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reptile scale-->feathers

   It is difficult to accept the traditional hypothesis that reptalian
scales gave rise, more-or-less, directly to avian feathers. Or, as can be
shown,  to most of the scales on birds. That is, the null hypothesis that
reptalian scales and feathers shared a close common ancestry is easily
   The evidence comes from the combined facts that 1) the early developmental
sequences are very different. 2) the genes that produce the structural proteins
 are vastly different in their coding sequences, control regions and
organization. Breifly, the reptilian epidermal structures are all derived
from the alpha-keratins, the proteins responsible for soft structures (==skin)
in vertebrates. Feathers, which includes all types of feathers such as flight
and contour feathers, adult and natal down, are produced from genes whith an
unique structure, probably assembled from 4 exons, and then subjected to
moderate duplication and perhaps a single deletion. This model explains the
genic structure of all feathers and the closely related scales, claw and beak
proteins. The avian structure mose like the reptile scale is the reticulate
scales on the dorsal surface of the foot. BTW, other reticulate scales, eg
on the ventral surface of penguin feet etc., are closely related to the  scute-
like structure.
    3). Additional evidence that feather-(phi) keratins in different from alpha
 keratin, is found in molecular size (all feather proteins are smaller than
those in skin and reptile scales),the amino-acid sequence, and shape. The
beta-pleated sheet os feather proteins is completely unlike the strucutre, size
, and sequence of the alpha-keratins.
    Both alpha and feather-keratins form filaments, but the organization and
structure of the filaments, the conditions required for filamentation, and the
process of filament formation are essentially dissimilar.
    It goes without saying (at least at this point) that feather follicles, the
 machines that produce feathers, are unique to birds: and quite unlike the
development of scales.
     I would agrue that feathers-and probably most of the other avian epidermal
structures are unique. Particurarly at the genic and molecular level. There
 is some convergence in morphology of scales and claws. But this is a
consequence of function, the components and conditions of assemply are
 different. Feathers, which share structural proteins, are produced differently
, have a different molecular structure and are unique.
      No single reptialian epidermal structure is "most like feathers". By the
same token, no reptilian epidermal protein is "most like" feather proteins:
nor are any set of reptalian genes "most like" avian feather keratin genes.
Substitute "closely related to" or "derived from" and the picture becomes
 increasingly clear that feathers (and the suite of avian epidermal structures)
are a novel evolutionary system

     Alan Brush                          fax:  203-486-3303
     Physiology & Neurobiology           phone: 203-486-6474
     University of Connecticut
     Storrs, CT