BMP signaling is involved in many aspects of metazoan development, with two of the most conserved functions being to pattern the dorsal-ventral axis and to specify neural versus epidermal fates. An active area of research within developmental biology asks how BMP signaling was modified over evolution to build disparate body plans. Animals belonging to the superclade Spiralia/Lophotrochozoa are excellent experimental subjects for studying the evolution of BMP signaling because a highly conserved, stereotyped early cleavage program precedes the emergence of distinct body plans. In this study we examine the role of BMP signaling in one representative, the slipper snail Crepidula fornicata. We find that mRNAs encoding BMP pathway components (including the BMP ligand decapentaplegic, and BMP antagonists chordin and noggin-like proteins) are not asymmetrically localized along the dorsal-ventral axis in the early embryo, as they are in other species. Furthermore, when BMP signaling is perturbed by adding ectopic recombinant BMP4 protein, or by treating embryos with the selective Activin receptor-like kinase-2 (ALK-2) inhibitor Dorsomorphin Homolog 1 (DMH1), we observe no obvious effects on dorsal-ventral patterning within the posterior (post-trochal) region of the embryo. Instead, we see effects on head development and the balance between neural and epidermal fates specifically within the anterior, pre-trochal tissue derived from the 1q1 lineage. Our experiments define a window of BMP signaling sensitivity that ends at approximately 44-48 h post fertilization, which occurs well after organizer activity has ended and the dorsal-ventral axis has been determined. When embryos were exposed to BMP4 protein during this window, we observed morphogenetic defects leading to the separation of the anterior, 1q lineage from the rest of the embryo. The 1q-derived organoid remained largely undifferentiated and was radialized, while the post-trochal portion of the embryo developed relatively normally and exhibited clear signs of dorsal-ventral patterning. When embryos were exposed to DMH1 during the same time interval, we observed defects in the head, including protrusion of the apical plate, enlarged cerebral ganglia and ectopic ocelli, but otherwise the larvae appeared normal. No defects in shell development were noted following DMH1 treatments. The varied roles of BMP signaling in the development of some other spiralians have recently been examined. We discuss our results in this context, and highlight the diversity of developmental mechanisms within spiral-cleaving animals.

中文翻译：

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BMP信号在腹足纲Crepidula fornicata中在前神经/头部发育中起作用，但在组织者活动中不起作用。

BMP信号传导涉及后生动物发育的许多方面，其中最保守的两个功能是对背腹轴进行模式设置，并指明神经命运与表皮命运。发育生物学领域的一个活跃研究领域询问如何在进化过程中修改BMP信号以建立不同的身体计划。属于超螺旋藻/风毛虫的动物是研究BMP信号转导的优秀实验对象，因为高度保守的定型早期卵裂程序先于独特的身体计划出现。在这项研究中，我们研究了BMP信号传导在蜗牛蜗牛Crepidula fornicata中的作用。我们发现，编码BMP途径成分的mRNA（包括BMP配体去Capcappleplegic，和BMP拮抗剂chordin和noggin样蛋白）在早期胚胎中并不像在其他物种中那样沿背腹轴不对称地定位。此外，当通过添加异位重组BMP4蛋白或通过使用选择性激活素受体样激酶2（ALK-2）抑制剂Dorsomorphin Homolog 1（DMH1）处理胚胎而干扰BMP信号传导时，我们观察到对背腹没有明显影响在胚胎的后部（行后）区域内形成图案。相反，我们看到了对头部发育的影响以及神经和表皮命运之间的平衡，特别是在源自1q1世系的前，前气管前组织内。我们的实验确定了BMP信号传导敏感性的窗口，该窗口在受精后大约44-48小时结束，发生在组织者活动结束并确定了背腹轴之后。当在此窗口内胚胎暴露于BMP4蛋白时，我们观察到形态发生缺陷，导致前1q谱系与其余胚胎分离。1q派生的类器官在很大程度上保持未分化状态，并呈放射状，而胚的后转子部分发育相对正常，并显示出背腹模式的明显迹象。当胚胎在同一时间间隔内暴露于DMH1时，我们观察到头部缺陷，包括顶板突出，脑神经节增大和异位卵泡，但幼虫看起来正常。DMH1处理后未发现壳发育缺陷。最近已经研究了BMP信号在其他螺旋虫发育中的各种作用。我们在这种情况下讨论我们的结果，并强调螺旋切割动物内的发展机​​制的多样性。