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

大多数现存的无脊椎动物都有一个复杂的生活史，幼虫和成虫阶段被一个后生阶段的过渡所分隔。 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，两个亚门的祖先：Linguliformea和Craniiformea产生了更衍生的亚门Rhynchonelliformea。现存的Linguliformea成员有长寿的营养型幼虫，吃浮游生物并在浮游生物中生长，而现存的Craniiformea有短命的营养型幼虫，不吃浮游生物。 Linguliformea和Craniiformea都有很好的化石记录，可以追溯到下古生代。 通过分析这些化石的壳的顶端，我们可以推断出形成第一个壳的地幔组织是何时形成的。这两个亚门中的所有科最初都有自养幼虫。 现生的喙水母目（Rhynchonelliformea）的幼虫均具石纹。 由于这一亚门起源于一个具有营养型幼虫的祖先，因此营养型是一种衍生状态。 该建议的一个目的是对Rhynchonelliformea中显示胚胎或幼虫地幔痕迹的化石进行研究，以追踪幼虫类型的进化。 这个建议的另一个目的是在现存腕足动物变态的背景下研究腕足动物的形成。（1）通过比较研究，发现在适当的外部刺激下，现存的亚门在何种程度上利用相同的信号传导机制来触发变态。2)实验将在正常变态期后饲养的幼虫中进行，以观察在没有变态的情况下，持续的发育事件是否导致幼虫后结构的形成，如摄食器、消化系统和壳。3)有一个候选基因，腕足动物的“enrailed”，可能在地幔分化中发挥作用。 将进行比较研究，以检查该基因在每个亚门的代表组织特异性表达。

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.