Shapiro and colleagues found that the Pitx1<\/em> and Tbx5<\/em> genes themselves are no different in breeds with and without foot muffs. Instead, they learned that nongene \u201cregulatory sequences\u201d of DNA near the genes act like switches to turn down the activity of hindlimb gene Pitx1<\/em> and turn up activity of forelimb gene Tbx5<\/em> in the embryos of pigeons with foot feathers.<\/p>\n

\u201cOur experiments show that these switches work differently in birds with feathered feet relative to the ones with normal, scaled feet,\u201d Shapiro says.<\/p>\n

The two genes were previously known in all vertebrate animals, and they are linked to limb and other defects in people. The new study showed how they \u201cprovide a mechanism by which scaled and feathered skin regions can be controlled,\u201d Shapiro says.<\/p>\n

 <\/p>\n

Of pigeons, dinosaurs and early birds<\/b><\/p>\n

Most birds have scales on the feet and toes and feathers elsewhere. Some \u2013 like snowy owls, golden eagles and ptarmigans \u2013 have foot feathers. Only chickens and domestic pigeons \u2013 with 146 and over 350 breeds, respectively \u2013 have breeds with and without foot feathers in a single species.<\/p>\n

\u201cA major change that occurred in the evolution of birds from dinosaurs was the replacement of scales by feathers throughout the body,\u201d Shapiro says. \u201cPigeons give us an unusual chance to understand the genetic and developmental mechanisms that might control this change because there are pigeon breeds with and without foot feathers.\u201d<\/p>\n

Some dinosaurs had foot feathers, which may have been common in meat-eating theropods, including raptors and Tyrannosaurus rex<\/i>.<\/p>\n

\u201cWe don\u2019t know if Pitx1<\/em> and Tbx5<\/em> were specifically involved in feather-footed dinosaurs, but this gives us a genetic and developmental mechanism that could have done it,\u201d he adds. \u201cWe have no direct way of knowing how that change occurred in dinosaurs. However, we see a lot of striking anatomical similarities between dinosaurs\u2019 feathered legs and feet and pigeons with feathered legs and feet. So pigeons might give us some insights into how the skin knows to develop scales or feathers.\u201d<\/p>\n

\u201cBased on what we found in pigeons, the change from scales to feathers can be genetically very simple,\u201d he says. \u201cThis can give us some clues about not only how pigeons get feathered feet, but perhaps about how ancient birds lost foot feathers.\u201d<\/p>\n

The new study was funded by the National Science Foundation, Burroughs Wellcome Fund and the National Institutes of Health.<\/p>\n

Shapiro performed the study with University of Utah postdoctoral researcher Eric Domyan; human genetics doctoral student Zev Kronenberg; biology graduate students Anna Vickrey, Rebecca Bruders and Sydney Stringham; undergraduate student Michael Guernsey; and human genetics professors Gabrielle Kardon and Mark Yandell. Other co-authors were at the University of Georgia.<\/p>\n

 <\/p>\n

Genes that help define wings and legs<\/b><\/p>\n

\u201cThe genes we found that control feathered feet determine forelimb versus hindlimb identity,\u201d Shapiro says, and were already known for other functions besides their newfound role in foot feather development:<\/p>\n

— \u201cTbx5 <\/i>is critical for proper forelimb development in all vertebrates \u2013 fish [fins], chickens, mice and even humans,\u201d he adds. \u201cMutations in the gene in humans cause Holt-Oram Syndrome, which results in arm defects and heart defects in newborns.\u201d<\/p>\n

— \u201cPitx1<\/i> has been known for a long time to be critical for hindlimb development in vertebrates,\u201d including people, Shapiro says. \u201cIf Pitx1<\/em> is inactivated, mice get severe hindlimb defects. The bones change size and shape. They also get severe brain and jaw defects. Mutations in Pitx1<\/em> in humans can lead to clubfoot\u201d and Liebenberg syndrome, which causes elbow and wrist defects that make the arms more leglike.<\/p>\n

Shapiro and colleagues showed that to get feathers on pigeons\u2019 hindlimbs or legs, the normal hindlimb signal from Pitx1<\/em> goes down, making the leg less like a hindlimb, and the forelimb signal from Tbx5<\/em> goes up, making the leg more like a wing.<\/p>\n

\u201cIn pigeons when Tbx5<\/em> is expressed in the leg, the leg takes on some forelimb or wingike characteristics, including feathers,\u201d he says. \u201cIn some breeds with feathered feet, the fibula or outer shin bone becomes much bigger\u201d and more like a bone in the wing.<\/p>\n

 <\/p>\n

Finding foot-feathering genes<\/b><\/p>\n

Shapiro and coworkers did breeding experiments in which they crossed muffed pigeons named Pomeranian pouters with scale-footed pigeons known as Scandaroons. Then they crossed the offspring.<\/p>\n

\u201cIn the grandkids, some birds had scaled feet, others had big muffs and others were in between, with a range of scales and feathers,\u201d he says. \u201cThey usually had both. This told us very few genes were involved in this trait, as we expected.\u201d (If most of the birds were intermediate, that would indicate many genes were involved.)<\/p>\n

PHOTO CREDIT: <\/span>Sydney Stringham, University of Utah<\/p>

A collage of four breeds of pigeons shows two with scaled feet (top) and two with feathered feet, known as muffs (bottom). Top left is a racing homer. Top right is an English Pouter. Bottom left is an Indian fantail. Bottom right is a fairy swallow. Biologists at the University of Utah have shown how foot feathers develop in pigeon breeds in which a hindlimb development gene becomes less active and a forelimb or wing development gene becomes more active in the legs.<\/p>