The Comparative Experimental Embryology of Brachiopods

9513068弗里曼发育生物学领域的一个目标是了解负责多细胞生物体空间组织的机制的利用在进化过程中如何以及为什么发生变化。这项提案将在Phoronida和相关的腕足纲中研究这个问题。腕足类作为化石并不为人所知；然而，由于它们的壳，腕足类拥有极好的化石记录，这使得古生物学家能够相当令人信服地重建这一门的历史。一些现存的腕足类物种起源于祖先，并与之非常相似，这些祖先代表着这一系统发展中的重要节点。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，表明幼虫从卵子和早期胚胎的特定部分发育而来的区域。然后在发育的不同阶段分离胚胎的不同区域，并将它们的分化能力与整个胚胎中相同区域的分化能力进行比较，如命运图所示。通过分离卵子和胚胎的不同部分，结合其他部分和不同的发育阶段，人们可以推断某个地区的发育是自主的还是有条件的。通过比较位于系统发育不同节点的一系列物种在胚胎发育过程中区域指定的时间和方式，可以证明区域指定机制的利用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，尽管这些物种没有密切的亲缘关系，而另一些物种的幼虫关系密切，但有非常不同的幼虫具有不同的生活史策略。这些物种之间的比较将使人们能够剖析幼虫进化与成虫结构进化在导致涉及早期胚胎发生的发育变化方面所起的作用。***

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. ***