Biochemical and genomic underpinnings of carotenoid color variation across a hybrid zone between South Asian flameback woodpeckers

Coloration and patterning have been implicated in lineage diversification
across various taxa, as color 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 color variation in naturally
hybridizing Dinopium flamebacks in tropical South Asia. We employed
reflectance spectrometric analysis to quantify species-specific plumage
coloration, 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 color variation in flamebacks.
Our analysis revealed that the red mantle feathers of D. psarodes
primarily contain 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,
coloration 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 coloration.
This gene exhibited significant allele variation and evidence of multiple
copies in flamebacks. These findings contribute to the growing knowledge
of avian carotenoid metabolism and highlight how genomic architecture can
influence phenotypic diversity.