Background Spiral cleavage is a conserved, early developmental mode found in several phyla of Lophotrochozoans resulting in highly diverse adult body plans. While the cleavage pattern has clearly been broadly conserved, it has also undergone many modifications in various taxa. The precise mechanisms of how different adaptations have altered the ancestral spiral cleavage pattern are an important ongoing evolutionary question, and adequately answering this question requires obtaining a broad developmental knowledge of different spirally cleaving taxa. In flatworms (Platyhelminthes), the spiral cleavage program has been lost or severely modified in most taxa. Polyclad flatworms, however, have retained the pattern up to the 32-cell stage. Here we study early embryogenesis of the cotylean polyclad flatworm Maritigrella crozieri to investigate how closely this species follows the canonical spiral cleavage pattern and to discover any potential deviations from it. Results Using live imaging recordings and 3D reconstructions of embryos, we give a detailed picture of the events that occur during spiral cleavage in M. crozieri. We suggest, contrary to previous observations, that the four-cell stage is a product of unequal cleavages. We show that that the formation of third and fourth micromere quartets is accompanied by strong blebbing events; blebbing also accompanies the formation of micromere 4d. We find an important deviation from the canonical pattern of cleavages with clear evidence that micromere 4d follows an atypical cleavage pattern, so far exclusively found in polyclad flatworms. Conclusions Our findings highlight that early development in M. crozieri deviates in several important aspects from the canonical spiral cleavage pattern. We suggest that some of our observations extend to polyclad flatworms in general as they have been described in both suborders of the Polycladida, the Cotylea and Acotylea.

中文翻译：

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重新研究扁虫 Maritigrella crozieri 的早期胚胎发生突出了在多壳扁虫中发现的独特的螺旋分裂程序。

背景螺旋劈裂是一种保守的早期发育模式，在 Lophotrochozoans 的几个门中发现，导致高度多样化的成人身体计划。虽然切割模式显然已被广泛保存，但它在各种分类群中也经历了许多修改。不同适应如何改变祖先螺旋分裂模式的确切机制是一个重要的持续进化问题，充分回答这个问题需要获得不同螺旋分裂类群的广泛发育知识。在扁虫（Platyhelminthes）中，螺旋分裂程序在大多数分类群中已经丢失或严重修改。然而，Polyclad 扁虫将这种模式保留到 32 细胞阶段。在这里，我们研究了子叶多叶扁虫 Maritigrella crozieri 的早期胚胎发生，以研究该物种遵循规范螺旋劈裂模式的程度，并发现与它的任何潜在偏差。结果使用实时成像记录和胚胎的 3D 重建，我们详细描述了 M. crozieri 螺旋分裂过程中发生的事件。我们认为，与之前的观察相反，四细胞阶段是不等分裂的产物。我们表明，第三和第四个微孔四重奏的形成伴随着强烈的起泡事件；起泡也伴随着微孔 4d 的形成。我们发现与典型的劈裂模式存在重要偏差，有明确的证据表明微粒 4d 遵循非典型的劈裂模式，迄今为止仅在多囊扁虫中发现。结论 我们的研究结果强调了 M. crozieri 的早期发育在几个重要方面偏离了典型的螺旋分裂模式。我们建议将我们的一些观察结果扩展到一般的多枝扁虫，因为它们已在多枝亚目、子叶亚目和子叶亚目中进行了描述。