Biology subjects

Palmer, B. A.

Publications and source records attributed to Palmer, B. A..

4 recordsLinked to original sources

bioRxiv2025

Damselflies Overcome Color Saturation Barriers of Photonic Glasses via Structural Dispersion and Pigment Loading

Biologys strategies for manipulating light offer rich inspiration for the design of sustainable replacements to conventional pigments in paints, coatings, and displays. For these applications, where angle-independent color is required, photonic glasses, composed of random arrangements of dielectric spheres, offer a promising solution. However, their intrinsic disorder, particularly from particle polydispersity, fundamentally limits their color saturation and practical utility. In contrast, insects like damselflies and dragonflies exhibit surprisingly vivid, non-iridescent structural colors, despite relying on disordered photonic structures. Here, we show how damselflies combine compositional and structural dispersion to overcome color saturation limits of photonic glasses. Firstly, doping transparent particles with yellow pigments dramatically enhances blue-green structural resonances by the coupled effects of narrowband absorption and refractive index (material) dispersion. Secondly, the refractive index of the nanospheres varies with their size and crystallinity. This gives rise to a structural dispersion which maintains consistent optical path lengths in polydisperse assemblies, preserving high color purity. Finally, we show how damselflies tune their structural colors during maturation by precisely modulating the size of the nanospheres. Remarkably, the tuning of particle size, refractive index and pigment loading, arises naturally during the development of the pigment cells - where the pteridine nanospheres undergo a process of densification, crystallization and metabolic maturation.

animal behavior and cognition↗

bioRxiv2022

The Formation of Biogenic Guanine Crystals Closely Resembles Melanosome Morphogenesis

Animals precisely control the morphology and assembly of highly reflective guanine crystals to produce diverse optical phenomena used in coloration and vision. However, little is known about how organisms regulate crystallization to produce optically useful crystal morphologies which express highly reflective crystal faces. Guanine crystals form inside iridosome vesicles within specialized chromatophore cells called iridophores. By following the formation of iridosomes in developing scallop eyes we show that, despite their different physical properties, the morphogenesis of iridosomes bears a striking resemblance to melanosome morphogenesis in vertebrates. A key finding is that pre-assembled intravesicular fibrillar sheets template crystal nucleation, growth, and orientation, dictating the formation of optically functional, but thermodynamically disfavored plate-like habits. The common control mechanisms for melanin and guanine formation inspire new approaches for manipulating the morphologies and properties of molecular materials.

cell biology↗

bioRxiv2022

Ontogenetic Color Switching in Lizards as a by-Product of Guanine Cell Development

Many animals undergo dramatic changes in colour during development1,2. Changes in predation risk during ontogeny are associated with spectacular switches in defensive colours, typically involving the replacement of skin or the production of new pigment cells3. Ontogenetic colour systems are ideal models for understanding the evolution and formation mechanisms of animal colour which remain largely enigmatic2. We show that defensive colour switching in lizards arises by reorganization of a single photonic system, as an incidental by-product of chromatophore maturation. The defensive blue tail colour of hatchling A. beershebensis lizards is produced by light scattering from premature guanine crystals in underdeveloped iridophore cells. Camouflaged adult tail colours emerge upon reorganization of the guanine crystals into a photonic reflector during chromatophore maturation. The substituent guanine crystals form by the attachment of individual nanoscopic plates, which coalesce during growth to form single crystals. Our results show that the blue colour of hatchlings is a fortuitous, but necessary, precursor to the development of adult colour. Striking functional colours in animals can thus arise not as distinct evolutionary innovations but via exploitation of the timing of naturally occurring changes in chromatophore cell development.

developmental biology↗

bioRxiv2017

Isoxanthopterin: An Optically Functional Biogenic Crystal in the Eyes of Decapod Crustaceans

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye-units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the underlying photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close to physiological conditions. We show that the distal mirror consists of three or four layers of sparse plate-like nano-crystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen bonded layers of the molecules results in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.\n\nSignificanceAquatic animals use reflectors in their eyes either to form images or to increase photon capture. Guanine is the most widespread molecular component of these reflectors. Here we show that crystals of isoxanthopterin, a pteridine analogue of guanine, form both the image-forming distal mirror and the intensity-enhancing tapetum reflector in the compound eyes of some decapod crustaceans. The crystal structure of isoxanthopterin was determined, providing an explanation for why these crystals are so well suited for efficient reflection. Pteridines were previously known only as pigments and our discovery raises the question of which other organic molecules may be used to form crystals with superior reflective properties either in organisms or in artificial optical devices.

bioengineering↗