Schott Ryan K, Müller Johannes, Yang Clement G Y, Bhattacharyya Nihar, Chan Natalie, Xu Mengshu, Morrow James M, Ghenu Ana-Hermina, Loew Ellis R, Tropepe Vincent, Chang Belinda S W
Department of Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada M5S 3B2;
Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, 10115 Berlin, Germany;
Proc Natl Acad Sci U S A. 2016 Jan 12;113(2):356-61. doi: 10.1073/pnas.1513284113. Epub 2015 Dec 29.
Vertebrate retinas are generally composed of rod (dim-light) and cone (bright-light) photoreceptors with distinct morphologies that evolved as adaptations to nocturnal/crepuscular and diurnal light environments. Over 70 years ago, the "transmutation" theory was proposed to explain some of the rare exceptions in which a photoreceptor type is missing, suggesting that photoreceptors could evolutionarily transition between cell types. Although studies have shown support for this theory in nocturnal geckos, the origins of all-cone retinas, such as those found in diurnal colubrid snakes, remain a mystery. Here we investigate the evolutionary fate of the rods in a diurnal garter snake and test two competing hypotheses: (i) that the rods, and their corresponding molecular machinery, were lost or (ii) that the rods were evolutionarily modified to resemble, and function, as cones. Using multiple approaches, we find evidence for a functional and unusually blue-shifted rhodopsin that is expressed in small single "cones." Moreover, these cones express rod transducin and have rod ultrastructural features, providing strong support for the hypothesis that they are not true cones, as previously thought, but rather are modified rods. Several intriguing features of garter snake rhodopsin are suggestive of a more cone-like function. We propose that these cone-like rods may have evolved to regain spectral sensitivity and chromatic discrimination as a result of ancestral losses of middle-wavelength cone opsins in early snake evolution. This study illustrates how sensory evolution can be shaped not only by environmental constraints but also by historical contingency in forming new cell types with convergent functionality.
脊椎动物的视网膜通常由具有不同形态的视杆（暗光）和视锥（亮光）光感受器组成，这些形态是为适应夜间/晨昏和昼间光照环境而进化形成的。七十多年前，“转化”理论被提出以解释一些光感受器类型缺失的罕见例外情况，这表明光感受器在细胞类型之间可能发生进化转变。尽管研究已在夜行性壁虎中为该理论提供了支持，但全视锥视网膜的起源，比如在昼行性游蛇科蛇类中发现的那些，仍然是个谜。在这里，我们研究了一种昼行性束带蛇视杆的进化命运，并测试了两种相互竞争的假说：（i）视杆及其相应的分子机制已经丢失；（ii）视杆在进化上被修饰成类似于视锥并发挥视锥的功能。通过多种方法，我们发现了一种功能性且异常蓝移的视紫红质的证据，它在小型单个“视锥”中表达。此外，这些视锥表达视杆转导蛋白并具有视杆超微结构特征，这为它们并非如之前所认为的是真正的视锥，而是经过修饰的视杆这一假说提供了有力支持。束带蛇视紫红质的几个有趣特征表明其具有更类似视锥的功能。我们提出，这些类似视锥的视杆可能是由于早期蛇类进化过程中中波长视锥视蛋白的祖先性丢失而进化形成，以重新获得光谱敏感性和颜色辨别能力。这项研究说明了感觉进化不仅可以由环境限制塑造，还可以由形成具有趋同功能的新细胞类型过程中的历史偶然性塑造。
