Mazet Françoise, Shimeld Sebastian M
School of Animal and Microbial Sciences, The University of Reading, P.O. Box 228, Whiteknights, Reading, RG6 6AJ, United Kingdom.
Dev Biol. 2002 Nov 15;251(2):258-70. doi: 10.1006/dbio.2002.0831.
Amphioxus is the closest relative to vertebrates but lacks key vertebrate characters, like rhombomeres, neural crest cells, and the cartilaginous endoskeleton. This reflects major differences in the developmental patterning of neural and mesodermal structures between basal chordates and vertebrates. Here, we analyse the expression pattern of an amphioxus FoxB ortholog and an amphioxus single-minded ortholog to gain insight into the evolution of vertebrate neural segmentation. AmphiFoxB expression shows cryptic segmentation of the cerebral vesicle and hindbrain, suggesting that neuromeric segmentation of the chordate neural tube arose before the origin of the vertebrates. In the forebrain, AmphiFoxB expression combined with AmphiSim and other amphioxus gene expression patterns shows that the cerebral vesicle is divided into several distinct domains: we propose homology between these domains and the subdivided diencephalon and midbrain of vertebrates. In the Hox-expressing region of the amphioxus neural tube that is homologous to the vertebrate hindbrain, AmphiFoxB shows the presence of repeated blocks of cells along the anterior-posterior axis, each aligned with a somite. This and other data lead us to propose a model for the evolution of vertebrate rhombomeric segmentation, in which rhombomere evolution involved the transfer of mechanisms regulating neural segmentation from vertical induction by underlying segmented mesoderm to horizontal induction by graded retinoic acid signalling. A consequence of this would have been that segmentation of vertebrate head mesoderm would no longer have been required, paving the way for the evolution of the unsegmented head mesoderm seen in living vertebrates.
文昌鱼是与脊椎动物亲缘关系最近的物种，但缺乏关键的脊椎动物特征，如菱脑节、神经嵴细胞和软骨内骨骼。这反映了基础脊索动物和脊椎动物在神经和中胚层结构发育模式上的主要差异。在这里，我们分析了文昌鱼FoxB直系同源基因和文昌鱼单 minded 直系同源基因的表达模式，以深入了解脊椎动物神经节段化的进化。AmphiFoxB 的表达显示出脑泡和后脑的隐性节段化，这表明脊索动物神经管的神经节段化起源于脊椎动物出现之前。在前脑，AmphiFoxB 的表达与 AmphiSim 以及其他文昌鱼基因表达模式相结合表明，脑泡被分为几个不同的区域：我们提出这些区域与脊椎动物细分的间脑和中脑具有同源性。在与脊椎动物后脑同源的文昌鱼神经管的Hox表达区域，AmphiFoxB显示沿前后轴存在重复的细胞块，每个细胞块与一个体节对齐。这些以及其他数据使我们提出了一个脊椎动物菱脑节段化进化的模型，其中菱脑节的进化涉及调节神经节段化的机制从由下方分节的中胚层进行垂直诱导转变为由梯度视黄酸信号进行水平诱导。这样做的一个结果是不再需要脊椎动物头部中胚层的节段化，为现存脊椎动物中未分节的头部中胚层的进化铺平了道路。
