Abstract:

Background & Aims: Animal coloration has long been a topic of interest. As one of the most colorful groups in the world, birds exhibit a wide diversity of plumage pigmentation patterns. Carotenoid, an important component contributing to vivid colors in many avian species, can be deposited in avian integument and produce red, orange, yellow, pink and purple coloration, which can serve as an honest signal of individual condition in mediating social and mating interaction. Therefore, carotenoid pigmentation is an ideal phenotypic trait for understanding the diversity of plumage coloration under a variety of evolutionary pressures and constraints. Most significantly, this complex coloration mechanism provides a variety of opportunities for the evolution of plumage coloration driven by natural and sexual selection. In this review, we highlight the current advances in the mechanisms underlying carotenoid-based coloration in four physiological processes involved in carotenoid coloration in birds. We also introduce the interaction of carotenoid pigment and microstructural coloration, and the ecological forces that drive the evolution of carotenoid coloration.

Progresses: We summarized the biochemical pathways of dietary carotenoids absorption, transportation, metabolism and deposition in birds, all of which may affect feather coloration as condition-dependent traits. We further reported a biochemical modification pathway of the yellow dietary carotenoids and conversion to red ketocarotenoids in vivo, which gives birds more opportunities to become colorful. Still, the physiological and genetic mechanisms related to carotenoid processing remain uncertain. With the development of high-throughput sequencing technology, recent breakthroughs have revealed some genes that control carotenoid pigmentation, which has shed light on the molecular basis of carotenoid-based coloration. These genes, such as CYP2J19, BCO2, SCARB1, play an important role in the carotenoid coloration in feathers, and lay the foundation for our understanding of the genetic mechanism of carotenoids in the process of feather coloration.

Prospects: This review provides a helpful foundation for understanding the biochemical mechanisms underlying bird coloration. In addition, we have put forward questions in the field that require urgent attention: (1) How do different physiological pathways interact to form and maintain the color diversity in birds? (2) What factors regulate the metabolism and deposition of carotenoids during the growth of bird feathers, which makes the occurrence of carotenoid plumage present a specific temporal and spatial pattern? (3) How does the plumage dichromatism caused by the difference in carotenoid coloration occur and maintain?

Key words: carotenoid, plumage color, CYP2J19, BCO2, SCARB1, metabolism