Simple explanation for complex pattern of feather development

Biologists testing a mathematical model of the mechanism birds use to control the growth of complex feathers found that plumed feather structures involve the coordination of at least two genes that activate and that inhibit barb growth.

“Understanding these mechanisms of feather growth gives a whole new perspective on the unique beauty of feathers,” said Richard Prum, senior author on the study. Prum is the William Robertson Coe Professor of Ornithology, and Curator of Ornithology and Vertebrate Zoology at Yale’s Peabody Museum of Natural History.

An eclectic team of biologists used a combination of mathematical and molecular methods to reveal some of the secrets of branched feather growth, and propose how the unique complexity of feathers may have evolved. Ornithologist Prum led a team including anatomists Matthew Harris and John Fallon at the University of Wisconsin, statistician Scott Williamson at Cornell and Hans Meinhardt at the Max Plank Institute.

Their findings provide the best experimental evidence for a classical theory for growth of complex biological structures. In the 1950’s, Alan Turing, mathematician, pioneering computer scientist and code-breaker, proposed that repeated patterns could emerge through the interactions among chemical morphogens or molecules that cause things to develop — an activator that makes things happen, and an inhibitor that suppresses the activator.

To test the model in feathers, Harris forced expression of the activator, Shh, or the inhibitor, Bmp2, in the skin of six-day old chick embryos by injecting them with a retrovirus. The results were seen in localized patches and demonstrated that a simple relationship between developmental genes could be the basis for formation of feather structures. This was the first documentation, in any plant or animal, that signaling molecules in development can actually behave as envisioned by Turing 50 years ago.

This work provides a key to some of these most basic questions of biology. The findings also indicate that more complex shafted feathers evolved from the simpler downy tufts by the addition of new players to the original activator-inhibitor pair. Prum is now following up on several clues in the search for these other molecular signals.