Rhodopsin, the light-sensitive visual pigment expressed in rod photoreceptors, is specialized for vision in dim light environments. Aquatic environments are particularly challenging for vision due to the spectrally-dependent attenuation of light, which can differ greatly in marine and freshwater systems. Among fish lineages that have successfully colonized freshwater habitats from ancestrally marine environments, croakers are known as highly visual benthic predators. In this study, we isolate rhodopsins from a diversity of freshwater and marine croakers, and find that strong positive selection in rhodopsin is associated with a marine to freshwater transition in South American croakers. In order to determine if this is accompanied by significant shifts in visual abilities, we resurrected ancestral rhodopsin sequences, and tested the experimental properties of ancestral pigments bracketing this transition using in vitro spectroscopic assays. We found the ancestral freshwater croaker rhodopsin is red-shifted relative to its marine ancestor, with mutations that recapitulate ancestral amino acid changes along this transitional branch resulting in faster kinetics that are likely to be associated with more rapid dark adaptation. This could be quite advantageous in freshwater due to the red-shifted spectrum and relatively narrow interface and frequent transitions between bright and dim light environments. This study is the first to experimentally demonstrate that positively selected substitutions in ancestral visual pigments alter protein function to freshwater visual environments following a transition from an ancestrally marine state, and provides insight into the molecular mechanisms underlying some of the physiological changes associated with this major habitat transition.

中文翻译：

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重新创建的祖先视蛋白与海洋到淡水黄花鱼的入侵有关，揭示了动力学和光谱适应性

视紫红质是杆状感光体中表达的一种光敏视觉色素，专门用于昏暗环境下的视觉。由于光的光谱依赖性衰减，水生环境对于视觉尤其具有挑战性，在海洋和淡水系统中，光的衰减可能有很大差异。在成功地定居于祖先海洋环境的淡水栖息地的鱼类谱系中，黄鱼被称为高度视觉底栖捕食者。在这项研究中，我们从多种淡水和海洋黄花鱼中分离了视紫红质，发现在南美洲黄花鱼中，视紫红质中强烈的正选择与海洋向淡水的过渡有关。为了确定这是否伴随视觉能力的重大变化，我们复活了祖传视紫红质序列，体外光谱分析。我们发现祖先的淡水黄花鱼视紫红质相对于其海洋祖先发生了红移，突变沿该过渡分支概括了祖先的氨基酸变化，从而导致更快的动力学，这可能与更快的黑暗适应有关。由于红移光谱和相对狭窄的界面以及明亮和暗光环境之间的频繁转换，这在淡水中可能是非常有利的。这项研究首次通过实验证明，祖先视觉色素中的阳性选择替代物会在从祖先海洋状态转变为淡水视觉环境后将蛋白质功能改变为淡水视觉环境，