The Developmental Basis for Evolutionary Changes in the Larval Biology of Brachiopods

在大多数现存的门中，无脊椎动物都有一个复杂的生活史，幼虫期和成虫期被一个变质过渡分开。人们认为，在这种转变的背景下发生了大量的进化变化。对腕足动物的系统发育研究表明，两个亚门：扁足亚门和颅足亚门的祖先产生了更衍生的舌足亚门。现存的linguliforma成员有长寿命的浮游营养型幼虫，它们以浮游生物为食并在其中生长，而现存的craniiforma成员有短寿命的不吃东西的电营养型幼虫。舌形目和颅形目都有很好的化石记录，可以追溯到下古生代。通过分析这些化石壳的顶端，人们可以推断出奠定第一个壳的地幔组织是什么时候形成的。这两个亚门的所有科最初都有浮游营养性幼虫。现存的rhynchonelli亚的成员都有嗜电性的幼虫。由于这一亚门起源于具有浮游营养性幼虫的祖先，因此嗜脂肪是一种衍生状态。这一提议的目的之一是对显示胚胎或幼虫地幔痕迹的纹状体目化石进行研究，以追踪幼虫类型的进化。这个建议的另一个目的是在现存的腕足动物的变态的背景下检查地幔的形成。1)将进行一项比较调查，以找出在暴露于适当的外部线索后，现有亚门的代表在多大程度上使用相同的信号转导机制来触发变态。2)实验将在超过正常变态时间的幼虫饲养时进行，以观察在没有变态的情况下，持续的发育事件是否会导致幼虫后结构的形成，如摄食装置、消化系统和外壳。3)候选基因腕足类“engrailed”可能在地幔分化中起作用。将进行一项比较研究，以检查该基因在每个亚门代表中的组织特异性表达。

Invertebrate animals in most extant phyla have a complex life historywith larval and adult phases that are separated by a metamoprhictransition. It is thought that a great deal of evolutionary change hasoccurred in the context of this transition. Phylogenetic studies onbrachiopods indicate that an ancestor of two subphyla: Linguliformea andCraniiformea gave rise to the more derived subphylum Rhynchonelliformea.Extant members of the Linguliformea have long lived planktotrophic larvaethat eat and grow in the plankton, while extant Craniiformea have shortlived lecithotrophic larvae that do not eat. Both the Linguliformea andCraniiformea have an excellent fossil record that extends back to the LowerPaleozoic. By analyzing the apex of the shells of these fossils one caninfer when the mantle tissue that laid down the first shell was formed.All of the families in these two subphyla initially had planktotrophiclarvae. The extant members of the Rhynchonelliformea all havelecithotriophic larvae. Since this subphylum originated from an ancestorwith planktotrophic larvae, lecithotrophy is a derived state. One aim ofthis proposal is to do a study of fossils in the Rhynchonelliformea thatshow traces of an embryonic or larval mantle in order to trace theevolution of larval type. The other aim of this proposal is to examinemantle formation in the context of metamorphosis in extant brachiopods. 1)A comparative survey will be done in order to find out to what extentrepresentatives of extant subphyla use the same signal transductionmechansisms to trigger metamorphosis following exposure to an appropriateexternal cue. 2) Experiments will be done where larvae are reared well pastthe normal time of metamorphosis in order to see if continuingdevelopmental events lead to the formation of post-larval structures suchas a feeding apparatus, alimentary system and shell in the absence ofmetamorphosis. 3) There is a candidate gene , brachiopod "engrailed" thatmay play a role in mantle differentiation. A comparative study will bedone to examine tissue specific expression of this gene in representativesof each subphylum.