In the basal chordate amphioxus (Branchiostoma), somites extend the full length of the body. The anteriormost somites segment during the gastrula and neurula stages from dorsolateral grooves of the archenteron. The remaining ones pinch off, one at a time, from the tail bud. These posterior somites appear to be homologous to those of vertebrates, even though the latter pinch off from the anterior end of bands of presomitic mesoderm rather than directly from the tail bud. To gain insights into the evolution of mesodermal segmentation in chordates, we determined the expression of ten genes in nascent amphioxus somites. Five (Uncx4.1, NeuroD/atonal-related, IrxA, Pcdhdelta2-17/18, and Hey1) are expressed in stripes in the dorsolateral mesoderm at the gastrula stage and in the tail bud while three (Paraxis, Lcx, and Axin) are expressed in the posterior mesendoderm at the gastrula and neurula stages and in the tail bud at later stages. Expression of two genes (Pbx and OligA) suggests roles in the anterior somites that may be unrelated to initial segmentation. Together with previous data, our results indicate that, with the exception that Engrailed is only segmentally expressed in the anterior somites, the genetic mechanisms controlling formation of both the anterior and posterior somites are probably largely identical. Thus, the fundamental pathways for mesodermal segmentation involving Notch-Delta, Wnt/beta-catenin, and Fgf signaling were already in place in the common ancestor of amphioxus and vertebrates although budding of somites from bands of presomitic mesoderm exhibiting waves of expression of Notch, Wnt, and Fgf target genes was likely a vertebrate novelty. Given the conservation of segmentation gene expression between amphioxus and vertebrate somites, we propose that the clock mechanism may have been established in the basal chordate, while the wavefront evolved later in the vertebrate lineage.

中文翻译：

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Somphite分割基因在文昌鱼中的表达：没有波前的时钟？

在基底的绒毛膜双歧杆菌（Branchiostoma）中，体节延伸到整个身体。在腹股沟和神经节阶段，最前的节段从原肠的背外侧沟开始。其余的一次从尾芽上pin下来。这些后牙节似乎与脊椎动物的后牙节同源，即使后者从早熟中胚层带的前端而不是直接从尾芽上捏下来也是如此。为了深入了解中胚层中的中胚层分割的进化过程，我们确定了新生的两栖类松散体中十个基因的表达。五个（Uncx4.1，NeuroD / atonal相关，IrxA，Pcdhdelta2-17 / 18和Hey1）在下腹部中胚层和尾芽的条纹中表达，而三个（Paraxis，Lcx，（和Axin）在胃胚和神经末梢的后中胚层以及后期的尾芽中表达。两个基因（Pbx和OligA）的表达表明在前牙节中的作用可能与初始分割无关。与以前的数据一起，我们的结果表明，除了Engrailed仅在节段前部分表达外，控制前节段和后节段形成的遗传机制可能基本相同。因此，在两栖动物和脊椎动物的共同祖先中，涉及Notch-Delta，Wnt /β-catenin和Fgf信号传导的中胚层分割的基本途径已经存在，尽管早熟中胚层带中的卵节出芽并表现出Notch表达波， Wnt，Fgf靶基因可能是脊椎动物的新奇事物。考虑到在两栖类和脊椎动物体节之间的节段基因表达的保守性，我们建议时钟机制可能已经建立在基脉中，而波前则在脊椎动物谱系中发展。