Background Eyes have evolved and been lost multiple times during animal evolution, however, the process of eye loss has only been reconstructed in a few cases. Mollusks exhibit eyes as varied as the octopod camera eye or the gastropod cup eye and are ideal systems for studying the evolution of eyes, photoreceptors, and opsins. Results Here, we identify genes related to photoreceptor formation and function in an eyeless conchiferan mollusk, the scaphopod Antalis entalis, and investigate their spatial and temporal expression patterns during development. Our study reveals that the scaphopod early mid-stage trochophore larva has putative photoreceptors in a similar location and with a similar gene expression profile as the trochophore of polyplacophoran mollusks. The apical and post-trochal putative photoreceptors appear to co-express go-opsin, six1/2, myoV, and eya, while expression domains in the posterior foot and pavilion (posterior mantle opening) show co-expression of several other candidate genes but not go-opsin. Sequence analysis reveals that the scaphopod Go-opsin amino acid sequence lacks the functionally important lysine (K296; Schiff base) in the retinal-binding domain, but has not accumulated nonsense mutations and still exhibits the canonical G-protein activation domain. Conclusions The scaphopod Go-opsin sequence reported here is the only known example of a bilaterian opsin that lacks lysine K296 in the retinal-binding domain. Although this may render the Go-opsin unable to detect light, the protein may still perform sensory functions. The location, innervation, development, and gene expression profiles of the scaphopod and polyplacophoran apical and post-trochal photoreceptors suggest that they are homologous, even though the scaphopod post-trochal photoreceptors have degenerated. This indicates that post-trochal eyes are not a polyplacophoran apomorphy but likely a molluscan synapomorphy lost in other mollusks. Scaphopod eye degeneration is probably a result of the transition to an infaunal life history and is reflected in the likely functional degeneration of Go-opsin, the loss of photoreceptor shielding pigments, and the scarce expression of genes involved in phototransduction and eye development. Our results emphasize the importance of studying a phylogenetically broad range of taxa to infer the mechanisms and direction of body plan evolution.

中文翻译：

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没有眼睛的软体动物中的祖先幼虫眼睛的残余物？scaphopod Antalis entalis 中光感受器的分子特征。

背景 在动物进化过程中，眼睛已经进化并多次丢失，但是，仅在少数情况下重建了眼睛丢失的过程。软体动物的眼睛与章鱼相机眼或腹足杯眼一样多变，是研究眼睛、感光器和视蛋白进化的理想系统。结果 在这里，我们在无眼的贝壳类软体动物、舟足类 Antalis entalis 中鉴定了与光感受器形成和功能相关的基因，并研究了它们在发育过程中的空间和时间表达模式。我们的研究表明，舟足类早期中期的滋养层幼虫在与多腹纲软体动物的滋养层相似的位置和基因表达谱上具有推定的光感受器。顶端和滑轮后假定的光感受器似乎共同表达 go-opsin，Six1/2、myoV 和 eya，而后足和凉亭（后地幔开口）中的表达域显示其他几个候选基因的共表达，但不显示 go-opsin。序列分析表明，scaphopod Go-opsin 氨基酸序列在视网膜结合结构域中缺乏功能重要的赖氨酸（K296；席夫碱），但没有积累无义突变，仍然表现出典型的 G 蛋白激活结构域。结论 这里报道的舟足类 Go-视蛋白序列是唯一已知的在视网膜结合域中缺乏赖氨酸 K296 的双侧视蛋白的例子。尽管这可能会使 Go-opsin 无法检测到光，但该蛋白质仍可能执行感官功能。位置，支配，发展，scaphopod 和 polyplacophoran 顶端和 trochal 后感光器的基因表达谱表明它们是同源的，即使 scaphopod post-trochal 感光器已经退化。这表明滑轮后眼不是多足纲畸形，而可能是其他软体动物中丢失的软体动物突触。舟足类眼睛退化可能是过渡到动物生命史的结果，并反映在可能的 Go-opsin 功能退化、感光器屏蔽色素的丧失以及参与光转导和眼睛发育的基因表达稀少。我们的结果强调了研究广泛的系统发育分类群以推断身体计划进化的机制和方向的重要性。即使 scaphopod post-trochal 感光器已经退化。这表明滑轮后眼不是多足纲畸形，而可能是其他软体动物中丢失的软体动物突触。舟足类眼睛退化可能是过渡到动物生命史的结果，并反映在可能的 Go-opsin 功能退化、感光器屏蔽色素的丧失以及参与光转导和眼睛发育的基因表达稀少。我们的结果强调了研究广泛的系统发育分类群以推断身体计划进化的机制和方向的重要性。即使 scaphopod post-trochal 感光器已经退化。这表明滑轮后眼不是多足纲畸形，而可能是其他软体动物中丢失的软体动物突触。舟足类眼睛退化可能是过渡到动物生命史的结果，并反映在可能的 Go-opsin 功能退化、感光器屏蔽色素的丧失以及参与光转导和眼睛发育的基因表达稀少。我们的结果强调了研究广泛的系统发育分类群以推断身体计划进化的机制和方向的重要性。舟足类眼睛退化可能是过渡到动物生命史的结果，并反映在可能的 Go-opsin 功能退化、感光器屏蔽色素的丧失以及参与光转导和眼睛发育的基因表达稀少。我们的结果强调了研究广泛的系统发育分类群以推断身体计划进化的机制和方向的重要性。舟足类眼睛退化可能是过渡到动物生命史的结果，并反映在可能的 Go-opsin 功能退化、感光器屏蔽色素的丧失以及参与光转导和眼睛发育的基因表达稀少。我们的结果强调了研究广泛的系统发育分类群以推断身体计划进化的机制和方向的重要性。