Phylogenomics reveals deep molluscan relationships.

Evolutionary relationships among the eight major lineages of Mollusca have remained unresolved despite their diversity and importance. Previous investigations of molluscan phylogeny, based primarily on nuclear ribosomal gene sequences or morphological data, have been unsuccessful at elucidating these relationships. Recently, phylogenomic studies using dozens to hundreds of genes have greatly improved our understanding of deep animal relationships. However, limited genomic resources spanning molluscan diversity has prevented use of a phylogenomic approach. Here we use transcriptome and genome data from all major lineages (except Monoplacophora) and recover a well-supported topology for Mollusca. Our results strongly support the Aculifera hypothesis placing Polyplacophora (chitons) in a clade with a monophyletic Aplacophora (worm-like molluscs). Additionally, within Conchifera, a sister-taxon relationship between Gastropoda and Bivalvia is supported. This grouping has received little consideration and contains most (>95%) molluscan species. Thus we propose the node-based name Pleistomollusca. In light of these results, we examined the evolution of morphological characters and found support for advanced cephalization and shells as possibly having multiple origins within Mollusca.

尽管软体动物具有多样性且十分重要，但其八个主要类群之间的进化关系仍未明确。先前主要基于核糖体基因序列或形态学数据对软体动物系统发育的研究，未能阐明这些关系。最近，利用数十到数百个基因进行的系统基因组学研究极大地增进了我们对动物深层关系的理解。然而，涵盖软体动物多样性的基因组资源有限，阻碍了系统基因组学方法的应用。在此，我们利用来自所有主要类群（单板纲除外）的转录组和基因组数据，得出了一个有充分支持的软体动物拓扑结构。我们的研究结果有力地支持了有壳亚门假说，即将多板纲（石鳖）与单系的无板纲（蠕虫状软体动物）归为一个分支。此外，在有壳类中，腹足纲和双壳纲之间的姐妹群关系得到了支持。这一分类群很少被考虑到，但包含了大多数（>95%）的软体动物物种。因此，我们提出了基于节点的名称“多源软体动物类”。鉴于这些结果，我们研究了形态特征的进化，发现了支持头部集中化和贝壳在软体动物中可能有多个起源的证据。