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Characterization of melanosomes involved in production of non-iridescent structural colors in bird plumage (Aves) and their detection in fossil record

Colors in bird plumage are either structural or pigmentary. Pigmentary colors are produced by absorption and reflection of light from pigment molecules. Structural colors are produced by scattering of light on nanostructural assemblages in bird plumage, and are classified as iridescent and non-iridescent. The color producing nanostructure in non-iridescent structural colours (NISC) is β-keratin and air-filled matrix (or a “spongy layer”) of the feather barbs. Even though a spongy layer is responsible for color production in NISC, a layer of melanosomes placed underneath color producing structures has been observed. These melanosomes are not directly involved in color production as they absorb incoherently scattered light relative to the coherently scattered light produced by the overlying spongy layer. Without them, i.e. in amelanotic birds, the NISC is found to be washed out. It has been shown that fossil melanosomes in bird feathers are preserved as layers of spherical and cylindrical structures. Melanosomes vary in shape and have been correlated to some categories of colours. However, what types of melanosomes are involved in generating NISC and will there be overlap with other melanin based colors? To address the question on whether NISC can be detected in the fossil record, melanosomes from extant NISC colored feathers were extracted and their shapes determined. One-way ANOVA and Turkeys’ post hoc test were used to compare them to other color categories. Results indicate a significant overlap with melanosomes from gray color, being very large and wide. While NISC appear to be relatively consistent in its morphology, gray category is becoming less distinct. Quadratic discriminant analysis was used to determine the ability to predict coloration from unknown melanosome samples and applied to a fossil member of the Coraciidae, Eocoracias brachyptera from the Messel pit in Germany. This fossil was chosen due to the high occurrence of NISC in the Coraciidae. The plumage color of Eocoracias brachyptera was predicted to be dominated by NISC as expected. To further illuminate categorization of melanosome shape, ancestral state reconstruction of plumage color was performed to discriminate between gray and NISC. In this analysis, Eocoracias was also predicted to have NISC colouration. Our study demonstrates that NISC is not conflated with most melanin based colors and has classificatory strength, which, at least in certain cases, allows for detecting NISC in the fossil record based on melanosome morphology alone. To explain the results of melanosome shape determination a TEM images of feathers where taken. These results pointed toward suitability of large shape of melanosomes that overlap in shape with grey colour melanosomes to create underlying melanosome layer in non-iridescent structural colour regarding self-assembly processes that take place during feather growth.

non-iridescent structural colors in bird plumage, melanosomes, fossil record, Eocoracias brachyptera

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