The comparative embryology of Chelicerata has greatly advanced in recent years with the integration of classical studies and genetics, prominently spearheaded by developmental genetic works in spiders. Nonetheless, the understanding of the evolution of development and polarization of embryological characters in Chelicerata is presently limited, as few non-spider species have been well studied. A promising focal species for chelicerate evo-devo is the daddy-long-legs (harvestman) Phalangium opilio, a member of the order Opiliones. Phalangium opilio, breeds prolifically and is easily accessible in many parts of the world, as well as tractable in a laboratory setting. Resources for this species include developmental transcriptomes, a draft genome, and protocols for RNA interference, but a modern staging system is critically missing for this emerging model system. We present a staging system of P. opilio embryogenesis that spans the most important morphogenetic events with respect to segment formation, appendage elongation and head development. Using time-lapse imaging, confocal microscopy, colorimetric in situ hybridization, and immunohistochemistry, we tracked the development of synchronous clutches from egg laying to adulthood. We describe key events in segmentation, myogenesis, neurogenesis, and germ cell formation. Considering the phylogenetic position of Opiliones and the unduplicated condition of its genome (in contrast to groups like spiders and scorpions), this species is poised to serve as a linchpin for comparative studies in arthropod development and genome evolution. The staging system presented herein provides a valuable reference for P. opilio that we anticipate being useful to the arthropod evo-devo community, with the goal of revitalizing research in the comparative development of non-spider arachnids.

中文翻译：

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从卵到长腿：一种新兴模式蜘蛛——收割者奥皮利奥 (Opiliones) 的胚胎阶段


近年来，随着经典研究和遗传学的融合，螯肢动物的比较胚胎学取得了巨大进展，其中以蜘蛛发育遗传学研究为先导。尽管如此，目前对螯肢动物胚胎学特征的发育进化和极化的了解还很有限，因为很少有非蜘蛛物种得到了充分的研究。螯肢进化-发育的一个有前途的焦点物种是长腿爸爸（收割者）Phalangium opilio，它是 Opiliones 目的成员。 Phalangium opilio 繁殖力强，在世界许多地方都很容易获得，而且在实验室环境中也很容易处理。该物种的资源包括发育转录组、基因组草案和 RNA 干扰方案，但这一新兴模型系统严重缺乏现代分期系统。我们提出了 P. opilio 胚胎发生的分期系统，涵盖了节段形成、附属物伸长和头部发育等最重要的形态发生事件。使用延时成像、共焦显微镜、比色原位杂交和免疫组织化学，我们追踪了同步离合器从产卵到成年的发育过程。我们描述了分割、肌生成、神经发生和生殖细胞形成中的关键事件。考虑到 Opiliones 的系统发育位置及其基因组的不重复状态（与蜘蛛和蝎子等群体相比），该物种有望成为节肢动物发育和基因组进化比较研究的关键。本文提出的分期系统为 P 提供了有价值的参考。 opilio 表示，我们预计对节肢动物进化-发育群体有用，目标是振兴非蜘蛛蜘蛛类动物的比较发育研究。