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Feather Symmetry from Asymmetry Through Wnt3a Gradient

  Yue Y.-c., Jiang T.-x., R. B. Widelitz, and Chuong C.-m. 2006.
   Wnt3a gradient converts radial to bilateral feather symmetry
   via topological arrangement of epithelia. _Proceedings of the
   National Academy of Sciences, Philadelphia_ 103(4):951-955.

  "The evolution of bilaterally symmetric feathers is a
   fundamental process leading toward flight. One major unsolved
   mystery is how the feathers of a single bird can form radially
   symmetric downy feathers and bilaterally symmetric flight
   feathers. In developing downy feather follicles, barb ridges
   are organized parallel to the long axis of the feather
   follicle. In developing flight-feather follicles, the barb
   ridges are organized helically toward the anterior region,
   leading to the fusion and creation of a rachis. Here we
   discover an anterior--posterior molecular gradient of wingless
   int (Wnt3)a in flight but not downy feathers. Global inhibition
   of the Wnt gradient transforms bilaterally symmetric feathers
   into radially symmetric feathers. Production of an ectopic
   local Wnt3a gradient reoriented barb ridges toward the source
   and created an ectopic rachis. We further show that the
   orientation of the Wnt3a gradient is dictated by the dermal
   papilla (DP). Swapping DPs between wing covert and breast downy
   feathers demonstrates that both feather symmetry and molecular
   gradients are in accord with the origin of the DP. Thus the
   fates of feather epidermal cells are not predetermined through
   some molecular codes but can be modulated. Together, our data
   suggest feathers are shaped by a DP -> Wnt gradient -> helical
   barb ridge organization -> creation of rachis -> bilateral
   symmetry sequence. We speculate diverse feather forms can be
   achieved by adjusting the orientation and slope of molecular
   gradients, which then shape the topological arrangements of
   feather epithelia, thus linking molecular activities to organ
   forms and novel functions."


Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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