In cephalochordates (amphioxus), the notochord runs along the dorsal to the anterior tip of the body. In contrast, the vertebrate head is formed anterior to the notochord, as a result of head organizer formation in anterior mesoderm during early development. A key gene for the vertebrate head organizer, goosecoid (gsc), is broadly expressed in the dorsal mesoderm of amphioxus gastrula. Amphioxus gsc expression subsequently becomes restricted to the posterior notochord from the early neurula. This has prompted the hypothesis that a change in expression patterns of gsc led to development of the vertebrate head during chordate evolution. However, molecular mechanisms of head organizer evolution involving gsc have never been elucidated. To address this question, we compared cis-regulatory modules of vertebrate organizer genes between amphioxus, Branchiostoma japonicum, and frogs, Xenopus laevis and Xenopus tropicalis. Here we show conservation and diversification of gene regulatory mechanisms through cis-regulatory modules for gsc, lim1/lhx1, and chordin in Branchiostoma and Xenopus. Reporter analysis using Xenopus embryos demonstrates that activation of gsc by Nodal/FoxH1 signal through the 5′ upstream region, that of lim1 by Nodal/FoxH1 signal through the first intron, and that of chordin by Lim1 through the second intron, are conserved between amphioxus and Xenopus. However, activation of gsc by Lim1 and Otx through the 5′ upstream region in Xenopus are not conserved in amphioxus. Furthermore, the 5′ region of amphioxus gsc recapitulated the amphioxus-like posterior mesoderm expression of the reporter gene in transgenic Xenopus embryos. On the basis of this study, we propose a model, in which the gsc gene acquired the cis-regulatory module bound with Lim1 and Otx at its 5′ upstream region to be activated persistently in anterior mesoderm, in the vertebrate lineage. Because Gsc globally represses trunk (notochord) genes in the vertebrate head organizer, this cooption of gsc in vertebrates appears to have resulted in inhibition of trunk genes and acquisition of the head organizer and its derivative prechordal plate.

中文翻译：

---

脊索动物头部组织基因goosecoid的顺式调控模块的进化：Branchiostoma和Xenopus之间的比较

在头脊动物（文昌鱼）中，脊索沿着背侧延伸到身体的前尖端。相比之下，脊椎动物的头部在脊索之前形成，这是在早期发育过程中前中胚层中头部组织形成的结果。脊椎动物头部组织者的关键基因 goosecoid (gsc) 在文昌鱼原肠胚背中胚层中广泛表达。文昌鱼 gsc 的表达随后从早期神经元开始局限于后脊索。这提示了这样一种假设，即 gsc 表达模式的变化导致脊索动物进化过程中脊椎动物头部的发育。然而，涉及 gsc 的头部组织者进化的分子机制从未被阐明。为了解决这个问题，我们比较了文昌鱼之间脊椎动物组织基因的顺式调控模块，Branchiostoma japonicum 和青蛙、非洲爪蟾和热带爪蟾。在这里，我们通过顺式调控模块展示了基因调控机制的保护和多样化，这些模块用于鳃孔和爪蟾中的 gsc、lim1/lhx1 和 chordin。使用非洲爪蟾胚胎的报告分析表明，Nodal/FoxH1 信号通过 5' 上游区域激活 gsc，Nodal/FoxH1 信号通过第一个内含子激活 lim1，Lim1 通过第二个内含子激活 chordin，在文昌鱼之间是保守的和爪蟾。然而，Lim1 和 Otx 通过 非洲爪蟾 的 5' 上游区域激活 gsc 在文昌鱼中是不保守的。此外，文昌鱼 gsc 的 5' 区域概括了转基因非洲爪蟾胚胎中报告基因的文昌鱼样后中胚层表达。在这项研究的基础上，我们提出了一个模型，其中 gsc 基因在其 5' 上游区域获得了与 Lim1 和 Otx 结合的顺式调节模块，以便在脊椎动物谱系的前中胚层中持续激活。由于 Gsc 全局抑制脊椎动物头部组织者中的躯干（脊索）基因，因此 gsc 在脊椎动物中的这种共同选择似乎导致了躯干基因的抑制和头部组织者及其衍生的前弦板的获得。