Developmental Mechanisms Involved in Animal Diversity : Molecular and Developmental Biological Studies on the Origin and Evolution of Chordates

在本研究中，我们已经调查的分子和发育机制的起源和进化的脊索动物。我们首先通过分子系统发育分析确定棘皮动物、半索动物和脊索动物有着共同的祖先，构成后口动物的单系类群，脊索动物最显著的特征是脊索，脊索动物的名称来源于脊索。在海鞘蝌蚪幼虫，脊索是由正好40个细胞，和规格的脊索细胞诱导在32细胞阶段由相邻的内胚层细胞发出的信号。在诱导后，海鞘Brachyury基因仅在脊索细胞中表达，并且该基因对脊索形成发挥主控制作用。Brachyury的表达似乎激活了大约40个与脊索形成相关的下游基因。我们通过确定cDNA的完整核苷酸序列来表征其中的20个。这些基因编码与脊索的形成和功能相关的广泛的不同蛋白质，但很少有基因编码转录factors.Hemichordates占据了一个关键的系统发育中的位置后口动物，因为他们表现出棘皮动物样的幼虫形态和脊索动物样的成年形态。因此，半索动物发育基因的表达和功能的分析提供了深入了解的进化后口动物的身体计划。我们的特点的表达Brachyury基因，forkhead/HNF-3基因（负责内胚层和脊索形成的基因在脊索动物胚胎）和Pax 1/9基因（负责咽内胚层形成的基因在脊索动物胚胎）。这些基因的表达模式暗示了脊索动物体平面的进化。

In the present research, we have investigated molecular and developmental mechanisms underlying the origin and evolution of chordates. We first determined by molecular phylogenetic analyses that echinoderms, hemichordates and chordates share a common ancestor and form the monophyletic group of deuterostomes.Notochord is the most prominent feature of chordates, as the name of chordates originated from this organ. In ascidian tadpole larvae, notochord is composed of exactly 40 cells, and specification of the notochord cells is induced during the 32-cell stage by signals emanating from adjacent endoderm cells. Immediately after the induction, an ascidian Brachyury gene is expressed solely in the notochord cells, and the gene exerts a master control over the notochord formation. The Brachyury expression seems to activate about 40 downstream genes that are associated with the notochord formation. We characterized twenty of them by determining the complete nucleotide sequences of the cDNAs. These genes encode a broad spectrum of divergent proteins associated with notochord formation and function, but very few genes encoding transcription factors.Hemichordates occupy a critical phylogenetic position among deuterostomes since they exhibit echinoderm-like larval morphology and chordate-like adult morphology. Analyses of the expression and function of hemichordate developmental genes therefore provide insight into the evolution of deuterostome body plan. We characterized the expression of Brachyury gene, forkhead/HNF-3 gene (the gene responsible for endoderm and notochord formation in chordate embryos) and Pax 1/9 gene (the gene responsible for pharyngeal endoderm formation in chordate embryos). The expression patterns of these genes are suggestive for the evolution of chordate body plan.