Molecular mechanism of feather formation found | Science Blog:
The results suggested that a simple interaction between Shh and Bmp2 is sufficient to model the creation and patterning of barbs in feather development. The team then tested whether such interactions truly exist in the developing feather. In the first steps of feather development, cells exposed to essentially the same levels of Bmp2 and Shh grow from a small bud to form a uniform ring. Shh then is expressed in specific spots along the ring, giving rise to bumps, seen microscopically as longitudinal stripes demarcating the edges of ridges in the developing barb.
"Each barb ridge grows in length by recruiting new cells, which proliferate at the growing base of the feather germ, to join the base of that barb ridge," Harris says. "The variations in the initial number of barb ridges will directly affect the shape, and consequent function, of the feather."
To test the activator-inhibitor model, Harris injected retroviruses to force the expression of either Shh or Bmp2 into the skin of six-day-old chick embryos. The virus infected only small patches of cells and allowed Harris to locally examine the effects of the treatment on barb patterning during feather development.
To assess the specific role of Bmp2 in regulating Shh expression, Harris tricked the cells into believing that Bmp2 was signaling them continuously by altering receptors in the cells. The over-expression of Bmp2 signaling via the altered receptors led to ongoing down-regulation of normal Shh expression needed to form the barbs.
Harris and colleagues used a similar experiment to test whether Shh could up-regulate its own expression during barb formation, and found that it could. Similarly, they found that regional expression of Shh led to detectable up-regulation of Bmp2 in feather buds as they first grew.
The underlying assumptions of the model were found to be true in developing feathers. These findings suggest that simple relationships between developmental genes can provide the basis for the formation of complex forms.
The researchers predict that a more complicated version of the model can be applied to the formation of more complex feathers. Termed pennaceous, these feathers occur in the duck and other birds, including adult chickens, and are not characterized as downy. The more primitive young chicken feathers, which are downy, are called plumalaceous.
"We don't have empirical evidence for this yet, but mathematical analyses lead us to believe that the addition of a third signaling factor leads to the development of the more complex pennaceous feather," Fallon says. " Our model supports paleontologic evidence that pennaceous feathers are more advanced than plumalaceous feathers."
Evolutionary hypotheses generated from a combination of fossils and molecular biology.