Ochadai Academic Production, Ochanomizu University, Ohtsuka 2-1-1, Bunkyo, Tokyo, Japan.
BMC Evol Biol. 2011 Jun 24;11:180. doi: 10.1186/1471-2148-11-180.
Coleoid cephalopods (squids and octopuses) have evolved a camera eye, the structure of which is very similar to that found in vertebrates and which is considered a classic example of convergent evolution. Other molluscs, however, possess mirror, pin-hole, or compound eyes, all of which differ from the camera eye in the degree of complexity of the eye structures and neurons participating in the visual circuit. Therefore, genes expressed in the cephalopod eye after divergence from the common molluscan ancestor could be involved in eye evolution through association with the acquisition of new structural components. To clarify the genetic mechanisms that contributed to the evolution of the cephalopod camera eye, we applied comprehensive transcriptomic analysis and conducted developmental validation of candidate genes involved in coleoid cephalopod eye evolution.
We compared gene expression in the eyes of 6 molluscan (3 cephalopod and 3 non-cephalopod) species and selected 5,707 genes as cephalopod camera eye-specific candidate genes on the basis of homology searches against 3 molluscan species without camera eyes. First, we confirmed the expression of these 5,707 genes in the cephalopod camera eye formation processes by developmental array analysis. Second, using molecular evolutionary (dN/dS) analysis to detect positive selection in the cephalopod lineage, we identified 156 of these genes in which functions appeared to have changed after the divergence of cephalopods from the molluscan ancestor and which contributed to structural and functional diversification. Third, we selected 1,571 genes, expressed in the camera eyes of both cephalopods and vertebrates, which could have independently acquired a function related to eye development at the expression level. Finally, as experimental validation, we identified three functionally novel cephalopod camera eye genes related to optic lobe formation in cephalopods by in situ hybridization analysis of embryonic pygmy squid.
We identified 156 genes positively selected in the cephalopod lineage and 1,571 genes commonly found in the cephalopod and vertebrate camera eyes from the analysis of cephalopod camera eye specificity at the expression level. Experimental validation showed that the cephalopod camera eye-specific candidate genes include those expressed in the outer part of the optic lobes, which unique to coleoid cephalopods. The results of this study suggest that changes in gene expression and in the primary structure of proteins (through positive selection) from those in the common molluscan ancestor could have contributed, at least in part, to cephalopod camera eye acquisition.
头足类软体动物（鱿鱼和章鱼）已经进化出了一个相机眼，其结构与脊椎动物非常相似，被认为是趋同进化的经典例子。然而，其他软体动物拥有镜像、针孔或复眼，所有这些眼睛的结构和参与视觉回路的神经元的复杂程度都与相机眼不同。因此，在从共同的软体动物祖先中分化出来后，在头足类动物眼中表达的基因可能通过与获得新的结构成分相关而参与眼睛的进化。为了阐明促成头足类相机眼进化的遗传机制，我们应用了全面的转录组分析，并对头足类软体动物眼睛进化中涉及的候选基因进行了发育验证。
我们比较了 6 种软体动物（3 种头足类和 3 种非头足类）的眼睛中的基因表达，并基于对头足类无相机眼的 3 种软体动物的同源搜索，选择了 5707 个作为头足类相机眼特异性候选基因。首先，我们通过发育基因芯片分析证实了这些 5707 个基因在头足类相机眼形成过程中的表达。其次，通过对头足类谱系的分子进化（dN/dS）分析来检测正选择，我们在头足类从软体动物祖先分化出来后功能似乎发生变化的 156 个基因中鉴定出这些基因，并促进了结构和功能的多样化。第三，我们选择了 1571 个在头足类和脊椎动物的相机眼中都有表达的基因，这些基因可能在表达水平上独立获得了与眼睛发育相关的功能。最后，作为实验验证，我们通过原位杂交分析，对头足类幼体进行了研究，鉴定出了三个与头足类视神经叶形成有关的功能新颖的头足类相机眼基因。
我们从表达水平对头足类相机眼特异性进行了分析，鉴定出了头足类谱系中 156 个正选择的基因和 1571 个在头足类和脊椎动物相机眼中共同发现的基因。实验验证表明，头足类相机眼特异性候选基因包括那些在头足类特有的视神经叶外部表达的基因。这项研究的结果表明，来自共同软体动物祖先的基因表达和蛋白质一级结构（通过正选择）的变化至少部分促成了头足类相机眼的获得。
