Queensland Brain Institute, The University of Queensland, Brisbane, Queensland 4072, Australia.
Red Sea Research Center, Biological and Environmental Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
Sci Adv. 2017 Nov 8;3(11):eaao4709. doi: 10.1126/sciadv.aao4709. eCollection 2017 Nov.
Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides' habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.
大多数脊椎动物的视网膜是双焦点的，包括两种感光细胞类型：杆状细胞用于暗光（暗视）视觉，锥状细胞用于强光（明视）和色觉。然而，深海鱼类只在暗光条件下活动；因此，大多数深海鱼类已经失去了它们的锥状细胞，转而形成了由单纯的杆状细胞组成的单焦点视网膜。尽管珍珠鱼属的鱼类具有这种纯粹的杆状视网膜，但它们的行为与这种单焦点视觉系统不符。与其他深海鱼类不同，珍珠鱼主要在黄昏和黎明时分在靠近水面的地方活动，那里的光线强度处于中间水平（黄昏或中间光），需要同时使用杆状细胞和锥状细胞感光器。这项研究通过证明珍珠鱼的视网膜不仅具有杆状细胞感光器，而且几乎完全由转化的锥状细胞感光器组成，阐明了这一悖论。这些转化细胞将杆状细胞感光器的形态特征与锥状细胞视蛋白和锥状细胞光转导级联结合起来，形成了一种独特的感光器类型，即对珍珠鱼栖息地光条件（中间光强度下的蓝移光）具有特殊调谐的杆状锥状细胞。将两种感光器类型的特性结合到一个单一的细胞类型中，而不是在中间光条件下使用不能充分发挥作用的两种感光器类型，可能是优化视觉性能的最有效和最经济的解决方案。这些结果挑战了脊椎动物双焦点视网膜的功能和进化的现有范式，并强调需要更全面地评估一般视觉系统。
