in the brain as part of the circuitry controlling food intake. That’s because the systems that control body functions were preserved in the distant sites as vertebrates evolved from simple organisms with simple gut and reproductive systems to ones with nervous systems remote from the gut. 


The protein works at two levels in the brain as well as in the gut to control feed intake. What Dunn found interesting was that genes in the brain’s feeding centre were altered between high growth and low growth chickens. 


Historically, it appears that this effect was present in the heavy chicken breeds from which modern broilers are descended. Equally interesting is the fact that all species have a genetic variation in the interpretation of biological signals relating to feeling full. 


They can regulate their appetites to take in the right amount of proteins and fats to maintain body weight, but as chickens have demonstrated, external influences such as selective breeding can change the level of those biological signals. 


So does this same protein level issue apply to other domestic animals?


“It certainly looks as if this is the case in pigs, from work done here some years ago … although the function was not worked out. It just remained an association,” Dunn said.


“I believe from the literature that there is a good chance size in horses and cattle and possibly people may also be influenced by this locus, but there is a lot of work to do to prove that association in the way we have for chickens.”


Whether feed manufacturers can develop chicken grower pellets that would reduce excessive growth is still speculation. 


“We don’t know, is the real answer,” he said. “However, we believe it is possible.” 


He says the diets are not economical due to increased production and transportation costs. 


Researchers are also evaluating how different diets may improve satiety and if diluted diets work.


“This new work is an important part of understanding the factors we need to measure and of course how different birds may respond.”