Evolving a Protofeather and Feather Diversity1

Homology and Potential Cellular and Molecular Mechanisms for the Development of Unique Feather Morphologies in Early Birds

At least two lineages of Mesozoic birds are known to have possessed a distinct feather morphotype for which there is no neornithine (modern) equivalent. The early stepwise evolution of apparently modern feathers occurred within Maniraptora, basal to the avian transition, with asymmetrical pennaceous feathers suited for flight present in the most basal recognized avian, Archaeopteryx lithographica. The number of extinct primitive feather morphotypes recognized among non-avian dinosaurs continues to increase with new discoveries; some of these resemble feathers present in basal birds. As a result, feathers between phylogenetically widely separated taxa have been described as homologous. Here we examine the extinct feather morphotypes recognized within Aves and compare these structures with those found in non-avian dinosaurs. We conclude that the "rachis dominated" tail feathers of Confuciusornis sanctus and some enantiornithines are not equivalent to the "proximally ribbon-like" pennaceous feathers of the juvenile oviraptorosaur Similicaudipteryx yixianensis. Close morphological analysis of these unusual rectrices in basal birds supports the interpretation that they are modified pennaceous feathers. Because this feather morphotype is not seen in living birds, we build on current understanding of modern feather molecular morphogenesis to suggest a hypothetical molecular developmental model for the formation of the rachis dominated feathers of extinct basal birds.

Developmental integration of feather growth and pigmentation and its implications for the evolution of diet-derived coloration

Variation in avian coloration is produced by coordinated pigmentation of thousands of growing feathers that vary in shape and size. Although the functional consequences of avian coloration are frequently studied, little is known about its developmental basis, and, specifically, the rules that link feather growth to pigment uptake and synthesis. Here, we combine biochemical, modeling, and morphometric techniques to examine the developmental basis of feather pigmentation in house finches (Carpodacus mexicanus)--a species with extensive variation in both growth dynamics of ornamental feathers and their carotenoid pigmentation. We found that the rate of carotenoid uptake was constant across a wide range of feather sizes and shapes, and the relative pigmented area of feathers was independent of the total amount of deposited carotenoids. Analysis of the developmental linkage of feather growth and pigment uptake showed that the mechanisms behind partitioning the feather into pigmented and nonpigmented parts and the mechanisms regulating carotenoid uptake into growing feathers are partially independent. Carotenoid uptake strongly covaried with early elements of feather differentiation (the barb addition rate and diameter), whereas the pigmented area was most closely associated with the rate of feather growth. We suggest that strong effects of carotenoid uptake on genetically integrated mechanisms of feather growth and differentiation provide a likely route for genetic assimilation of diet-dependent coloration.

Refractive index and dispersion of butterfly chitin and bird keratin measured by polarizing interference microscopy

Using Jamin-Lebedeff interference microscopy, we measured the wavelength dependence of the refractive index of butterfly wing scales and bird feathers. The refractive index values of the glass scales of the butterfly Graphium sarpedon are, at wavelengths 400, 500 and 600 nm, 1.572, 1.552 and 1.541, and those of the feather barbules of the white goose Anas anas domestica are 1.569, 1.556 and 1.548, respectively. The dispersion spectra of the chitin in the butterfly scales and the keratin in the bird barbules are well described by the Cauchy equation n(λ) = A + B/λ(2), with A = 1.517 and B = 8.80·10(3) nm(2) for the butterfly chitin and A = 1.532 and B = 5.89·10(3) nm(2) for the bird keratin.

A diverse assemblage of Late Cretaceous dinosaur and bird feathers from Canadian amber

The fossil record of early feathers has relied on carbonized compressions that lack fine structural detail. Specimens in amber are preserved in greater detail, but they are rare. Late Cretaceous coal-rich strata from western Canada provide the richest and most diverse Mesozoic feather assemblage yet reported from amber. The fossils include primitive structures closely matching the protofeathers of nonavian dinosaurs, offering new insights into their structure and function. Additional derived morphologies confirm that plumage specialized for flight and underwater diving had evolved in Late Cretaceous birds. Because amber preserves feather structure and pigmentation in unmatched detail, these fossils provide novel insights regarding feather evolution.

Cell structure of barb ridges in down feathers and juvenile wing feathers of the developing chick embryo: Barb ridge modification in relation to feather evolution

The present study deals with the cell structure and three-dimensional organization of barb and barbule cells within barb ridges of down feathers and juvenile feathers in the chick embryo. Juvenile feathers represent the second generation of feathers in the wing, and replace down feathers some weeks after hatching. Within the follicle of juvenile feathers, at 16-18 days of embryonic development, barb ridges are more numerous than in down feathers. Barb ridges of juvenile feathers contain more cells in their barbule and axial plates with respect to barb ridges of down feathers. This condition determines the formation of longer barbules inserted in the rami of juvenile feathers than barbules of down feathers. Barb ridges of juvenile feathers merge with the rachidial ridge so that pennaceous feathers are formed. Barbule cells are surrounded by cytoplasmic elongation from barb vane ridge cells located in the axial plate, which constitute most of the axial plate. The degeneration of supportive cells among barbule cells branching from barbs determine the formation of spaces between barbules. The study emphasizes that, in addition to the size of the dermal papilla, it is the length of barb ridges and the infiltration of barb ridge vane cells among barbule cells that determine the size and length of feathers. The knowledge of the cell structure of barb ridges allows understanding not only of how feathers develop but also gives insights into their evolution. Based on changes of the process of barb ridge morphogenesis some hypotheses on the evolution of plumulaceous and pennaceous feathers are presented. Feathers derived from the process of carving-out supportive cells within barb ridges and from the specific pattern of fusion of barb/barbule cells. This process initially produced variably branched down feathers and later, after barb ridge fusion, a rachis. From the modulation in the pattern of barb ridge formation various pennaceous feathers later evolved.

Gene co-option in physiological and morphological evolution

Co-option occurs when natural selection finds new uses for existing traits, including genes, organs, and other body structures. Genes can be co-opted to generate developmental and physiological novelties by changing their patterns of regulation, by changing the functions of the proteins they encode, or both. This often involves gene duplication followed by specialization of the resulting paralogous genes into particular functions. A major role for gene co-option in the evolution of development has long been assumed, and many recent comparative developmental and genomic studies have lent support to this idea. Although there is relatively less known about the molecular basis of co-option events involving developmental pathways, much can be drawn from well-studied examples of the co-option of structural proteins. Here, we summarize several case studies of both structural gene and developmental genetic circuit co-option and discuss how co-option may underlie major episodes of adaptive change in multicellular organisms. We also examine the phenomenon of intraspecific variability in gene expression patterns, which we propose to be one form of material for the co-option process. We integrate this information with recent models of gene family evolution to provide a framework for understanding the origin of co-optive evolution and the mechanisms by which natural selection promotes evolutionary novelty by inventing new uses for the genetic toolkit.