For The Rooster, Size Matters

 

Rooster and his comb

Credit: Wikipedia

Compared with the original wild jungle hen, domestic hens have larger combs as well as denser bones. This influences egg-laying, as the hen’s bone tissues provide calcium for the eggshells. The greater the bone mass, the more eggs she can lay.

After having spotted a clear correlation between comb size and bone mass in chickens from a cross between red junglefowl and domestic chicken, the research group – under the leadership of evolutionary geneticist Dominic Wright – set up a study where such chickens were crossed for several generations. In this way the genome was split up into smaller and smaller regions, which allowed the “mapping” of the functions of individual genes.

In the eighth generation, the researchers found an area that had a strong effect on the weight of the comb – but also on bone mass and fertility.

Rooster (left) and hen (right)

Credit: Wikipedia

The genetic variation has gradually decreased over the course of domestication. In domestic chickens there are now some 40 known small regions with stable genes that potentially govern their typical “domestic” characteristics. LiU researchers have now discovered two “pleiotropic” genes: two genes connected to each other that influence several characteristics simultaneously. By regulating the production of cartilage, they influence combs (which consist of cartilage throughout) as well as bone growth (where cartilage is the base material) and, ultimately, egg production.

“The original hens have smaller combs, thinner legs, and lay fewer eggs. When people bred for the characteristic of laying many eggs, the comb grew automatically,” Dominic Wright says.

In nature, the comb is an example of a sexual ornament. Individuals – often males – with the most impressive ornaments are favoured by females, thereby obtaining more numerous offspring than their competitors. In domesticated animals, sexual selection – like natural selection – has lost its role, as it was humans who determine breeding. 

The Red Junglefowl (Gallus gallus) is a tropical member of the Pheasant family. It is thought to be ancestral to the domestic chicken, with some hybridisation with the Grey Junglefowl . The Red Junglefowl was first domesticated at least five thousand years ago in Asia, then taken around the world, and the domestic form is kept globally as a very productive food source of both meat and eggs.

Credit: Wikipedia

Article: A sexual ornament in chickens is affected by pleiotropic alleles at HA01 and BMP2, selected during domestication by M Johnsson, I Gustafson, C-J Rubin, A-S Sahlqvist, K B Jonsson, S Kerje, O Ekwall, O Kämpe, L Andersson, P Jensen and D Wright. PLoS Genetics 8(8) August 30, 2012.

Dominic Wright
Linköping University