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placophoran和polyplacophoran软体动物的系统发育关系和钙质坚硬部分的进化。盘虫是一种类似蠕虫的深海软体动物，它们没有壳，但被碳酸钙矿物文石制成的针状体所覆盖。石鳖是扁平的长形软体动物，背部有八个壳板，嵌在针状的地幔带内。据推测，无placophorans与多placophorans关系密切，这两个类群代表了软体动物中最基础的类群（基于它们相对简单的特征），它们的文石针状体反映了一种比其他软体动物更“原始”的碳酸钙沉积类型。然而，无placophora和多placophora的进化关系尚未解决。由于缺乏良好的系统发育支持，对软体动物形态和矿物沉积的进化变化假说提出了质疑。利用高度保守的核编码基因ef1 - α和POL-II的分子序列数据，构建40个分类群的系统发育框架，包括软体动物主要纲的代表物种以及Sipuncula和Annelida的外群物种。这些群体的针尖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。研究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.