Evolutionary Origin of the Feather Epidermis
Roger H. Sawyer,1* Loren Rogers,1 Lynette Washington,1
Travis C. Glenn,2 and Loren W. Knapp1
The formation of scales and feathers in reptiles and birds has fascinated
biologists for decades. How might the developmental processes involved
in the evolution of the amniote ectoderm be interpreted to shed light
on the evolution of integumental appendages? An Evo-Devo approach to this
question is proving essential to understand the observation that there
is homology between the transient embryonic layers covering the scale
epidermis of alligators and birds and the epidermal cell populations of
embryonic feather filaments. Whereas the embryonic layers of scutate scales
are sloughed off at hatching, that their homologues persist in feathers
demonstrates that the predecessors of birds took advantage of the ability
of their ectoderm to generate embryonic layers by recruiting them to make
the epidermis of the embryonic feather filament. Furthermore, observations
on mutant chickens with altered scale and feather development (Abbott
and Asmundson [1957] J. Hered. 18:63-70; Abbott [1965] Poult. Sci. 44:1347;
Abbott [1967] Methods in developmental biology. New York: Thomas Y. Crowell)
suggest that the ectodermal placodes of feathers, which direct the formation
of unique dermal condensations and subsequently appendage outgrowth, provided
the mechanism by which the developmental processes generating the embryonic
layers diverged during evolution to support the morphogenesis of the epidermis
of the primitive feather filament with its barb ridges. Developmental
Dynamics 282:256-267, 2005. @ 2005 Wiley-Liss, Inc.
Key words: feather origin; evolution; alligator; avian
mutant; ectoderm; epidermis; ectodermal placode; embryonic layers; B-keratins
Received 11 June 2004; Revised 15 August 2004; Accepted 2 November 2004
SREL Reprint #2813
Sawyer, R. H., L. Rogers, L. Washington, T. C. Glenn and L. W. Knapp. 2005. Evolutionary origin of the feather epidermis. Developmental Dynamics 232:256-267.
