Department of Invertebrate Zoology, Faculty of Biology, Saint-Petersburg State University, Universitetskaja nab. 7/9, 199034, St. Petersburg, Russia.
Faculty of Physics, Saint-Petersburg State University, Universitetskaja nab. 7/9, 199034, St. Petersburg, Russia.
Mol Genet Genomics. 2019 Jun;294(3):715-727. doi: 10.1007/s00438-019-01543-1. Epub 2019 Mar 8.
Among invertebrates, only a few groups still have uncertain phylogenetic position, Orthonectida, a small group of rare multi-cellular parasites of marine invertebrates, being one of them. Recent molecular and morphological findings suggest that orthonectids belong to Lophotrochozoa and are close to Annelida. Nevertheless, phylogenetic relationships between orthonectids and annelids are unclear, and the phylogeny within the group itself has never been studied. Sequencing of mitochondrial genomes is used here to clarify this issue. Complete mt genomes of the orthonectids Intoshia variabili and Rhopalura litoralis were characterized and compared with Intoshia linei mt genome. Our results show that Orthonectida mt genomes have undergone reduction and gene loss, and that they have complicated organization revealed in strand asymmetry in nucleotide composition, in some features of intergenic non-coding regions, tRNA duplication and folding. Moreover, all species of Orthonectida have a unique gene order with complicated rearrangement landscape. Significant differences in mitochondrial genomes in the three orthonectid species could be explained by the fact that their host species belong to different taxa (flat worms, nemertines and gastropods). Among the analyzed mt genomes of Orthonectida, I. linei possesses the closest gene order to the ancestral genome. All Orthonectida species are monophyletic, and in the phylogenetic tree are close to Pleistoannelida, and specifically, to Clitellata.
在无脊椎动物中，只有少数几个群体的系统发育位置仍不确定，其中之一是 Orthonectida，这是一组罕见的海洋无脊椎动物多细胞寄生虫。最近的分子和形态学发现表明，orthonectids 属于 Lophotrochozoa，并且与环节动物密切相关。然而，orthonectids 和环节动物之间的系统发育关系尚不清楚，而且该群体内部的系统发育关系从未被研究过。本研究通过测序线粒体基因组来阐明这个问题。我们对多细胞寄生虫 Intoshia variabili 和 Rhopalura litoralis 的线粒体基因组进行了测序和特征分析，并与 Intoshia linei 的 mt 基因组进行了比较。我们的结果表明，Orthonectida 的 mt 基因组经历了减少和基因丢失，并且其组织复杂，在核苷酸组成的链不对称、基因间非编码区的某些特征、tRNA 复制和折叠方面都有所体现。此外，所有 Orthonectida 物种都具有独特的基因排列，具有复杂的重排景观。三种 orthonectid 物种的线粒体基因组存在显著差异，可以用其宿主物种属于不同分类群（扁形动物、纽形动物和腹足动物）来解释。在分析的 Orthonectida mt 基因组中，I. linei 的基因排列与祖先基因组最为接近。所有 Orthonectida 物种都是单系的，在系统发育树中与 Pleistoannelida 关系密切，特别是与 Clitellata 关系密切。
