Genetics of Craniofacial Variation in the Domestic Pigeon.

The domestic rock pigeon (Columba livia) comprises more than 350 breeds with spectacular variation in a variety of anatomical and behavioral traits. Diversity in the beak skeleton is particularly striking, and ranges from the short, blunt beak of the "owl" breeds to the elongated, curved beak of the Scandaroon. We used a combination of laboratory crosses and whole-genome resequencing to map the genetic architecture of beak variation among pigeon breeds. We found that beak reduction mapped to a region of the Z chromosome that explains more than half of the variance in craniofacial morphology in a laboratory cross. Fine-mapping across numerous breeds showed that this region includes ROR2, which encodes a receptor tyrosine kinase protein that plays key roles in skeletal development. Short-beak pigeons have a missense mutation in ROR2 that is predicted to be deleterious. In contrast, the genetic architecture of beak elaboration was more complex, with several loci controlling smaller proportions of size and shape differences. Some of these loci control coordinated changes between different structures, while others explain variation in the size and shape of specific skull and jaw regions. In this and other studies of both wild and domestic avian species, major shifts in beak morphology can result from changes to a single gene and appear to be relatively simple, or they can be complex with detectable effects throughout the genome.