Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA.
Nature. 2011 Oct 26;480(7377):364-7. doi: 10.1038/nature10526.
Molluscs (snails, octopuses, clams and their relatives) have a great disparity of body plans and, among the animals, only arthropods surpass them in species number. This diversity has made Mollusca one of the best-studied groups of animals, yet their evolutionary relationships remain poorly resolved. Open questions have important implications for the origin of Mollusca and for morphological evolution within the group. These questions include whether the shell-less, vermiform aplacophoran molluscs diverged before the origin of the shelled molluscs (Conchifera) or lost their shells secondarily. Monoplacophorans were not included in molecular studies until recently, when it was proposed that they constitute a clade named Serialia together with Polyplacophora (chitons), reflecting the serial repetition of body organs in both groups. Attempts to understand the early evolution of molluscs become even more complex when considering the large diversity of Cambrian fossils. These can have multiple dorsal shell plates and sclerites or can be shell-less but with a typical molluscan radula and serially repeated gills. To better resolve the relationships among molluscs, we generated transcriptome data for 15 species that, in combination with existing data, represent for the first time all major molluscan groups. We analysed multiple data sets containing up to 216,402 sites and 1,185 gene regions using multiple models and methods. Our results support the clade Aculifera, containing the three molluscan groups with spicules but without true shells, and they support the monophyly of Conchifera. Monoplacophora is not the sister group to other Conchifera but to Cephalopoda. Strong support is found for a clade that comprises Scaphopoda (tusk shells), Gastropoda and Bivalvia, with most analyses placing Scaphopoda and Gastropoda as sister groups. This well-resolved tree will constitute a framework for further studies of mollusc evolution, development and anatomy.
软体动物（蜗牛、章鱼、蛤及其亲属）的身体结构差异很大，在动物中，只有节肢动物的物种数量超过它们。这种多样性使软体动物成为研究最多的动物之一，但它们的进化关系仍未得到很好的解决。悬而未决的问题对软体动物的起源以及该群体内部的形态进化具有重要意义。这些问题包括无壳的、蠕虫状的无壳软体动物是否在有壳软体动物（腕足动物）起源之前就已经分化，还是它们是次级失去了外壳。最近，当多板纲动物（石鳖）与单板纲动物（腹足类）一起被提议构成一个名为 Serialia 的进化枝时，它们才被包括在分子研究中，这反映了这两个群体中身体器官的连续重复。当考虑到寒武纪化石的巨大多样性时，试图理解软体动物的早期进化变得更加复杂。这些化石可能有多个背壳板和骨片，也可能没有壳，但具有典型的软体动物齿舌和连续重复的鳃。为了更好地解析软体动物之间的关系，我们为 15 个物种生成了转录组数据，这些数据与现有的数据相结合，首次代表了所有主要的软体动物群体。我们分析了多个数据集，这些数据集包含多达 216402 个位点和 1185 个基因区域，使用了多种模型和方法。我们的结果支持包含有刺的三个软体动物群体但没有真正壳的 Aculifera 进化枝，并且支持腕足动物的单系性。单板纲动物不是其他腕足动物的姐妹群，而是头足动物的姐妹群。发现一个包含箭石（角贝）、腹足类和双壳类的进化枝得到了强烈支持，大多数分析将箭石和腹足类作为姐妹群。这个解决得很好的树将构成进一步研究软体动物进化、发育和解剖学的框架。
