Friedman Ofir, Böhm Alexander, Rechav Katya, Pinkas Iddo, Brumfeld Vlad, Pass Günther, Weiner Steve, Addadi Lia
Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel.
Department of Integrative Zoology, University of Vienna, Djerassiplatz 1, 1030 Vienna, Austria.
J Struct Biol. 2022 Mar;214(1):107834. doi: 10.1016/j.jsb.2022.107834. Epub 2022 Jan 22.
Biogenic purine crystals function in vision as mirrors, multilayer reflectors and light scatterers. We investigated a light sensory organ in a primarily wingless insect, the jumping bristletail Lepismachilis rozsypali (Archaeognatha), an ancestral group. The visual system of this animal comprises two compound eyes, two lateral ocelli, and a median ocellus, which is located on the front of the head, pointing downwards to the ground surface. We determined that the median ocellus contains crystals of xanthine, and we obtained insights into their function. To date, xanthine biocrystals have only been found in the Archaeognatha. We performed a structural analysis, using reflection light microscopy, cryo-FIB-SEM, microCT and cryo-SEM. The xanthine crystals cover the bottom of a bowl-shaped volume in the median ocellus, in analogy to a tapetum, and reflect photons to light-sensitive receptors that are spread in the volume without apparent order or preferential orientation. We infer that the median ocellus operates as an irregular multifocal reflector, which is not capable of forming images. A possible function of this organ is to improve photon capture, and by so doing assess distances from the ground surface when jumping by determining changes in the intensity and contrast of the incident light.
生物源嘌呤晶体在视觉中起到镜子、多层反射器和光散射体的作用。我们研究了一种主要无翅昆虫——跳跃缨尾虫Lepismachilis rozsypali（原尾目）（一个原始类群）的光感受器。这种动物的视觉系统包括两只复眼、两只侧单眼和一只位于头部前方、向下指向地面的中单眼。我们确定中单眼含有黄嘌呤晶体，并对其功能有了深入了解。迄今为止，黄嘌呤生物晶体仅在原尾目中被发现。我们使用反射光显微镜、冷冻聚焦离子束扫描电子显微镜、显微计算机断层扫描和冷冻扫描电子显微镜进行了结构分析。黄嘌呤晶体覆盖在中单眼碗状结构的底部，类似于反光色素层，将光子反射到散布在该结构内、无明显排列顺序或优先取向的光感受器上。我们推断中单眼作为一个不规则的多焦点反射器，无法形成图像。该器官的一个可能功能是改善光子捕获，从而在跳跃时通过确定入射光强度和对比度的变化来评估与地面的距离。
