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

棘皮动物是起源于寒武纪的无脊椎动物门之一，并存活到最近的海洋。该门的特点是在成人身体平面上呈五角形对称。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以及如何进化的，以及它的功能意义。化石记录；由于前寒武纪晚期Tribrachidium具有三聚体对称性，一些研究人员认为三聚体预示着棘皮动物的五倍体。然而，令人怀疑的是，Tribrachidium是棘皮动物。众所周知，寒武纪棘皮动物已经具有五角体对称性。寒武纪棘皮动物的五角形实际上是2-1-2型，其中三个分支中的两个分支成两个分支，形成五个分支。这一模式在后来的棘皮动物中广泛分布，因此，有理由认为真正的五角形模式是从2-1-2模式进化而来的。胚胎学：在棘皮动物的发育过程中，当水血管系统形成，水血管系统被分成五个突起时，五元对称性第一次出现。因此，详细观察水的维管系统的发育对于理解五元对称性的形成机制是很重要的。分子生物学；目前还不知道是什么基因控制着五元体计划的形成。从已知的同源基因在身体计划形成中的重要性来看，认为同源基因参与棘皮动物五倍体的形成并不是一种太大的推测。在这项研究中，设计了DNA引物来扩增某些棘球绦虫的同源盒的一个区域。结果，一些靶DNA片段成功地扩增出来。同源基因的表达与五倍体的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.