Warrington Rachael E, Davies Wayne I L, Hemmi Jan M, Hart Nathan S, Potter Ian C, Collin Shaun P, Hunt David M
Neuroscience Research Institute, University of California Santa Barbara, Santa Barbara, California, USA.
School of Biological Sciences, The University of Western Australia, Perth, Western Australia, Australia.
J Comp Neurol. 2021 Jun;529(9):2265-2282. doi: 10.1002/cne.25092. Epub 2020 Dec 30.
Lampreys are extant members of the agnathan (jawless) vertebrates that diverged ~500 million years ago, during a critical stage of vertebrate evolution when image-forming eyes first emerged. Among lamprey species assessed thus far, the retina of the southern hemisphere pouched lamprey, Geotria australis, is unique, in that it possesses morphologically distinct photoreceptors and expresses five visual photopigments. This study focused on determining the number of different photoreceptors present in the retina of G. australis and whether each cell type expresses a single opsin class. Five photoreceptor subtypes were identified based on ultrastructure and differential expression of one of each of the five different visual opsin classes (lws, sws1, sws2, rh1, and rh2) known to be expressed in the retina. This suggests, therefore, that the retina of G. australis possesses five spectrally and morphologically distinct photoreceptors, with the potential for complex color vision. Each photoreceptor subtype was shown to have a specific spatial distribution in the retina, which is potentially associated with changes in spectral radiance across different lines of sight. These results suggest that there have been strong selection pressures for G. australis to maintain broad spectral sensitivity for the brightly lit surface waters that this species inhabits during its marine phase. These findings provide important insights into the functional anatomy of the early vertebrate retina and the selection pressures that may have led to the evolution of complex color vision.
七鳃鳗是无颌脊椎动物现存的成员，它们在约5亿年前分化，处于脊椎动物进化的关键阶段，此时成像眼睛首次出现。在迄今为止评估的七鳃鳗物种中，南半球袋七鳃鳗（Geotria australis）的视网膜很独特，因为它拥有形态上不同的光感受器，并表达五种视觉视色素。本研究的重点是确定袋七鳃鳗视网膜中存在的不同光感受器的数量，以及每种细胞类型是否表达单一的视蛋白类别。基于已知在视网膜中表达的五种不同视觉视蛋白类别（lws、sws1、sws2、rh1和rh2）中每一种的超微结构和差异表达，鉴定出了五种光感受器亚型。因此，这表明袋七鳃鳗的视网膜拥有五种光谱和形态上不同的光感受器，具有形成复杂颜色视觉的潜力。每种光感受器亚型在视网膜中都有特定的空间分布，这可能与不同视线方向上光谱辐射率的变化有关。这些结果表明，对于袋七鳃鳗在其海洋阶段所栖息的明亮表层水域，存在强大的选择压力促使其保持广泛的光谱敏感性。这些发现为早期脊椎动物视网膜的功能解剖学以及可能导致复杂颜色视觉进化的选择压力提供了重要见解。
