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

棘皮动物是起源于寒武纪并存活到近代海洋的无脊椎动物门类之一。该门的特点是在成虫的身体平面上有五分对称。本项目的目的是从古生物学、胚胎学和分子生物学的角度阐明这一特征是何时、如何进化的，以及它的功能意义。化石记录；由于前寒武纪晚期的三角纲具有三聚对称，一些研究人员认为棘皮动物的三聚体预示着五聚体。然而，三角龙是否是棘皮动物是值得怀疑的。众所周知，寒武纪棘皮动物已经具有五元对称性。寒武纪棘皮动物的五分枝模式实际上是“2-1-2模式”，其中三个分支中的两个分支分支成两个分支，形成五个分支。这种模式在后来的棘皮动物中广泛存在，因此，有理由认为真正的五元模式是由2-1-2模式进化而来的。胚胎学;在棘皮动物的发育过程中，当水维管系统形成，水维管系统被分割成5个突起时，首先出现了五体对称。因此，对水维管系统发展的详细观察对于理解五体对称的形成机制是重要的。分子生物学；目前尚不清楚是哪种基因控制了五体计划的形成。从同种基因在体形形成中的已知意义来看，认为同种基因参与棘皮动物五分体的形成并不是一种过分的推测。在本研究中，设计了DNA引物，用于PCR扩增某些棘虫的同源盒区域。结果，一些目标DNA片段被成功扩增。均质基因的表达与五聚体形成之间的关系目前正在研究中。

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.