Researchers uncover multiple genetic paths to mottled feathers in chickens

The white-tipped feathers found in many chicken breeds may look like variations of the same decorative pattern, but new research shows that they do not all share the same genetic origin.

Researchers from the Texas A&M College of Veterinary Medicine and Biomedical Sciences (VMBS) found that at least three independently occurring mutations involving the same gene can produce mottling, a pigmentation pattern characterized by white feather tips.

The findings, published in Animal Genetics, provide insight into how selective breeding and human preferences have shaped the diversity seen among domestic chickens.

“Mottling is a bit unusual because there are at least three different mutations causing a very similar phenotype,” said Dr. Leif Andersson, a professor in the VMBS’ Department of Veterinary Integrative Biosciences.

The findings show that different genetic changes can converge on the same visible trait. In this case, the three mutations arose independently and were each preserved through selective breeding.

Mottling is found in chicken breeds around the world. On chickens with otherwise black feathers, the pattern typically appears as white tips that create a spotted appearance across the bird’s plumage.

The pattern develops when pigment cells do not migrate normally as feathers grow. If pigment cells do not reach the end of a developing feather, its tip remains white.

One gene, multiple mutations

Among domestic animals, a physical trait shared across multiple breeds can often be traced to a single mutation that spread as animals carrying it were bred with different populations; therefore, finding several mutations affecting the same gene was unexpected.

For example, Andersson said the dominant white coloration found in commercially important chicken populations, including White Leghorns, is associated with one specific mutation.

Mottling appears to have followed another evolutionary path. Rather than descending from one original mutation, similar patterns arose independently in different groups of chickens. People then selected birds with those patterns for breeding, allowing the separate mutations to become established in multiple breeds.

“These mutations occurred independently,” Andersson said. “Humans bred chickens carrying the different mutations because they liked the feather pattern, and the mutations had no negative effect on egg or meat production.”

This process is an example of parallel evolution, in which similar traits develop separately in different populations. It also demonstrates how selective breeding can preserve genetic diversity that might otherwise remain rare.

“Humans appreciate diversity among their domestic animals as long as a mutation does not cause harm to the animal,” Andersson said. “This is why mutations at the EDNRB2 locus have been selected multiple times during the evolution of domestic chickens.”

Following the genetics of mottling

To determine how different mutations could create the same feather pattern, the researchers examined the genetic origins of mottling across several chicken breeds.

Previous research had connected mottling in several Japanese chicken breeds to a particular mutation in EDNRB2, a gene that helps regulate pigmentation in birds. However, when Andersson and his colleagues examined other mottled breeds, they quickly encountered a problem with that explanation.

“We tested the specific sequence variant reported by the Japanese group and immediately found that the mutation was not present in the Mottled Houdan, for instance,” Andersson said.

The Mottled Houdan, a breed that originated in France, displays the same distinctive white-tipped feathers. Its lack of the previously identified mutation suggested another genetic change had produced the pattern.

To investigate, the researchers bred Mottled Houdan chickens with non-mottled Black Langshan chickens and used genetic mapping to locate the region associated with the trait. The analysis narrowed the search to a section of chromosome 4 containing EDNRB2.

Whole-genome sequencing then identified a different mutation in EDNRB2 that was strongly associated with mottling in Houdans and several other non-Japanese breeds. The researchers also found evidence that additional mutations near the same gene contribute to mottling in other breeds.

Together, the results indicate that at least three versions of EDNRB2, known as alleles, can create similar mottled patterns. Although the alleles involve the same gene, they do not contain the same mutation. Japanese breeds carry one mutation, while Mottled Houdans and several other non-Japanese breeds carry another. The precise mutation or mutations responsible for mottling in several other breeds have not yet been identified.  

At the center of these different genetic paths is EDNRB2, a gene that produces a receptor for endothelin 3, a protein that supports the growth and movement of pigment-producing cells.

“Reduced EDNRB2 function leads to reduced pigmentation,” Andersson said. “If the gene is inactivated completely, there is probably an almost complete lack of pigmentation. Thus, mottling is a mild mutation that reduces EDNRB2 signaling.”

Because many mutations could mildly disrupt EDNRB2 signaling, separate genetic changes can ultimately produce nearly identical mottling patterns.

Understanding pigmentation across species

The study’s primary significance lies in helping researchers understand the genetics and molecular mechanisms that regulate pigmentation patterns.

The work may also contribute to a broader understanding of related biological pathways. Birds have an EDNRB2 gene that specializes in pigmentation, but humans have a corresponding gene called EDNRB that has a wider role in development.

Mutations in human EDNRB can cause Hirschsprung disease, a condition involving a lack of nerve cells in parts of the colon that can also include pigmentation changes.

Although mottling in chickens and Hirschsprung disease are very different conditions, studying how related genes function across species can help researchers better understand their roles in cell development and migration.

The findings also demonstrate how a feather pattern that appears simple on the surface can conceal a much more complicated genetic history. Even with the origins of mottling now better understood, many of the colors and patterns seen among domestic chickens still hold unanswered genetic questions.  

“Our work on mottling is complete, but we are continuing to study several other gene variants that affect pigmentation patterns in chickens,” Andersson said.

By Camryn Haines, Texas A&M University College of Veterinary Medicine and Biomedical Sciences