Holland N D, Panganiban G, Henyey E L, Holland L Z
Marine Biology Research Division, Scripps Institute of Oceanography, La Jolla, CA 92093-0202, USA.
Development. 1996 Sep;122(9):2911-20. doi: 10.1242/dev.122.9.2911.
The dynamic expression patterns of the single amphioxus Distal-less homolog (AmphiDll) during development are consistent with successive roles of this gene in global regionalization of the ectoderm, establishment of the dorsoventral axis, specification of migratory epidermal cells early in neurulation and the specification of forebrain. Such a multiplicity of Distal-less functions probably represents an ancestral chordate condition and, during craniate evolution, when this gene diversified into a family of six or so members, the original functions evidently tended to be parcelled out among the descendant genes. In the amphioxus gastrula, AmphiDll is expressed throughout the animal hemisphere (presumptive ectoderm), but is soon downregulated dorsally (in the presumptive neural plate). During early neurulation, AmphiDll-expressing epidermal cells flanking the neural plate extend lamellipodia, appear to migrate over it and meet mid-dorsally. Midway in neurulation, cells near the anterior end of the neural plate begin expressing AmphiDll and, as neurulation terminates, these cells are incorporated into the dorsal part of the neural tube, which forms by a curling of the neural plate. This group of AmphiDll-expressing neural cells and a second group expressing the gene a little later and even more anteriorly in the neural tube demarcate a region that comprises the anterior three/fourths of the cerebral vesicle; this region of the amphioxus neural tube, as judged by neural expression domains of craniate Distal-less-related genes, is evidently homologous to the craniate forebrain. Our results suggest that craniates evolved from an amphioxus-like creature that had the beginnings of a forebrain and possibly a precursor of neural crest - namely, the cell population leading the epidermal overgrowth of the neural plate during early neurulation.
文昌鱼单个远端缺失同源基因（AmphiDll）在发育过程中的动态表达模式，与该基因在外胚层整体区域化、背腹轴建立、神经胚形成早期迁移表皮细胞的特化以及前脑特化中的连续作用相一致。远端缺失功能的这种多样性可能代表了一种原始的脊索动物状态，并且在有头类动物进化过程中，当该基因多样化成一个约有六个成员的家族时，其原始功能显然倾向于在后代基因中进行分配。在文昌鱼原肠胚中，AmphiDll在整个动物半球（假定的外胚层）表达，但很快在背侧（在假定的神经板中）下调。在神经胚形成早期，神经板两侧表达AmphiDll的表皮细胞伸出片状伪足，似乎在其上迁移并在背中部会合。在神经胚形成中期，神经板前端附近的细胞开始表达AmphiDll，并且随着神经胚形成的终止，这些细胞被并入神经管的背侧部分，神经管通过神经板的卷曲形成。这组表达AmphiDll的神经细胞以及稍后在神经管中更靠前表达该基因的第二组细胞划定了一个区域，该区域包括脑泡的前三分之四；根据有头类动物远端缺失相关基因的神经表达域判断，文昌鱼神经管的这个区域显然与有头类动物的前脑同源。我们的结果表明，有头类动物是从一种类似文昌鱼的生物进化而来的，这种生物已经有了前脑的雏形，可能还有神经嵴的前体——即神经胚形成早期引导神经板表皮过度生长的细胞群。
