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Roger H. SawyerExecutive Dean & Senior Associate Dean For Graduate Education, |
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Roger H. SawyerExecutive Dean & Senior Associate Dean For Graduate Education, |
The epidermal appendages of extant reptiles and birds, such as scales, claws and feathers, are constructed of beta (β) keratin, a unique fibrous protein, in which a filament-matrix structure is formed by each single β-keratin molecule, unlike the situation in mammalian epidermal appendages where alpha (α) keratin molecules interact with distinct matrix molecules to form cornified appendages, such as hair (see Fraser and Parry, 2008, 2010). The amino acid sequence (31-32 residues) of the central filament region of β-keratins is highly conserved throughout all reptiles and birds suggesting that this domain has changed little in ~285 Ma of evolution (Fraser and Parry, 2010).
In birds, the β-keratin multigene family has diverged into four major subfamilies, scale, claw, feather-like, and feather (Presland et al., '89 a, b). As the genomes of birds become available (i.e., chicken, turkey, and zebra finch), comparative studies of the genomic organization of the β keratin subfamilies provide information on how the genotype influences phenotype (Greenwold and Sawyer, 2010).
Furthermore, phylogenetic analyses of the avian β-keratin genes from the chicken and zebra finch genomes have shown that the avian scale β-keratin subfamily is closely related to the β-keratins of crocodilians and basal to the claw β-keratin subfamily, which is basal to the feather-like and feather β-keratin subfamilies (Greenwold and Sawyer, 2010).
Presently we are using molecular dating approaches, such as BEAST, to gain a better understanding of the how the molecular evolution of the avian β-keratins relates to the evolutionary origin of feathers. Our approach is to combine molecular dating techniques with knowledge of the sauropsid fossil record, feather development, molecular evolution of the avian β-keratins, and the biophysical properties of feathers to gain a better understanding of feather evolution.
Figure 2 and Table 1 from Greenwold and Sawyer (2011) demonstrate that the basal β-keratins of birds began diverging from their archosaurian ancestor ~216 million years ago, while the subfamily of feather β-keratins, as found in living birds, began diverging ~143 million years ago. Thus the evolutionary origin of feathers does not coincide with the molecular evolution of feather β-keratins found in modern birds. Recent biophysical studies of the β-keratins in today's feathers support the view that the appearance of the subfamily of feather β-keratins altered the biophysical nature of the feather establishing its role in powered flight.
![[Image: sawyer_graphic1]](image/sawyer_graphic1.png "sawyer_graphic1")
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![[Image: Sawyer_graphic2]](image/sawyer_graphic2.png "sawyer_graphic2")
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Figure 2 from Greenwold and Sawyer (2011). This figure illustrates the results of Bayesian molecular dating using β-keratins from the lizard, turtle, crocodile, and two bird species (chicken and zebra finch). The β-keratins are colored according to order and avian subfamily: grey = Squamata, teal = Testudines, black = Crocodylia, red = β-keratin from cultured keratinocytes, blue = scale, yellow = claw, magenta = feather-like and green = feather β-keratins. The feather β-keratins (green) are divided into two clades, in which A = genes on Chromosome 2 and B = genes on Microchromosome 25 and 27 of both bird species (Greenwold and Sawyer, 2010). The mean divergence dates and the 95% highest posterior density (HPD) for each numbered node are listed in Table 1. |
Table 1 from Greenwold and Sawyer (2011): The Results of the Molecular Dating of β-keratins. * Highest posterior density (HPD). ** Mean values were obtained from Shedlock and Edwards (2009) and the 95% standard deviation (SD) was calculated using a normal distribution with a standard deviation of ten. |
Greenwold, M.J. and R.H. Sawyer (2011) Linking the Molecular Evolution of Avian Beta (β) Keratins to the Evolution of Feathers. In Press: JEZ Part B: Molecular and Developmental Evolution.
Greenwold, M.J. and R.H. Sawyer. The Evolution of Feathers: Genomic Organization of the Beta-Keratin Multigene Families in the Chicken (Gallus gallus) and Zebra Finch (Taeniopygia guttata) and their Molecular Phylogenetics in the Archosaurians. Submitted to BMC Evolutionary Biology February, 2010.
Glenn, T.C., French, J.O., Heincelman, T.L., Jones, K.L., R.H. Sawyer (2008) Evolutionary relationships among copies of feather beta-keratin genes from several avian orders. Integrative and Comparative Biology 48(4):463-475.
Alibardi, L. and R.H. Sawyer (2006). Cell structure of developing down feathers in the zebra finch with emphasis on barb ridge morphogenesis. J. Anat. 208:621-642.
Alibardi, L., Knapp, L.W., R.H. Sawyer (2006) Beta-keratin localization in developing alligator scales and feathers in relation to the development and evolution of feathers. J Submicroscopic Cytology and Pathology 38(2-3):175-192.
Sawyer, R.H., T.C. Glenn, J.O. French and L.W. Knapp. (2005) Developing Antibodies to Synthetic Peptides Based on Comparative DNA sequencing of Multigene Families. In Methods in Enzymology395, Molecular Evolution: Producing the Biochemical Data, Part B. E.A. Zimmer and E.H. Roalson (eds.), Academic Press, San Diego. pp. 636-651.
Sawyer, R.H., L. Rogers, L. Washington, T.C. Glenn and L.W. Knapp. (2005) The Evolutionary Origin of the Feather Epidermis. Developmental Dynamics. 232:256-267.
Sawyer, R.H. and L.W. Knapp. (2003) Avian Skin Development and the Evolutionary origin of feathers. J. Exp. Zool. (Mol. Dev. Evol.) 298B:57-72.
Sawyer, R.H. Washington, L.D. Salvatore, B.A. Glenn, T.C. and L.W. Knapp. (2003) Origin of Archosaurian Integumentary Appendages: The Bristles of the Wild Turkey Beard Express Feather-Type ? Keratins. J. Exp. Zool. (Mol & Dev Evol) 297B: 27-34.
Sawyer, R.H., Salvatore, B.A., Potylicki, T-T. F., French, J.O., Glenn, T.C., and Knapp, L.K. (2003) Origin of Feathers: Feather Beta (?) Keratins are Expressed in Discrete Epidermal Cell Populations of embryonic Scutate Scales. J. Exp. Zool. (Mol. Dev. Evol) 295B: 12-24.
Sawyer, R.H. and L.W. Knapp. (2003) Embryonic Induction. In: Hall B.K., Olson W.M., eds. Keywords and concepts in evolutionary developmental biology. Harvard University Press. Cambridge, pp.102-108.
Davis, L.M., Glenn, T.C., Strickland, D.C., Guillette, L.J., Elsey, R.M., Rhodes, W.E., Dessauer, H.C., and R.H. Sawyer. (2002) Microsatellite DNA analyses support an east-west phylogeographic split of American Alligator populations. J. Exp. Zool. (Mol. Dev. Evol.) 294:352-372.
Glenn, T.C., Staton, J.L., Vu, A.T., Davis, L.M., Bremer, J.R., Rhodes, W.E., Brisbin, I.L., and R.H. Sawyer. (2002) Low mitochondrial DNA variation among American Alligators and a novel non-coding region in crocodilians. J. Exp. Zool. (Mol. Dev. Evol.) 294:312-324.
Alibardi L, Sawyer RH. 2002. Immunocytochemical analysis of beta keratin in the epidermis of chelonians, lepidosaurians and archosaurians. J. Exp. Zool. 293: 27-38.
Homer BL, Li C, Berry KH, Denslow ND, Jacobson ER, Sawyer RH, Williams JE. (2001) Soluble scute proteins of health and ill desert tortoises (Gopherus agassizii). Am. J Veterinary Res. 62:104-110.
Davis,L.M., Glenn,T.C., Elsey, R.M., Dessauer, H. R.H. Sawyer. (2001) Multiply Paternity and Mating Patterns in the American Alligator, Alligator mississippiensis. Molecular Ecology 10: 1011-1024.
Sawyer, R.H., Glenn, T.C., French, J.O., Mays, B., Shames, R.B., Barnes, G.L., Rhodes, W. and Y. Ishikawa. (2000) The Expression of Beta Keratins in the Epidermal Appendages of Reptiles and Birds. In: Symposium on the Origin of Feathers. Am. Zool., 40:530-539.
Davis, L. M., T. C. Glenn, R. M. Elsey, I. L. Brisbin Jr., W. E. Rhodes, H. C. Dessauer and R. H. Sawyer. (2000). Genetic structure of six populations of American alligators: A microsatellite analysis. Pages 38-50 In Crocodilian Biology and Evolution, (G. C. Grigg, F. Seebacher, and C. E. Franklin, eds.), Surrey Beatty and Sons. Australia.
Song, H.K. and R.H. Sawyer (1996) The dorsal dermis of the scaleless (sc/sc) embryo directs normal feather pattern formation until day 8 of development. Dev. Dynamics 205, 92-99.
Barnes, G.L. and R.H. Sawyer (1995) Histidine-rich protein B of embryonic feathers is present in the transient embryonic layers of scutate scales. J. Exp. Zool. 271, 307-314.
Shames, R.B., B.C. Bade, and R.H. Sawyer. (1994) Role of epidermal-dermal tissue interactions in regulating tenascin expression during development of the chick scutate scale. J. Exp. Zool. 269, 366-349.
Knapp, L.W., R.B. Shames, G.B. Barnes, and R.H. Sawyer. (1993) Region specific patterns of beta keratin expression during avian skin development. Dev. Dynamics 196, 283-290.
Song, H.K., W.E. Carver, and R.H. Sawyer. (1993) Pattern formation in chick feather development: distribution of B1-integrin in normal and scaleless embryos. Dev. Dynamics 200, 129-143.
Zeltinger, J.K. and R.H. Sawyer. (1992) Avian Scale Development XVII. The epidermis of the scaleless (sc/sc) anterior metatarsal skin is determined, but the dermis lacks permissive cues for the patterned expression of the determined state. Dev. Dynamics 193:58-69.
Zeltinger, J.K. and R.H. Sawyer. (1992) Avian Scale Development XVI. Epidermal commitment to terminal differentiation is prior to definitive scale ridge formation. Dev. Biol. 149:55-65.
Shames, R.B., A.G. Jennings and R.H. Sawyer. (1991) The initial expression and patterned appearance of tenascin in scutate scales is absent from the dermis of the scaleless (sc/sc) chicken. Dev. Biol. 147:174-186.