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Optically Functional Isoxanthopterin Crystals in the Mirrored Eyes of Decapod Crustaceans

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Optically Functional Isoxanthopterin Crystals in the Mirrored Eyes of Decapod Crustaceans Optically functional isoxanthopterin crystals in the mirrored eyes of decapod crustaceans Benjamin A. Palmera,1,2, Anna Hirschb,1, Vlad Brumfeldc, Eliahu D. Aflalod, Iddo Pinkasc, Amir Sagid,e, Shaked Rosennef, Dan Orong, Leslie Leiserowitzb, Leeor Kronikb, Steve Weinera, and Lia Addadia,2 aDepartment of Structural Biology, Weizmann Institute of Science, 7610001 Rehovot, Israel; bDepartment of Materials and Interfaces, Weizmann Institute of Science, 7610001 Rehovot, Israel; cDepartment of Chemical Research Support, Weizmann Institute of Science, 7610001 Rehovot, Israel; dDepartment of Life Sciences, Ben-Gurion University of the Negev, 8410501 Beer-Sheva, Israel; eThe National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev, 8410501 Beer-Sheva, Israel; fDepartment of Organic Chemistry, Weizmann Institute of Science, 7610001 Rehovot, Israel; and gDepartment of Physics of Complex Systems, Weizmann Institute of Science, 7610001 Rehovot, Israel Contributed by Lia Addadi, January 31, 2018 (sent for review December 27, 2017; reviewed by Joanna Aizenberg and Nicholas W. Roberts) The eyes of some aquatic animals form images through reflective a “clear zone” and is brought to focus as a single upright image on optics. Shrimp, lobsters, crayfish, and prawns possess reflecting the convex-shaped retina (Fig. 1C). The superposition of light rays superposition compound eyes, composed of thousands of square- from multiple eye units effectively increases the pupil size, making faceted eye units (ommatidia). Mirrors in the upper part of the eye this an extremely light-sensitive device, which is well adapted for (the distal mirror) reflect light collected from many ommatidia dim-light habitats. Reflection of light from the ommatidia operates onto the photosensitive elements of the retina, the rhabdoms. A in two regimes: Grazing incidence light, with angles of incidence second reflector, the tapetum, underlying the retina, back-scatters less than ∼15° relative to the ommatidial axis, is internally reflected dispersed light onto the rhabdoms. Using microCT and cryo-SEM from the walls of the ommatidia (12, 14). This is made possible by imaging accompanied by in situ micro–X-ray diffraction and micro- the refractive index contrast in the upper parts of the ommatidium Raman spectroscopy, we investigated the hierarchical organization (n = 1.41 inside and n = 1.34 outside the ommatidium in the region and materials properties of the reflective systems at high resolution of the “crystalline cone”;Fig.1C)(12,14).Atlargeranglesof and under close-to-physiological conditions. We show that the distal incidence, this index contrast is too small to lead to significant mirror consists of three or four layers of plate-like nanocrystals. The reflectivity. Light impinging at higher angles is thus reflected by a tapetum is a diffuse reflector composed of hollow nanoparticles high-refractive-index square mirror (“distal mirror”;Fig.1C), constructed from concentric lamellae of crystals. Isoxanthopterin, which surrounds each eye unit. Vogt postulated (11, 15) that the a pteridine analog of guanine, forms both the reflectors in the distal distal mirror of crustacean eyes is a multilayer reflector, but using mirror and in the tapetum. The crystal structure of isoxanthopterin conventional electron-microscopy methods, he was unable to retain was determined from crystal-structure prediction calculations and the in vivo structure. The square arrangement of the mirror acts verified by comparison with experimental X-ray diffraction. The ex- like a corner reflector, whereby light incident from oblique planes tended hydrogen-bonded layers of the molecules result in an ex- will be reflected from two orthogonal mirrors by a total of 180° and tremely high calculated refractive index in the H-bonded plane, n = will return parallel to its original direction and be brought to focus 1.96, which makes isoxanthopterin crystals an ideal reflecting ma- on the retina (12, 14). A second reflector, the tapetum, lies terial. The crystal structure of isoxanthopterin, together with a de- tailed knowledge of the reflector superstructures, provide a Significance rationalization of the reflective optics of the crustacean eye. Some aquatic animals use reflectors in their eyes either to form isoxanthopterin | eyes | crystal | reflection | mirror images or to increase photon capture. Guanine is the most wide- spread molecular component of these reflectors. Here, we show CHEMISTRY any spectacular optical phenomena exhibited by animals that crystals of isoxanthopterin, a pteridine analog of guanine, Mare produced by the reflection of light from organic (1, 2) form both the image-forming “distal” mirror and the intensity- or inorganic crystals (3). A fascinating manifestation of such bi- enhancing tapetum reflector in the compound eyes of some ological reflectors is in vision (4). Certain animals use mirrors in- decapod crustaceans. The crystal structure of isoxanthopterin steadoflensestoformimages.Thisstrategy is particularly useful in was determined, providing an explanation for why these crys- aquatic environments, where the reduced refractive index contrast tals are so well suited for efficient reflection. Pteridines were in water makes conventional lens-based eyes less effective. Mirror- previously known only as pigments, and our discovery raises the ECOLOGY containing eyes are often extremely efficient light-collectors and question of which other organic molecules may be used to form are found in nocturnal animals or animals inhabiting dim-light crystals with superior reflective properties either in organisms or environments (5). Two types of image-forming reflective eyes are in artificial optical devices. known: concave mirrored eyes and reflection superposition com- pound eyes. The former is epitomized by the scallop eyes, which Author contributions: B.A.P., A.S., D.O., L.L., L.K., S.W., and L.A. designed research; B.A.P., produce well-resolved images by reflecting light from a concave, A.H., V.B., E.D.A., I.P., and S.R. performed research; B.A.P., A.H., I.P., D.O., L.L., and L.K. guanine crystal-based mirror located at the back of the eye onto the analyzed data; and B.A.P., A.H., L.L., L.K., S.W., and L.A. wrote the paper. retina above it (6, 7). Similar eyes are also found in deep-sea fish Reviewers: J.A., Harvard University; and N.W.R., University of Bristol. (8, 9) and crustaceans (10). The latter type of eye, the reflecting The authors declare no conflict of interest. superposition compound eye, is the focus of this study. Published under the PNAS license. The reflecting superposition compound eye is found in decapod Data deposition: CIF file of calculated isoxanthopterin crystal structure deposited in Cam- crustaceans (11, 12). Each compound eye is composed of thou- bridge Structural Database, www.ccdc.cam.ac.uk/ (deposition no. CCDC 1819283). sands of square-faceted eye units called ommatidia. An image is 1B.A.P. and A.H. contributed equally to this work. formed when light is reflected from the top of each ommatidium 2To whom correspondence may be addressed. Email: [email protected] or onto the retina, comprising the photosensitive “rhabdoms” (Fig. 1) [email protected]. (13). In contrast to apposition compound eyes, where each om- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. matidium acts as an isolated optical unit, in the reflecting com- 1073/pnas.1722531115/-/DCSupplemental. pound eye, light collected from many ommatidia is reflected across Published online February 20, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1722531115 PNAS | March 6, 2018 | vol. 115 | no. 10 | 2299–2304 reflector formed from a few layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals which back- scatter light to the retina. By solving the crystal structure of iso- xanthopterin, together with a detailed analysis of the hierarchical organization of the reflectors, we provide a rationalization for the reflective optics in this unique visual system. Results We first determined the 3D organization of the eye components Fig. 1. The reflecting superposition compound eye. (A) X-ray microCT scan on the micrometer to millimeter scale, using X-ray microcomputed of a whole crayfish eye (Procambarus clarkii). (B) Light microscopy image of tomography (X-ray microCT) measurements on fixed, dark-adapted the cornea looking down the eye axis. (C) Schematic of the compound eye eyes from a freshwater crayfish, Cherax quadricarinatus,anda viewed perpendicular to the eye axis. freshwater prawn, Machrobrachium rosenbergii. Similar observations were made on both eyes and are shown only for C. quadricarinatus in Fig. 2 A, B,andD. Two regions of high X-ray attenuation were immediately behind the retina and is responsible for the observed identified in the upper part of the eye: the cornea and the distal eye-shine of decapod crustaceans (16, 17). The tapetum reflects mirror. The cornea forms a smooth boundary around the eye. The light back through the retina, giving the retina a second chance of high X-ray attenuation of the cornea is likely due to the presence of absorbing light that was not absorbed on the first pass (4). some form of calcium carbonate (20). The cornea is formed from a Surprisingly, little is known about the nature of the reflective mosaic of weakly refracting square microlenses (21), ∼50 μmwide, materials in crustacean eyes. Using
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