Department of Invertebrate Zoology, Faculty of Biology, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia.
Department of Invertebrate Zoology, Faculty of Biology, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia; Zoological Institute RAS, Universitetskaya nab. 1, 199034 St. Petersburg, Russia.
Curr Biol. 2020 Apr 6;30(7):1292-1298.e3. doi: 10.1016/j.cub.2020.01.061. Epub 2020 Feb 20.
Orthonectida is a small, rare, and in many aspects enigmatic group of organisms with a unique life cycle and a highly simplified adult free-living stage parasitizing various marine invertebrates [1, 2]. Phylogenetic relationships of Orthonectida have remained controversial for a long time. According to recent data, they are close to Annelida, specifically to Clitellata [3-5]. Several studies have shown that parasitism can not only lead to a dramatic reduction of the body plan and morphological structures but also affect organisms at the genomic level [6, 7]. Comparative studies of parasites and closely related non-parasitic species could clarify the genome reduction degree and evolution of parasitism. Here, we report on the morphology, genome structure, and content of the smallest known Orthonectida species Intoshia variabili, inhabiting the flatworm Graffiellus croceus. This orthonectid with an extremely simplified nervous system demonstrates the smallest known genome (15.3 Mbp) and one of the lowest reported so far gene numbers (5,120 protein-coding genes) among metazoans. The genome is extremely compact, due to a significant reduction of gene number, intergenic regions, intron length, and repetitive elements. The small genome size is probably a result of extreme genome reduction due to their parasitic lifestyle, as well as of simplification and miniaturization of the free-living stages. Our data could provide further insights into the evolution of parasitism and could help to define a minimal bilaterian gene set.
纽形动物门是一类体型较小、罕见且在许多方面都很神秘的生物，具有独特的生命周期和高度简化的自由生活阶段，寄生在各种海洋无脊椎动物中[1,2]。纽形动物门的系统发育关系长期以来一直存在争议。根据最近的数据，它们与环节动物门关系密切，特别是与担轮动物亚门[3-5]。几项研究表明，寄生不仅会导致身体结构和形态结构的急剧减少，还会影响生物体的基因组水平[6,7]。寄生虫和密切相关的非寄生物种的比较研究可以阐明基因组减少的程度和寄生的进化。在这里，我们报告了栖息在扁平虫 Graffiellus croceus 中的最小已知纽形动物物种 Intoshia variabili 的形态、基因组结构和内容。这种神经系统极其简化的纽形动物具有已知最小的基因组（15.3 Mbp）和目前报道的最低基因数量之一（5120 个蛋白质编码基因），在后生动物中。由于基因数量、基因间区、内含子长度和重复元件的显著减少，基因组非常紧凑。小基因组大小可能是由于其寄生生活方式导致的极端基因组减少，以及自由生活阶段的简化和小型化。我们的数据可以为寄生的进化提供进一步的见解，并有助于确定最小的两侧对称动物基因集。
