Feral pigeons (Columba livia domestica), also known as rock doves, are often disparaged within urban areas as pests or vermin, but their domesticated cousins, known as 'fancy pigeons', are considered prize animals, bred and shown for their distinctive characteristics. Breeds are distinguished by many characteristics, including the presence or absence of 'muffs', or feathering on their feet.
Foot feathering is an uncommon trait among bird species, and most birds have scaly feet. Some raptors and boreal birds have feathered feet, and among chickens and pigeons, birds can have feathery or scaly feet, depending on their breeds. This intraspecific variability presents an opportunity for scientists to explore how breeding and genetics effect discrete changes in phenotypes.
Building on the defined and pedigreed history of pigeon breeding, biologist Mike Shapiro (University of Utah, Salt Lake City) and colleagues investigated the molecular basis for why some breeds have feathered feet and others have scales (eLife 5, e12115; 2016). To narrow down some target chromosome regions, they crossed a muffed breed, the Pomeanian pouter, with a scaled breed, the Scandaroon, and analyzed the genomes of second-generation offspring. They also carried out whole-genome scans and analysis of a variety of feather- and scale-footed birds. From these analyses, Shapiro et al. identified that foot feathering hinges on regulation of two genes, Pitx1 and Tbx5. These genes are generally involved in the development of forelimbs and hindlimbs, respectively, which suggests that feathering accompanies broader changes in limb development and identity.
In agreement with this finding, when they examined muscular and skeletal morphology they noticed discernible differences in that of muffed pigeons. In a press release, Shapiro explained that feathered feet express some characteristics that one might find in a forelimb: “It's not a complete transformation of a leg into a wing. Rather, components of the leg are more winglike, including feathers and a larger leg bone.”
As Shapiro and his coauthors note in their manuscript, these findings suggest that “limb-type identity is not a simple binary choice between two global fates” but rather a complex state that develops from a variety of factors, including regulation of Pitx1 and Tbx5. Now that they've pinpointed the sequences that mediate feathering in feet, Shapiro notes that future efforts could use pigeons to home in on how skin knows to develop scales or feathers. Such questions might someday even reveal how saurian ancestors first developed feathers from a scaly reptilian epidermis.
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Larsen, G. How fancy pigeons grow feathery feet. Lab Anim 45, 158 (2016). https://doi.org/10.1038/laban.1009