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Evolution of bird beaks and feathers
Two new articles:
Bhart-Anjan S. Bhullar, Zachary S. Morris, Elizabeth M. Sefton, Atalay
Tok, Masayoshi Tokita, Bumjin Namkoong, Jasmin Camacho, David A.
Burnham and Arhat Abzhanov (2015)
A molecular mechanism for the origin of a key evolutionary innovation,
the bird beak and palate, revealed by an integrative approach to major
transitions in vertebrate history.
Evolution (advance online publication)
The avian beak is a key evolutionary innovation whose flexibility has
permitted birds to diversify into a range of disparate ecological
niches. We approached the problem of the mechanism behind this
innovation using an approach bridging paleontology, comparative
anatomy, and experimental developmental biology. First we used fossil
and extant data to show the beak is distinctive in consisting of fused
premaxillae that are geometrically distinct from those of ancestral
archosaurs. To elucidate underlying developmental mechanisms, we
examined candidate gene expression domains in the embryonic face: the
earlier frontonasal ectodermal zone (FEZ) and the later midfacial
Wnt-responsive region, in birds and several reptiles. This permitted
the identification of an autapomorphic median gene expression region
in Aves. In order to test the mechanism, we used inhibitors of both
pathways to replicate in chicken the ancestral amniote expression.
Altering the FEZ altered later Wnt responsiveness to the ancestral
pattern. Skeletal phenotypes from both types of experiments had
premaxillae that clustered geometrically with ancestral fossil forms
instead of beaked birds. The palatal region was also altered to a more
ancestral phenotype. This is consistent with the fossil record and
with the tight functional association of avian premaxillae and palate
in forming a kinetic beak.
Jacob M. Musser, Günter P. Wagner and Richard O. Prum (2015)
Nuclear β-catenin localization supports homology of feathers, avian
scutate scales, and alligator scales in early development.
Evolution & Development 17 (3): 185–194
Feathers are an evolutionary novelty found in all extant birds.
Despite recent progress investigating feather development and a
revolution in dinosaur paleontology, the relationship of feathers to
other amniote skin appendages, particularly reptile scales, remains
unclear. Disagreement arises primarily from the observation that
feathers and avian scutate scales exhibit an anatomical
placode—defined as an epidermal thickening—in early development,
whereas alligator and other avian scales do not. To investigate the
homology of feathers and archosaur scales we examined patterns of
nuclear β-catenin localization during early development of feathers
and different bird and alligator scales. In birds, nuclear β-catenin
is first localized to the feather placode, and then exhibits a dynamic
pattern of localization in both epidermis and dermis of the feather
bud. We found that asymmetric avian scutate scales and alligator
scales share similar patterns of nuclear β-catenin localization with
feathers. This supports the hypothesis that feathers, scutate scales,
and alligator scales are homologous during early developmental stages,
and are derived from early developmental stages of an asymmetric scale
present in the archosaur ancestor. Furthermore, given that the
earliest stage of β-catenin localization in feathers and archosaur
scales is also found in placodes of several mammalian skin appendages,
including hair and mammary glands, we hypothesize that a common skin
appendage placode originated in the common ancestor of all amniotes.
We suggest a skin placode should not be defined by anatomical
features, but as a local, organized molecular signaling center from
which an epidermal appendage develops.