DNA region in chickens identified for disease resistance


One region in the DNA explains a large difference in possible disease resistance betweenchickens. This was discovered by researchers of Wageningen University & Research and Hendrix Genetics. This DNA region contains, amongst others, an important sensor for activating the immune system, which might explain why some chickens become ill and other do not. This discovery enables breeding organizations to select animals with a higher general disease resistance, which will result in a lower antibiotics use and higher animal welfare.
In poultry housing system, animals frequently get in contact with each other. This enables a pathogen, once it is present in a stable, to spread relatively easily among animals. For decades, the poultry industry is searching for more robust animals: animals with a good resistance against diseases. In addition, two recent management changes have been made: group housing of layer chickens, and reducing (preventive) usage of antibiotics. This increased the need for a more robust layer chicken. A possibility to obtain this robust chicken is to breed for animals with a higher general disease resistance.

Natural antibodies
Antibodies are proteins present in sick animals, which attack the pathogen. These antibodies are only made after the animal became infected with the pathogen. However, in addition to that, an animal also has natural antibodies (NAb). NAb have only recently been discovered in livestock. These antibodies are already present in healthy animals without (previous) exposure to a pathogen. NAb are important for fighting pathogens: they inhibit and prevent further infection in the body, but also warn and activate other parts of the immune system. Earlier studies show promising results: higher NAb levels in layers were associated to an increased chance to survive, and NAb levels are heritable and thus can be influenced by breeding.

Identification of DNA region
To get a better understand which DNA region(s) contribute to the heritable variation, researchers of Wageningen University & Research and Hendrix Genetics investigated the whole genome of more than 1,600 chickens. Tom Berghof, lead researcher of this study, explains: “We used genetic difference in the whole genome to identify DNA regions that occur more often in chickens with high NAb levels or in chickens with low NAb levels. One region had a very large effect on NAb level, which explained more than 60% of the genetic variation observed. Within this region one candidate gene could eventually be identified. Berghof explains: “The region contained a few genes. It is very difficult to identify the difference at DNA level that explains the difference in NAb level. Most likely this difference is due to the Toll-like receptor 1A (TLR1A) gene, which makes this our main candidate.”

Toll-like receptors
TLR1A is member of the TLR family, an important part of the immune system. This is a group of receptors, a kind of sensors, that recognize common structures on pathogens. Berghof: “They detect certain parts present on many bacteria or viruses. These sensors have therefore a very broad function.” In humans, genetic mutation in TLR have been associated with an increased or reduced risk for diseases. Based on the study of Berghof and colleagues, this seems to be the case as well in layer chickens. “But the association with NAb is new, independent of the specie.”, says Berghof.

Applications and future plans
This study offers direct applications for breeders to select layer chickens for increased disease resistance by selecting for this specific DNA region. Frans van Sambeek, director R&D Hendrix Genetics Layers, says: “Currently we are investigating how we can apply these research findings into the breeding programs of pure bred line. At this moment, we are running three field experiments with layers with high or low NAb levels. These hens will be monitored for livability and production.” In addition, plans are being made to investigate the TLR1A-sensor. “We have good indications that TLR1A is our candidate,” says Berghof, “however, this still needs to be proven.”
At this moment a research proposal is being written to continue the investigation on the role of TLR1A in the immune system of chickens. In the long term this could result in, for example, improved vaccines and health-promoting nutrition. Eventually this should lead to animals with a higher general disease resistance with lower antibiotic use, lower costs for farmers, and higher animal welfare.
Hendrix Genetics supplied animals and data for this research.
This research is part of the research programme “Divergent selection for natural antibodies in poultry (with project number 12208), which is financed by the Netherlands Organisation for Scientific Research (NWO). From Wageningen University and Research