Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Japan.
Sars International Centre for Marine Molecular Biology, University of Bergen, Norway.
Genome Biol Evol. 2019 Aug 1;11(8):2232-2243. doi: 10.1093/gbe/evz157.
Dicyemids, previously called "mesozoans" (intermediates between unicellular protozoans and multicellular metazoans), are an enigmatic animal group. They have a highly simplified adult body, comprising only ∼30 cells, and they have a unique parasitic lifestyle. Recently, dicyemids were shown to be spiralians, with affinities to the Platyhelminthes. In order to understand molecular mechanisms involved in evolution of this odd animal, we sequenced the genome of Dicyema japonicum and a reference transcriptome assembly using mixed-stage samples. The D. japonicum genome features a high proportion of repetitive sequences that account for 49% of the genome. The dicyemid genome is reduced to ∼67.5 Mb with 5,012 protein-coding genes. Only four Hox genes exist in the genome, with no clustering. Gene distribution in KEGG pathways shows that D. japonicum has fewer genes in most pathways. Instead of eliminating entire critical metabolic pathways, parasitic lineages likely simplify pathways by eliminating pathway-specific genes, while genes with fundamental functions may be retained in multiple pathways. In principle, parasites can stand to lose genes that are unnecessary, in order to conserve energy. However, whether retained genes in incomplete pathways serve intermediate functions and how parasites overcome the physiological needs served by lost genes, remain to be investigated in future studies.
双胚虫，以前被称为“中生动物”（单细胞原生动物和多细胞后生动物之间的中间体），是一种神秘的动物群。它们具有高度简化的成年体，仅由约 30 个细胞组成，并且具有独特的寄生生活方式。最近，双胚虫被证明是螺旋动物，与扁形动物有亲缘关系。为了了解这种奇异动物进化过程中涉及的分子机制，我们使用混合阶段样本对日本双胚虫的基因组和参考转录组组装进行了测序。日本双胚虫基因组具有高比例的重复序列，占基因组的 49%。双胚虫基因组减少到约 67.5Mb，包含 5012 个蛋白质编码基因。基因组中仅存在四个 Hox 基因，且没有聚类。KEGG 途径中的基因分布表明，日本双胚虫在大多数途径中的基因较少。寄生谱系可能通过消除特定途径的基因来简化途径，而不是消除整个关键代谢途径，而具有基本功能的基因可能在多个途径中保留。原则上，寄生虫可以失去不必要的基因，以节省能量。然而，在不完整途径中保留的基因是否具有中间功能，以及寄生虫如何克服失去的基因所提供的生理需求，仍有待未来研究调查。
