Origin and evolution of echinoderm pentamery, a combined paleontological and molecular biological approach

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Echinoderms constitute one of the invertebrate animal phyla that originated in the Cambrian and survived to the Recent seas. The phylum is characterized by the pentamerous symmetry in the adult body plan. The purpose of this project has been to clarify when and how this character evolved, as well as its functional significance, from the viewpoints of paleontology, embryology and molecular biology.1. Fossil records ; Because the late pre-Cambrian Tribrachidium has trimerous symmetry, some researchers consider that trimery presedes pentamery in echinoderms. However, it is dubious that Tribrachidium is an echinoderm. It is known that Cambrian echinoderms already had pentamerous symmetry. The pentamerous pattern of Cambrian echinoderms is in fact the '2-1-2 pattern', in which two of the three branches branches into two to make five branches. This pattern is wide spread in later echinoderms, therefore, it is reasonable to consider that the true pentamerous pattern evolved from the 2-1-2 pattern.2. Embryology ; In the development of echinoderms, pentamerous symmetry first appear when the water vascular system is formed and where the water vascular system is segmented into five projections. Detailed observation of the development of water vascular systems is therefore important to understand the mechanisms of the formation of pentamerous symmetry.3. Molecular biology ; It is not yet known what kind of genes control the formation of pentamerous body plan. From the known significance of homeogenes in body plan formation, it is not too much a speculation to consider that homeogenes are involved in the making of pentamery in echinoderms. In this study, DNA primers have been designed to PCR amplify a region of homeobox of some echinoids. As a result, some target DNA fragments have successfully amplified. The relationship between the expression of homeogenes and the formation of pentamery is now being investigated.