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

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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.