Canadian Institute for Advanced Research, Centre Robert-Cedergren, Département de Biochimie, Université de Montréal, Montréal, Québec, Canada.
PLoS One. 2007 Aug 8;2(8):e717. doi: 10.1371/journal.pone.0000717.
Acoel flatworms are small marine worms traditionally considered to belong to the phylum Platyhelminthes. However, molecular phylogenetic analyses suggest that acoels are not members of Platyhelminthes, but are rather extant members of the earliest diverging Bilateria. This result has been called into question, under suspicions of a long branch attraction (LBA) artefact. Here we re-examine this problem through a phylogenomic approach using 68 different protein-coding genes from the acoel Convoluta pulchra and 51 metazoan species belonging to 15 different phyla. We employ a mixture model, named CAT, previously found to overcome LBA artefacts where classical models fail. Our results unequivocally show that acoels are not part of the classically defined Platyhelminthes, making the latter polyphyletic. Moreover, they indicate a deuterostome affinity for acoels, potentially as a sister group to all deuterostomes, to Xenoturbellida, to Ambulacraria, or even to chordates. However, the weak support found for most deuterostome nodes, together with the very fast evolutionary rate of the acoel Convoluta pulchra, call for more data from slowly evolving acoels (or from its sister-group, the Nemertodermatida) to solve this challenging phylogenetic problem.
后生动物扁形虫是一种小型海洋蠕虫，传统上被认为属于扁形动物门。然而，分子系统发育分析表明，后生动物扁形虫并非扁形动物门的成员，而是最早分化的两侧对称动物的现存成员。这一结果受到了质疑，怀疑是由于长枝吸引（LBA）的人为产物。在这里，我们通过对后生动物扁形虫 Convoluta pulchra 的 68 种不同的蛋白质编码基因和 51 种属于 15 个不同门的后生动物物种的基因组学方法重新研究了这个问题。我们使用了一种名为 CAT 的混合模型，该模型在经典模型失败的地方被发现可以克服 LBA 人为产物。我们的结果明确表明，后生动物扁形虫不属于经典定义的扁形动物门，使得后者是多系的。此外，它们表明后生动物扁形虫与后口动物有亲缘关系，可能是所有后口动物、Xenoturbellida、环节动物门或甚至是脊索动物的姐妹群。然而，大多数后口动物节点的支持度很弱，加上后生动物扁形虫 Convoluta pulchra 的进化速度非常快，这就需要更多来自进化缓慢的后生动物（或其姐妹群 Nemertodermatida）的数据来解决这个具有挑战性的系统发育问题。
