About the Blog Author: Brinkley Thornton is a current accelerated bachelor’s to master’s student at the University of Alabama at Birmingham and will become a full time masters student in the Summer of 2022. She currently works in the Krueger-Hadfield Evolutionary Ecology Lab. Her research interests include population genetics and plant ecology and evolutionary biology.
In birds, we find the most diverse range of color phenotypes – plumage, eggs – across many different species! These colors play a role that is so much more significant than simply for looks – courtship, avoiding predators, communication between parents and their offspring, photoprotection, thermoregulation, bacterial resistance, and structural support (Price-Waldman & Stoddard 2021). What is the genetic and developmental bases for avian coloration?
Most research on coloration has centered on melanin pigments, but there are SO many more pigments involved that we need to understand to get the full picture of the phenotypic diversity in avian coloration.
Carotenoids, for example, are a class of pigments responsible for many of the yellow, red, or orange colors that must be obtained through a bird’s diet (Figure 1). Many genes are involved in carotenoid processing such as scavenger receptors, ketolases, and beta carotene oxygenases. Studies have linked testosterone levels to CYP2J19 expression in red-backed fairywrens in which males with higher levels of testosterone exhibit higher levels of CYP2J19, leading to ornamented males with red plumage (Kahlil et al. 2020).
Psittacofulvins are pigments that so far have only been found in species of parrots and give rise to a range of red, orange, and yellow colors (Figure 2) (Price-Waldman & Stoddard 2021). However, unlike carotenoids, psittacofulvins are synthesized within the parrots themselves. In a recent study, a polyketide synthase gene, MuKPS, was identified as a possible candidate for yellow psittacofulvins (Cook et al. 2017).
Coloration is not only produced through pigments but also through the interaction of light with microscopic surfaces of various refractive indexes (Price-Waldman & Stoddard 2021). These structural colors tend to be the major players involved in the coloration of plumage, skin, and irises of birds. For instance, iridescent plumage coloration is produced by the systematic scattering of light from arrays of keratin, air, and melanosomes in feather barbules (Price-Waldman & Stoddard 2021). The challenge with studying the genetic basis of structural colors lies in the fact that morphological variation happens at the nanoscale level and very little is known about the development of the nanostructures.
Structural colors and pigments also depend on their interactions with one another in shaping avian color phenotypes. The green plumage coloration in budgerigars is a result of this ‘color-mixing’. This green is a result of structural color reflecting both green and blue wavelengths of light, but a yellow pigment absorbs some of the blue light allowing a novel green to predominate (D’Alba et al. 2012).
Avian species are differentiated by the various patterning of their plumages as well. Micropatterning in feathers is likely the result of spatial and temporal changes in genotypic expression (Price-Waldman & Stoddard 2021). Recent work has shown that mechanisms of self-organization and instructional developmental cues work together to produce a wide variety of periodic patterns.
Diverse coloration can be seen in avian bare parts as well. The study of how this occurs is an up-and-coming area of avian coloration genetics, as most research has focused on coloration of plumage. Recent studies have shown that mutation at the site of the gene BCO2 allowed for carotenoid pigmentation to occur in bills of urucum canaries and allows for beak color polymorphism in finches (Gazda et al. 2020). Nevertheless, the genetic bases of bare part coloration remain relatively unexplored in relation to plumage coloration research.
Exciting areas of future research center on uncovering genetic links between avian visual systems and coloration and linking the vast diversity of avian coloration with speciation. Recent analyses have shown divergence in the genomes of closely related species at sites of genes related to pigmentation (Price-Waldman & Stoddard 2021). Birds are great models to further understand the evolution of phenotypic variation.
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