The stalked jellyfish (Staurozoa) is an extraordinary clade within medusozoan cnidarians in which the medusa is attached to the substrate unlike the pelagic jellyfishes which compose the rest of the medusozoans. Along with this remarkable feature, staurozoans are characterized by an extremely low number of cells (< 100) in the embryos and larvae. The aim of the present study is to explore early development of the staurozoan Lucernaria quadricornis and to elucidate morphogenetic events evolved to overcome the constraints imposed by low cell number. Using bright field, confocal, and electron microscopy, we create a normal table of development of Lucernaria, describe cell number dynamics, and visualize organization of embryos and larvae. From these data, we infer a crosstalk between cell reshaping, cell rearrangement, and mechanical stress, involved in gastrulation, anterior-posterior axis differentiation, and even locomotion of the larva. Our work also demonstrates that staurozoans convergently developed morphogenetic pathways similar to other very distant animals with low cell number in the early development. We consider Lucernaria as an EvoDevo model with potential for further research to answer the question of how evolutionary forces act on developmental trajectories.

中文翻译：

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如何打造不到一百个细胞的幼虫体？对有柄水母（Staurozoa，Cnidaria）早期发育的见解

有柄水母（Staurozoa）是水母刺胞动物中的一个非凡进化枝，与构成其余水母的远洋水母不同，水母附着在基质上。除了这一显着特征外，星形动物的特征还在于胚胎和幼虫中的细胞数量极少（< 100）。本研究的目的是探索 staurozoan Lucernaria quadricornis 的早期发育，并阐明进化的形态发生事件以克服低细胞数量施加的限制。使用明场、共聚焦和电子显微镜，我们创建了一个正常的琉璃苣发育表，描述了细胞数量动态，并可视化了胚胎和幼虫的组织。从这些数据中，我们推断出细胞重塑、细胞重排和机械应力之间的串扰，参与原肠胚形成、前后轴分化，甚至幼虫的运动。我们的工作还表明，在早期发育过程中，星形动物会趋同地发展出类似于其他非常遥远的具有低细胞数量的动物的形态发生途径。我们认为 Lucernaria 是一种 EvoDevo 模型，具有进一步研究的潜力，可以回答进化力如何作用于发育轨迹的问题。