Annelids are a diverse group of segmented worms within Spiralia, whose embryos exhibit spiral cleavage and a variety of larval forms. While most modern embryological studies focus on species with unequal spiral cleavage nested in Pleistoannelida (Sedentaria + Errantia), a few recent studies looked into Owenia fusiformis, a member of the sister group to all remaining annelids and thus a key lineage to understand annelid and spiralian evolution and development. However, the timing of early cleavage and detailed morphogenetic events leading to the formation of the idiosyncratic mitraria larva of O. fusiformis remain largely unexplored. Owenia fusiformis undergoes equal spiral cleavage where the first quartet of animal micromeres are slightly larger than the vegetal macromeres. Cleavage results in a coeloblastula approximately 5 h post-fertilization (hpf) at 19 °C. Gastrulation occurs via invagination and completes 4 h later, with putative mesodermal precursors and the chaetoblasts appearing 10 hpf at the dorso-posterior side. Soon after, at 11 hpf, the apical tuft emerges, followed by the first neurons (as revealed by the expression of elav1 and synaptotagmin-1) in the apical organ and the prototroch by 13 hpf. Muscles connecting the chaetal sac to various larval tissues develop around 18 hpf and by the time the mitraria is fully formed at 22 hpf, there are FMRFamide+ neurons in the apical organ and prototroch, the latter forming a prototrochal ring. As the mitraria feeds, it grows in size and the prototroch expands through active proliferation. The larva becomes competent after ~ 3 weeks post-fertilization at 15 °C, when a conspicuous juvenile rudiment has formed ventrally. Owenia fusiformis embryogenesis is similar to that of other equal spiral cleaving annelids, supporting that equal cleavage is associated with the formation of a coeloblastula, gastrulation via invagination, and a feeding trochophore-like larva in Annelida. The nervous system of the mitraria larva forms earlier and is more elaborated than previously recognized and develops from anterior to posterior, which is likely an ancestral condition to Annelida. Altogether, our study identifies the major developmental events during O. fusiformis ontogeny, defining a conceptual framework for future investigations.

中文翻译：

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环节动物 Owenia fusiformis 的早期胚胎发生和器官发生

环节动物是螺旋纲内不同节段的蠕虫，其胚胎表现出螺旋卵裂和多种幼虫形态。虽然大多数现代胚胎学研究都集中在 Pleistoannelida（Sedentaria + Errantia）中具有不等螺旋卵裂的物种，但最近的一些研究着眼于 Owenia fusiformis，它是所有剩余环节动物的姐妹群成员，因此是了解环节动物和螺旋动物的关键谱系演变和发展。然而，导致O. fusiformis 特异质的mitraria 幼虫形成的早期卵裂时间和详细的形态发生事件在很大程度上仍未被探索。Owenia fusiformis 经历相等的螺旋分裂，其中动物微团的第一四重体略大于植物大团。19°C 受精后 (hpf) 约 5 小时，卵裂形成腔母细胞。原肠胚形成通过内陷发生，并在 4 小时后完成，假定的中胚层前体和毛毛细胞在 10 hpf 时出现在背侧后侧。不久之后，在 11 hpf 时，顶端簇出现，然后到 13 hpf 时，顶端器官和原支座中出现第一个神经元（如 elav1 和 synaptotagmin-1 的表达所揭示）。连接毛管囊和各种幼虫组织的肌肉在 18 hpf 左右发育，当 22 hpf 完全形成时，顶端器官和原转子中存在 FMRFamide+ 神经元，后者形成原转子环。当米特拉利亚进食时，它的尺寸会变大，原轮也会通过活跃的增殖而扩张。幼虫在 15°C 受精后约 3 周后具有能力，此时腹侧已形成明显的幼虫雏形。Owenia fusiformis 胚胎发生与其他等螺旋分裂环节动物相似，支持等分裂与环节动物中腔母细胞的形成、内陷原肠胚形成以及摄食的支轮样幼虫有关。米特拉幼虫的神经系统比以前认识的更早形成，也更复杂，并且从前部到后部发育，这可能是环节动物的祖先条件。总而言之，我们的研究确定了 O. fusiformis 个体发育过程中的主要发育事件，为未来的研究定义了概念框架。