Onai Takayuki
Department of Anatomy, School of Medical Sciences, University of Fukui, 23-3, Matsuokashimoaizuki, Eiheiji-cho, Yoshida-gun, Fukui, 910-1193, Japan.
Life Science Innovation Center, University of Fukui, 23-3, Matsuokashimoaizuki, Eiheiji-cho, Yoshida-gun, Fukui, 910-1193, Japan.
Theory Biosci. 2018 Apr;137(1):1-16. doi: 10.1007/s12064-018-0260-y. Epub 2018 Feb 27.
One of the definitive characteristics of chordates (cephalochordates, vertebrates) is the somites, which are a series of paraxial mesodermal blocks exhibiting segmentation. The presence of somites in the basal chordate amphioxus and in vertebrates, but not in tunicates (the sister group of vertebrates), suggests that the tunicates lost the somites secondarily. Somites are patterned from anterior to posterior during embryogenesis. How such a segmental pattern evolved from deuterostome ancestors is mysterious. The classic enterocoel theory claims that chordate mesoderm evolved from the ancestral deuterostome mesoderm that organizes the trimeric body parts seen in extant hemichordates. Recent progress in molecular embryology has been tremendous, which has enabled us to test this classic theory. In this review, the history of the study on the evolution of the chordate mesoderm is summarized. This is followed by a review of the current understanding of genetic mapping on anterior/posterior (A/P) mesodermal patterning between chordates (cephalochordates, vertebrates) and a direct developing hemichordate (Saccoglossus kowalevskii). Finally, a possible scenario about the evolution of the chordate mesoderm from deuterostome ancestors is discussed.
脊索动物（头索动物、脊椎动物）的一个决定性特征是体节，体节是一系列表现出分节现象的轴旁中胚层块。在基部脊索动物文昌鱼和脊椎动物中存在体节，但在被囊动物（脊椎动物的姐妹类群）中不存在，这表明被囊动物是在进化过程中次生地失去了体节。在胚胎发育过程中，体节是从前向后形成模式的。这种分节模式是如何从后口动物祖先演化而来的，仍然是个谜。经典的肠体腔理论认为，脊索动物的中胚层是从原始后口动物的中胚层进化而来的，原始后口动物的中胚层构成了现存半索动物中可见的三体节身体部分。分子胚胎学最近取得了巨大进展，这使我们能够检验这一经典理论。在这篇综述中，总结了脊索动物中胚层进化研究的历史。随后，回顾了目前对脊索动物（头索动物、脊椎动物）和直接发育的半索动物（柯氏囊舌虫）之间前后（A/P）中胚层模式的遗传图谱的理解。最后，讨论了从后口动物祖先演化出脊索动物中胚层的一种可能情况。
