BACKGROUND 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. RESULTS 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. CONCLUSIONS 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.

中文翻译：

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双孔中生动物的系统发育位置提供了螺旋进化的见解。

背景技术获得神秘分类群的植物学数据对于更好地了解动物进化至关重要。Dicyemids由于其高度简化的身体组织而长期以来一直使生物学家着迷，但其生命周期仍然知之甚少。基于发现编码双螺旋肽的dicyemid DoxC基因，发现dicyemids是螺旋藻的成员。其他研究表明，双麦草醚可能与软体动物和肘类动物有更密切的联系。然而，双嘧菌类的系统发育位置仍然是一个有争议的问题，导致螺旋形进化的模棱两可。结果：在本研究中，来自日本双胞胎的新测序转录组数据与已发表的转录组数据或来自29个螺旋形，蜕皮动物，和氘骨动物物种，生成用于系统生物学分析的数据集（数据集1），其中包含348个直系同源物和58,124个氨基酸。除此数据集外，为消除系统错误，还通过从数据集1中删除成分异质或快速发展的位点和直系同源物，创建了两个其他子数据集，这可能会导致成分异质性和长分支吸引伪像。最大似然法和贝叶斯推断分析法都将日本地锦（Dicyema japonicum）（地锦（Dicyemida））与线虫Intoshia linei（Orthonectida）放在一起，得到了强有力的统计支持。此外，最大似然分析将双翅目+直翅目科分类为Gastrotricha，而在贝叶斯推断分析中，该分类为Gastrotricha + Platyhelminthes的姐妹组。结论无论哪种情况，在所有分析中，双翅目科，直翅目科，胃科和侧柏科构成了一个单系群，是软体动物+纳涅利进化枝的姊妹群。根据目前的植物遗传学分析，双嘧菌胺与直链鱼类有密切的亲和力，它们可能与胃食小动物和蠕虫，而不是与软体动物和肘类动物有共同的祖先。关于螺线虫的系统发育，如先前的研究所表明的那样，Gnathifera形成了Rouphozoa和Lophotrochozozoa的姐妹群。因此，我们的分析支持了螺旋虫的近微观，非腔突的共同祖先的传统的类胚粉-扁平体假说。双麦草胺与直链鱼类有密切的亲和力，它们可能与胃食动物和蠕虫类动物而不是软体动物和肘类动物有共同的祖先。关于螺线虫的系统发育，如先前的研究所表明的那样，Gnathifera形成了Rouphozoa和Lophotrochozozoa的姐妹群。因此，我们的分析支持了螺旋虫的近微观，非腔突的共同祖先的传统的类胚粉-扁平体假说。双麦草胺与直链鱼类有密切的亲和力，它们可能与胃食动物和蠕虫类动物而不是软体动物和肘类动物有共同的祖先。关于螺线虫的系统发育，如先前的研究所表明的那样，Gnathifera形成了Rouphozoa和Lophotrochozozoa的姐妹群。因此，我们的分析支持了螺旋虫的近微观，非腔突的共同祖先的传统的类胚粉-扁平体假说。