The Comparative Experimental Embryology of Brachiopods

发育生物学领域的一个目标是了解多细胞生物空间组织机制的利用如何以及为什么在进化过程中发生变化。本文拟在腕足动物门及其相关门腕足动物中研究这一问题。Phoronida是未知的化石；然而，由于它们的壳，腕足类动物有一个很好的化石记录，这使得古生物学家能够做一个相当令人信服的工作来重建这个门的历史。许多现存的腕足动物物种都是由祖先衍生而来的，并且与祖先非常相似，这些祖先代表了这一系统发育的重要节点。将对这些物种的胚胎进行实验研究，以确定在发育过程中，胚胎的不同部分何时被指定，并确定负责区域指定的机制。这些实验首先为每个胚胎构建一个命运图，表明幼虫从卵和早期胚胎的特定部位发育而来的区域。然后，胚胎的不同区域将在不同的发育阶段被分离出来，它们的分化能力将与命运图所示的整个胚胎中相同区域的分化能力进行比较。将不同发育阶段的部分卵子和胚胎与其他部分相结合，可以推断出某一特定区域的发育是自主的还是有条件的。通过比较处于不同系统发育节点的一系列物种胚胎发育过程中区域规范的时间和模式，可以记录区域规范利用机制随进化变化而发生的变化。其中一些物种的幼虫形态相似，生活史相似，但这些物种并不密切相关，而另一些物种关系密切，但幼虫的生活史策略不同。这些物种之间的比较将使人们能够解剖出幼虫进化与成虫结构进化在带来涉及早期胚胎发生的发育变化中的作用。＊＊＊

9513068 Freeman One goal of the field of developmental biology is to understand how and why the utilization of the mechanisms responsible for the spatial organization of multicellular organisms changes during the course of evolution. This proposal will study this problem in the Phoronida and the related phylum, Brachiopoda. The Phoronida are unknown as fossils; however, because of their shell, brachiopods have an excellent fossil record which has allowed paleontologists to do a rather convincing job of reconstructing the history of this phylum. A number of extant brachiopod species are derived from, and closely resemble, ancestors that represent important nodes in this phylogeny. Experimental studies will be performed on the embryos of these species in order to establish when, during development, different parts of the embryo become specified and to identify the mechanisms that are responsible for regional specification. These experiments begin with the construction of a fate map for each embryo indicating the regions of hte larva that develop from specific parts of the egg and early embryo. Then diffenrent regions of the embryo will be isolated at various stages of development, and their ability to differentiate will be compared to that of the same region in a whole embryo as indicated by the fate map. By isolating parts of eggs and embryos in combination with other parts and at different developmental stages, one can infer whether the development of a given region is autonomous or conditional. By comparing the timing and mode of regional specification during embryogenesis for a series of species at different nodes in the phyylogeny, one can document the changes in the utilization of mechanisms for regional specification that accompany evolutionary change. Some of these species have larvae that are morphologically similar and have similar life histories although these species are not closely related, while other species are closely related but have very different larvae with different life history strategies. Comparisons beteen these species will allow one to dissect out the role of larval evolution versus the evolution of adult structures in bringing about developmental changes involving early embryogenesis. ***