College of Life Sciences, Shaanxi Normal University, Xi'an, China.
College of Animal Science and Veterinary Medicine, Shenyang Agricultural University, Shenyang, China.
Mol Biol Evol. 2021 Dec 9;38(12):5726-5734. doi: 10.1093/molbev/msab262.
Rhodopsin comprises an opsin attached to a retinal chromophore and is the only visual pigment conferring dim-light vision in vertebrates. On activation by photons, the retinal group becomes detached from the opsin, which is then inactive until it is recharged. Of all vertebrate species, those that dive face unique visual challenges, experiencing rapid decreases in light level and hunting in near darkness. Here, we combine sequence analyses with functional assays to show that the rhodopsin pigments of four divergent lineages of deep-diving vertebrates have undergone convergent increases in their retinal release rate. We compare gene sequences and detect parallel amino acids between penguins and diving mammals and perform mutagenesis to show that a single critical residue fully explains the observed increases in retinal release rate in both the emperor penguin and beaked whale. At the same time, we find that other shared sites have no significant effect on retinal release, implying that convergence does not always signify adaptive significance. We propose that accelerated retinal release confers rapid rhodopsin recharging, enabling the visual systems of diving species to adjust quickly to changing light levels as they descend through the water column. This contrasts with nocturnal species, where adaptation to darkness has been attributed to slower retinal release rates.
视蛋白与视黄醛结合构成视紫红质，是唯一能赋予脊椎动物暗光视觉的视觉色素。光激活后，视黄醛基团与视蛋白分离，视蛋白直到重新充电才恢复活性。在所有脊椎动物中，潜水物种面临着独特的视觉挑战，它们经历着光强度的快速下降，并在几乎黑暗的环境中捕猎。在这里，我们结合序列分析和功能测定，表明四种不同的深潜脊椎动物谱系的视紫红质色素的视网膜释放率都经历了趋同增加。我们比较了基因序列，在企鹅和潜水哺乳动物中检测到平行氨基酸，并进行了诱变实验，结果表明，一个单一的关键残基可以完全解释帝企鹅和喙鲸中观察到的视网膜释放率的增加。同时，我们发现其他共享位点对视紫红质释放没有显著影响，这意味着趋同并不总是具有适应性意义。我们提出，加速的视网膜释放赋予了快速的视蛋白再充电能力，使潜水物种的视觉系统能够在它们通过水柱下降时快速适应光强度的变化。这与夜间物种形成对比，夜间物种的适应黑暗归因于较慢的视网膜释放率。
