Biochemical and Genomic Underpinnings of Carotenoid Colour Variation Across a Hybrid Zone Between South Asian Flameback Woodpeckers

Colouration and patterning have been implicated in lineage diversification across various taxa, as colour traits are heavily influenced by sexual and natural selection. Investigating the biochemical and genomic foundations of these traits therefore provides deeper insights into the interplay between genetics, ecology and social interactions in shaping the diversity of life. In this study, we assessed the pigment chemistries and genomic underpinnings of carotenoid colour variation in naturally hybridising Dinopium flamebacks in tropical South Asia. We employed reflectance spectrometric analysis to quantify species-specific plumage colouration, High-Performance Liquid Chromatography (HPLC) to elucidate the feather carotenoids of flamebacks across the hybrid zone, and Genome-Wide Association Study (GWAS) using next-generation sequencing data to uncover the genetic factors underlying carotenoid colour variation in flamebacks. Our analysis revealed that the red mantle feathers of D. psarodes primarily contained astaxanthin, with small amounts of other 4-keto-carotenoids. In contrast, the yellow mantle feathers of D. benghalense predominantly contained lutein and 3'-dehydro-lutein, alongside minor amounts of zeaxanthin, β-cryptoxanthin and canary-xanthophylls A and B. Hybrids with an intermediate, orange colouration deposited all of these pigments in their mantle feathers, with notably higher concentrations of carotenoids with ε-end rings. The GWAS analysis identified the CYP2J2 gene, which plays a role in carotenoid ketolation, as associated with the expression of carotenoid colouration. Read depth data suggested variation in copy number of this gene in flamebacks. These findings contribute to the growing knowledge of avian carotenoid metabolism and highlight how genomic architecture can influence phenotypic diversity.