Dissertation Research: Phylogeny of Aplacophora and Polyplacophora and Evolution of Molluscan Calcareous Hard Parts

在哈佛大学Andrew Knoll博士的指导下，研究生Akiko Okusu正在利用DNA序列、形态学和胚胎学的综合数据研究无板类和多板类软体动物的系统发育关系和钙质硬部的进化。 无板类是一种蠕虫状的深海软体动物，没有外壳，但被碳酸钙矿物文石制成的针状物覆盖。 多板壳动物（石鳖）是一种扁平细长的软体动物，有八个背壳板，覆瓦状地嵌在一个刺嵌入的外套带内。 据推测，无板动物和多板动物是密切相关的，这两个群体代表了软体动物中最基本的群体（基于它们相对简单的特征），它们的文石针状体反映了比其他软体动物更“原始”的碳酸钙沉积类型。 然而，无板动物门和多板动物门的进化关系尚未解决。 缺乏一个良好的支持性的进化论，使人们对软体动物形态和矿物沉积的进化变化假说提出了质疑。 从高度保守的核编码基因，EF 1-α和POL-II的分子序列数据，将被用来构建一个系统发育框架的40个分类群，包括主要的软体动物类和外群物种的星虫和环节动物的代表性物种。 在这些群体中的刺的发育和形态将在系统发育框架内进行分析，以评估同源性和进化变化的模式。 这些结果将对我们理解软体动物的起源和进化产生重大影响。

0073312Knoll and OkusuGraduate student Akiko Okusu, under the direction of Dr. Andrew Knoll, Harvard University, is studying the phylogenetic relationships and the evolution of calcareous hard parts of aplacophoran and polyplacophoran molluscs using integrated data from DNA sequences, morphology, and embryology. Aplacophorans are worm-like deep-sea molluscs that have no shells but are covered by spicules made of the calcium carbonate mineral aragonite. Polyplacophorans (chitons) are flat elongate molluscs with eight dorsal shell-plates imbricated within a spicule-embedded mantle girdle. It has been postulated that aplacophorans are closely related to polyplacophorans, that the two groups represent the most basal groups of molluscs (based on their relatively simple features), and that their aragonite spicules reflect a more "primitive" type of calcium carbonate deposition than that found in other molluscs. However, the evolutionary relationships of the Aplacophora and the Polyplacophora are unresolved. The lack of a well-supported phylogeny calls into question hypotheses of evolutionary changes in molluscan form and mineral deposition. Molecular sequence data from highly conserved nuclear coding genes, EF1-alpha and POL-II, will be used to construct a phylogenetic framework for 40 taxa including representative species from major molluscan classes and outgroup species from the Sipuncula and Annelida. Spicule development and morphology in these groups will be analyzed within the phylogenetic framework in order to assess homology and pattern of evolutionary change. The results will have a major impact on our understanding of the origin and evolution of molluscs.