The evolution of vertebrate smooth muscles is obscured by lack of identifiable smooth muscle-like cells in tunicates, the invertebrates most closely related to vertebrates. A recent evolutionary model was proposed in which smooth muscles arose before the last bilaterian common ancestor, and were later diversified, secondarily lost or modified in the branches leading to extant animal taxa. However, there is currently no data from tunicates to support this scenario. Here, we show that the axial columnar cells, a unique cell type in the adhesive larval papillae of the tunicate Ciona, are enriched for orthologs of vertebrate smooth/non-muscle-specific effectors of contractility, in addition to developing from progenitors that express conserved cardiomyocyte regulatory factors. We show that these cells contract during the retraction of the Ciona papillae during larval settlement and metamorphosis. We propose that the axial columnar cells of Ciona are a myoepithelial cell type required for transducing external stimuli into mechanical forces that aid in the attachment of the motile larva to its final substrate. Furthermore, they share developmental and functional features with vertebrate myoepithelial cells, vascular smooth muscle cells, and cardiomyocytes. We discuss these findings in the context of the proposed models of vertebrate smooth muscle and cardiomyocyte evolution.

中文翻译：

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海鞘收缩乳头中平滑肌样效应器和核心心肌细胞调节因子的表达。


脊椎动物平滑肌的进化因被囊动物（与脊椎动物关系最密切的无脊椎动物）中缺乏可识别的平滑肌样细胞而被掩盖。最近提出了一种进化模型，其中平滑肌在最后的两侧对称动物共同祖先之前出现，后来多样化，在导致现存动物分类群的分支中继发性丢失或修改。然而，目前没有来自被囊类动物的数据支持这种情况。在这里，我们表明，轴向柱状细胞是被囊动物粘着幼虫乳头中的一种独特细胞类型，除了从表达保守的祖细胞发育而来外，还富含脊椎动物平滑/非肌肉特异性收缩效应子的直系同源物。心肌细胞调节因子。我们发现这些细胞在幼虫定居和变态过程中玻璃海鞘乳头回缩时收缩。我们认为海鞘的轴向柱状细胞是一种肌上皮细胞类型，将外部刺激转化为机械力，帮助活动幼虫附着到其最终基质上。此外，它们与脊椎动物肌上皮细胞、血管平滑肌细胞和心肌细胞具有共同的发育和功能特征。我们在所提出的脊椎动物平滑肌和心肌细胞进化模型的背景下讨论这些发现。