RUI: Developmental Biology of Turtle Shell Formation

RUI：龟壳形成的发育生物学龟壳是一种进化上的新奇事物，由两个主要部分组成，上部甲壳和下部板。“使海龟成为海龟”的是肋骨向背部皮肤真皮层的迁移，而不是它们向下迁移形成胸腔。这项研究将使用细胞移植和基因标记技术来鉴定由肋骨分泌的分子，这些分子使皮肤变成骨头。我们预计肋骨分泌一种叫做骨形态发生蛋白（BMPs）的蛋白质，而骨形成的时间是由这些BMPs和真皮层中存在的BMPs抑制剂之间的相互作用调节的。形成板层的细胞起源于一个叫做主干神经嵴的胚胎结构。在其他现存物种身上，还没有发现干神经嵴长出骨骼。然而，颅神经嵴通常形成面部骨骼。当主干神经嵴细胞失去Hox基因的表达时，它们同时获得了形成骨架的能力。Hox基因的表达似乎抑制了神经嵴细胞成骨的能力。因此，这个项目的另一个主要目标是验证一个假设，即在海龟胚胎中，一群神经嵴细胞产生较晚，失去其Hox表达，并迁移形成板骨。如果这些假设得到证实，这项研究将进一步解释海龟是如何得到外壳的这个“古老”问题。它将为化石记录中一种新的解剖结构是如何进化得如此之快的问题提供一个解决方案，也将提供一个容易理解的例子，说明进化是如何产生新结构的。它将由本科生完成，并应使他们熟悉进化发育遗传学的基本技术。

Scott F. Gilbert Proposal #IOS-0748508 RUI: Developmental biology of turtle shell formationThe turtle shell is an evolutionary novelty composed of two main parts, the upper carapace and the lower plastron. What "makes a turtle a turtle" is the migration of the ribs into the dermis of the back skin rather than their migrating downward to form a rib cage. This study will use cell transplantation and gene labeling techniques to identify the molecules secreted by the ribs which cause the skin to turn into bone. We expect that the ribs are secreting proteins called Bone Morphogenetic Proteins (BMPs) and that the timing of bone formation is regulated by the interplay between these BMPs and inhibitors of BMPs that exist in the dermis. Cells forming the plastron originate in an embryonic structure called the trunk neural crest. The trunk neural crest has not been seen to produce bones on any other extant species. The cranial neural crest, however, usually makes facial bones. If trunk neural crest cells lose the expression of Hox genes, they simultaneously gain the ability to form skeleton. Hox gene expression appears to inhibit the ability of neural crest cells to form bone. So another major objective of this project is to test the hypothesis that in turtle embryos, a population of neural crest cells is produced late, loses its Hox expression, and migrates to form the plastron bones. If these hypotheses are validated, this research goes far in explaining the "age-old" question of how the turtle got its shell. It would provide a solution to the question of how a novel anatomical structure can evolve so quickly in the fossil record, and it would also provide a readily understood example how evolution works to make new structures. It will be done by undergraduates and should acquaint them with the basic techniques of evolutionary developmental genetics.