The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution.
The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution.
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DOI：
10.1186/s40851-017-0068-5
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发表时间：
2017
影响因子：
2.7
通讯作者：
Furuya H
中科院分区：
文献类型：
--
作者：
Lu TM;Kanda M;Satoh N;Furuya H
获得神秘类群的系统基因组数据对于更好地理解动物进化至关重要。由于其高度简化的身体组织，双叶虫长期以来一直吸引着生物学家，但它们的生命周期仍然鲜为人知。基于双叶门DoxC基因的发现，该基因编码一种螺旋肽，有人提出双叶门是螺旋门的成员。其他研究表明，双门动物可能与软体动物和环节动物有更密切的关系。然而，双叶门的系统发育位置仍然是一个有争议的问题，导致一个模棱两可的螺旋动物进化的画面。在本研究中，新测序的日本双叶虫转录组数据与来自29种螺旋动物、外生动物和后口动物的转录组数据或预测基因模型进行了补充，生成了一个用于系统基因组分析的数据集（数据集1），该数据集包含348个同源物种和58,124个氨基酸。除了该数据集之外，为了消除系统误差，通过从数据集1中删除可能导致成分异质性和长分支吸引伪影的组成异构或快速发展的位点和同源物，创建了两个额外的子数据集。最大似然分析和贝叶斯推理分析均将日本双叶蝇（Dicyema japonicum）与拟合叶蝇属（Intoshia lineei）归为一个分支，并得到了强有力的统计支持。此外，最大似然分析将双尾目+正孔目分支置于腹虫目，而在贝叶斯推断分析中，该分支是腹虫目+ Platyhelminthes分支的姐妹群。无论哪种情况，在所有的分析中，双足纲、直孔纲、腹虫纲和扁形纲构成了一个单一的类群，是软体动物+环节动物分支的姐妹类群。根据目前的系统基因组学分析，双叶纲与正连纲有密切的亲缘关系，它们可能与腹虫和扁形虫有共同的祖先，而不是与软体动物和环节动物有共同的祖先。关于螺旋动物的系统发育，Gnathifera是Rouphozoa和Lophotrochozoa的姐妹类群，这是以前的研究提出的；因此，我们的分析支持了传统的锥体-锥体假说，即螺旋体有一个近微观的、非体位的共同祖先。
Obtaining phylogenomic data for enigmatic taxa is essential to achieve a better understanding of animal evolution. Dicyemids have long fascinated biologists because of their highly simplified body organization, but their life-cycles remain poorly known. Based on the discovery of the dicyemid DoxC gene, which encodes a spiralian peptide, it has been proposed that dicyemids are members of the Spiralia. Other studies have suggested that dicyemids may have closer affinities to mollusks and annelids. However, the phylogenetic position of dicyemids has remained a matter of debate, leading to an ambiguous picture of spiralian evolution. In the present study, newly sequenced transcriptomic data from Dicyema japonicum were complemented with published transcriptomic data or predicted gene models from 29 spiralian, ecdysozoan, and deuterostome species, generating a dataset (Dataset 1) for phylogenomic analyses, which contains 348 orthologs and 58,124 amino acids. In addition to this dataset, to eliminate systematic errors, two additional sub-datasets were created by removing compositionally heterogeneous or rapidly evolving sites and orthologs from Dataset 1, which may cause compositional heterogeneity and long-branch attraction artifacts. Maximum likelihood and Bayesian inference analyses both placed Dicyema japonicum (Dicyemida) in a clade with Intoshia linei (Orthonectida) with strong statistical support. Furthermore, maximum likelihood analyses placed the Dicyemida + Orthonectida clade within the Gastrotricha, while in Bayesian inference analyses, this clade is sister group to the clade of Gastrotricha + Platyhelminthes. Whichever the case, in all analyses, Dicyemida, Orthonectida, Gastrotricha, and Platyhelminthes constitute a monophyletic group that is a sister group to the clade of Mollusca + Annelida. Based on present phylogenomic analyses, dicyemids display close affinity to orthonectids, and they may share a common ancestor with gastrotrichs and platyhelminths, rather than with mollusks and annelids. Regarding spiralian phylogeny, the Gnathifera forms the sister group to the Rouphozoa and Lophotrochozoa, as has been suggested by previous studies; thus our analysis supports the traditional acoeloid–planuloid hypothesis of a nearly microscopic, non-coelomate common ancestor of spiralians.