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

在本研究中，我们研究了脊索动物起源和进化的分子和发育机制。我们首先通过分子系统发育分析确定棘皮动物、半角动物和脊索动物具有共同的祖先，形成了后口的单系类群。脊索动物是脊索动物最显著的特征，因为脊索动物的名字起源于这个器官。在海鞘蝌蚪幼虫中，脊索正好由40个细胞组成，在32个细胞阶段，由相邻内胚层细胞发出的信号诱导指定脊索细胞。在诱导后，海鞘短缩基因只在脊索细胞中表达，该基因对脊索的形成起着主控作用。短链蛋白的表达似乎激活了大约40个与脊索形成相关的下游基因。我们通过测定cDNA的完整核苷酸序列来鉴定其中的20个基因。这些基因编码广泛的与脊索形成和功能相关的差异蛋白，但编码转录因子的基因很少。半角类在后口动物中占有关键的系统发育地位，因为它们表现出棘皮类幼虫的形态和脊索状的成体形态。因此，对半口发育基因的表达和功能的分析为后口体计划的进化提供了洞察力。我们对brachyury基因、forkhead/hnf-3基因(负责脊索胚胎内胚层和脊索形成的基因)和Pax1/9基因(负责脊索胚胎咽部内胚层形成的基因)的表达进行了研究。这些基因的表达模式提示了脊索体计划的进化。

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.