Arachnids are important components of cave ecosystems and display many examples of troglomorphisms, such as blindness, depigmentation, and elongate appendages. Little is known about how the eyes of arachnids are specified genetically, let alone the mechanisms for eye reduction and loss in troglomorphic arachnids. Additionally, duplication of Retinal Determination Gene Network (RDGN) homologs in spiders has convoluted functional inferences extrapolated from single-copy homologs in pancrustacean models. We investigated a sister species pair of Israeli cave whip spiders, Charinus ioanniticus and C. israelensis (Arachnopulmonata, Amblypygi), of which one species has reduced eyes. We generated embryonic transcriptomes for both Amblypygi species, and discovered that several RDGN homologs exhibit duplications. We show that duplication of RDGN homologs is systemic across arachnopulmonates (arachnid orders that bear book lungs), rather than being a spider-specific phenomenon. A differential gene expression (DGE) analysis comparing the expression of RDGN genes in field-collected embryos of both species identified candidate RDGN genes involved in the formation and reduction of eyes in whip spiders. To ground bioinformatic inference of expression patterns with functional experiments, we interrogated the function of three candidate RDGN genes identified from DGE using RNAi in the spider Parasteatoda tepidariorum. We provide functional evidence that one of these paralogs, sine oculis/Six1 A (soA), is necessary for the development of all arachnid eye types. Our work establishes a foundation to investigate the genetics of troglomorphic adaptations in cave arachnids, and links differential gene expression to an arthropod eye phenotype for the first time outside of Pancrustacea. Our results support the conservation of at least one RDGN component across Arthropoda and provide a framework for identifying the role of gene duplications in generating arachnid eye diversity.

中文翻译：

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蛛形纲动物视网膜决定网络同源物的系统旁系学和功能

蛛形纲动物是洞穴生态系统的重要组成部分，并展示了许多穴居动物的例子，例如失明、脱色和拉长的附属物。人们对蛛形纲动物的眼睛是如何在遗传上指定的知之甚少，更不用说穴居类蛛形纲动物眼睛缩小和丧失的机制了。此外，蜘蛛中视网膜测定基因网络 (RDGN) 同源物的重复已经使从 pancrustacean 模型中的单拷贝同源物外推的功能推断变得复杂。我们调查了一对以色列洞穴鞭蛛的姊妹物种 Charinus ioanniticus 和 C. israelensis (Arachnopulmonata, Amblypygi)，其中一个物种的眼睛缩小了。我们为这两种 Amblypygi 物种生成了胚胎转录组，并发现几个 RDGN 同源物表现出重复。我们表明，RDGN 同系物的重复在蛛形纲动物（具有书肺的蛛形纲目）中是全身性的，而不是蜘蛛特有的现象。差异基因表达 (DGE) 分析比较了现场收集的两个物种胚胎中 RDGN 基因的表达，确定了参与鞭蛛眼睛形成和减少的候选 RDGN 基因。为了通过功能实验对表达模式进行生物信息学推断，我们在蜘蛛 Parasteatoda tepidariorum 中使用 RNAi 从 DGE 中鉴定了三个候选 RDGN 基因的功能。我们提供的功能证据表明，这些旁系同源物之一，正弦眼/Six1 A (soA)，对于所有蜘蛛眼类型的发育都是必要的。我们的工作为研究洞穴蛛形纲动物的穴居适应遗传学奠定了基础，并首次将差异基因表达与 Pancrustacea 以外的节肢动物眼表型联系起来。我们的结果支持节肢动物中至少一个 RDGN 组件的保守性，并提供了一个框架来确定基因重复在产生蜘蛛眼多样性中的作用。