title: A complex carotenoid palette tunes avian colour vision.
abstract: The brilliantly coloured cone oil droplets of the avian retina function as long-pass cut-off filters that tune the spectral sensitivity of the photoreceptors and are hypothesized to enhance colour discrimination and improve colour constancy. Although it has long been known that these droplets are pigmented with carotenoids, their precise composition has remained uncertain owing to the technical challenges of measuring these very small, dense and highly refractile optical organelles. In this study, we integrated results from high-performance liquid chromatography, hyperspectral microscopy and microspectrophotometry to obtain a comprehensive understanding of oil droplet carotenoid pigmentation in the chicken (Gallus gallus). We find that each of the four carotenoid-containing droplet types consists of a complex mixture of carotenoids, with a single predominant carotenoid determining the wavelength of the spectral filtering cut-off. Consistent with previous reports, we find that the predominant carotenoid type in the oil droplets of long-wavelength-sensitive, medium-wavelength-sensitive and short-wavelength-sensitive type 2 cones are astaxanthin, zeaxanthin and galloxanthin, respectively. In addition, the oil droplet of the principal member of the double cone contains a mixture of galloxanthin and two hydroxycarotenoids (lutein and zeaxanthin). Short-wavelength-absorbing apocarotenoids are present in all of the droplet types, providing filtering of light in a region of the spectrum where filtering by hydroxy- and ketocarotenoids may be incomplete. Thus, birds rely on a complex palette of carotenoid pigments within their cone oil droplets to achieve finely tuned spectral filtering.
authors.affiliation: Department of Pathology and Immunology, Washington University School of Medicine, St Louis, MO 63110, USA.
authors.forename: Matthew B
authors.identifier: http://orcid.org/0000-0001-9184-197X
authors.initials: MB
authors.lastname: Toomey
authors.affiliation: Bioenergy and Defense Technologies, Sandia National Laboratories, Albuquerque, NM 87123, USA.
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Collins
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118-2526, USA.
authors.forename: Rikard
authors.initials: R
authors.lastname: Frederiksen
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118-2526, USA.
authors.forename: M Carter
authors.identifier: http://orcid.org/0000-0002-0847-939X
authors.initials: MC
authors.lastname: Cornwall
authors.affiliation: Bioenergy and Defense Technologies, Sandia National Laboratories, Albuquerque, NM 87123, USA.
authors.forename: Jerilyn A
authors.identifier: http://orcid.org/0000-0003-2953-1721
authors.initials: JA
authors.lastname: Timlin
authors.affiliation: Department of Pathology and Immunology, Washington University School of Medicine, St Louis, MO 63110, USA jcorbo@wustl.edu.
authors.forename: Joseph C
authors.identifier: http://orcid.org/0000-0002-9323-7140
authors.initials: JC
authors.lastname: Corbo
mesh_terms: D000818:Animals; D002338:Carotenoids / Q000737:chemistry*; D002645:Chickens; D002851:Chromatography, High Pressure Liquid; D003116:Color; D055253:Color Vision / Q000502:physiology*; D008027:Light; D014975:Lutein / Q000737:chemistry; D015591:Microspectrophotometry; D009821:Oils / Q000737:chemistry; D010858:Pigmentation / Q000502:physiology*; D012160:Retina / Q000502:physiology; D017949:Retinal Cone Photoreceptor Cells; D014466:Ultraviolet Rays; D024341:Xanthophylls / Q000737:chemistry; D065146:Zeaxanthins / Q000737:chemistry
chemical_list: D009821:Oils; D024341:Xanthophylls; D065146:Zeaxanthins; C000609456:galloxanthin; D002338:Carotenoids; C005948:astaxanthine; D014975:Lutein
keywords: carotenoid; hyperspectral microscopy; microspectrophotometry; vision