Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA.
Integr Comp Biol. 2013 Jul;53(1):68-77. doi: 10.1093/icb/ict063. Epub 2013 Jun 7.
Evolutionary biologists have long been interested in how expansions of the photosensory system might contribute to morphological differentiation of animals. Comparative studies in vertebrate and arthropod lineages have provided considerable insight into how the duplication of opsin, the first gene of the phototransduction pathway, have led to functional differentiation and new ecological opportunities; however, this relationship cannot be examined in many invertebrate groups as we have yet to characterize their opsin content. Scallops (Pectinidae) are a promising molluscan model to study the evolution of opsin and its potential role in speciation. Recently, we discovered a second Gq-coupled, or r-, opsin gene expressed in the eyes of two scallop species. To investigate the evolutionary origin of this opsin, we screened 12 bivalve species from 4 families, representing both mobile and sessile species, with and without eyes. Although only one ortholog was recovered from the genome of the eyeless, immobile oyster, we found both genes to have been retained in 3 families comprising the order Pectinoida. Within this clade, non-mobile species of scallops appear to have lost one gene. Phylogeny-based tests of selection indicate different degrees of purifying selection following duplication. These data, in conjunction with highly divergent gene sequences and ortholog-specific retention, suggest functional differences. Our results are congruent with a Gq-opsin gene duplication in an oyster-Pectinoida ancestor, approximately 470 Mya, and suggest the likelihood of retaining both genes is associated with either the presence of eyes and/or degree of mobility. The identification of two highly divergent Gq-opsin genes in scallops is valuable for future functional investigations and provides a foundation for further study of a morphologically and ecologically diverse clade of bivalves that has been understudied with respect to visual ecology and diversification of opsin.
进化生物学家长期以来一直对光感受器系统的扩展如何促进动物形态分化感兴趣。脊椎动物和节肢动物谱系的比较研究为我们提供了大量的见解，了解视蛋白（光转导途径的第一个基因）的复制如何导致功能分化和新的生态机会；然而，由于我们尚未描述许多无脊椎动物群体的视蛋白含量，因此无法在这些群体中研究这种关系。扇贝（扇贝科）是研究视蛋白进化及其在物种形成中潜在作用的有前途的软体动物模型。最近，我们在两种扇贝物种的眼睛中发现了第二个 Gq 偶联的 r-视蛋白基因。为了研究这种视蛋白的进化起源，我们筛选了 12 种来自 4 个科的双壳类物种，代表有眼和无眼的移动和固着物种。虽然从无眼、固着牡蛎的基因组中只回收了一个同源物，但我们发现这两个基因在包括扇贝目在内的 3 个科中都被保留了下来。在这个分支中，无眼的扇贝物种似乎失去了一个基因。基于系统发育的选择测试表明，复制后存在不同程度的纯化选择。这些数据与高度不同的基因序列和同源基因特异性保留一起，表明存在功能差异。我们的结果与大约 4.7 亿年前牡蛎-扇贝目中 Gq 视蛋白基因的重复是一致的，并表明保留两个基因的可能性与眼睛的存在和/或移动程度有关。在扇贝中鉴定出两个高度不同的 Gq 视蛋白基因，这对于未来的功能研究是有价值的，并为进一步研究在视觉生态学和视蛋白多样化方面研究不足的形态和生态多样化的双壳类群提供了基础。
