Two closely-linked genes influence both the comb size and bone mass of hens, and thus their ability to lay eggs and win the affection of roosters, researchers have found.
A research group at Sweden’s Linköping University spotted a clear correlation between comb size and bone mass in chickens from a cross between red junglefowl and domestic chicken. Compared with wild jungle hens, domestic hens have larger combs as well as denser bones more suitable for egg-laying, since the hens’ bone tissues provide calcium for the eggshells. The greater the bone mass, the more eggs she can lay.
The team, led by evolutionary geneticist Dominic Wright, then set up a study where such chickens were crossed for several generations, allowing for the division of the genome into smaller and smaller regions. That division, in turn, allowed for the mapping of the functions of individual genes.
In the eighth generation, the researchers using expression QTL analysis to discover a locus with a pair of pleiotropic genes (HAO1 and BMP2) regulating the production of cartilage, thus controlling multiple aspects of the chickens’ domestication phenotype. Those aspects ranged from a sexual ornament—namely the comb—to multiple life history traits, including bone mass and fertility.
BMP genes have been shown to stimulate bone formation, while HAO1 on the other hand is a novel candidate for altering bone and cartilage deposition, to date principally shown to be expressed in the liver metabolic pathways as a peroxisomal enzyme, according to the team.
“This demonstrates the importance of pleiotropy (or extremely close linkage) in controlling these genetic changes,” the research team concluded, in its study published in PLOS Genetics. “The nature of this pleiotropy also provides insights into how this sexual ornament could be maintained in wild populations.”