Evolutionary Analysis of Mitogenomes from Parasitic and Free-Living Flatworms

Mitochondrial genomes (mitogenomes) are useful and relatively accessible sources of molecular data to explore and understand the evolutionary history and relationships of eukaryotic organisms across diverse taxonomic levels. The availability of complete mitogenomes from Platyhelminthes is limited; of the 40 or so published most are from parasitic flatworms (Neodermata). Here, we present the mitogenomes of two free-living flatworms (Tricladida): the complete genome of the freshwater species Crenobia alpina (Planariidae) and a nearly complete genome of the land planarian Obama sp. (Geoplanidae). Moreover, we have reanotated the published mitogenome of the species Dugesia japonica (Dugesiidae). This contribution almost doubles the total number of mtDNAs published for Tricladida, a species-rich group including model organisms and economically important invasive species. We took the opportunity to conduct comparative mitogenomic analyses between available free-living and selected parasitic flatworms in order to gain insights into the putative effect of life cycle on nucleotide composition through mutation and natural selection. Unexpectedly, we did not find any molecular hallmark of a selective relaxation in mitogenomes of parasitic flatworms; on the contrary, three out of the four studied free-living triclad mitogenomes exhibit higher A+T content and selective relaxation levels. Additionally, we provide new and valuable molecular data to develop markers for future phylogenetic studies on planariids and geoplanids.线粒体基因组（mitogenomes）是探索并解析真核生物跨多样分类阶元的演化历史与亲缘关系的实用且相对易获取的分子数据来源。扁形动物门（Platyhelminthes）完整线粒体基因组的可用数据较为匮乏；已发表的约40条完整线粒体基因组中，绝大多数来自寄生扁形动物（新壁动物亚门Neodermata）。本研究报道了两种自由生活的三肠目（Tricladida）扁形动物的线粒体基因组：淡水物种高山真涡虫Crenobia alpina（真涡虫科Planariidae）的完整线粒体基因组，以及陆地扁形动物Obama sp.（地扁虫科Geoplanidae）的近完整线粒体基因组。此外，我们还重新注释了已发表的日本真涡虫Dugesia japonica（涡虫科Dugesiidae）的线粒体基因组。本研究贡献的新数据几乎使三肠目线粒体基因组的总发表量翻番——三肠目是一个物种丰富的类群，包含模式生物与具有经济重要性的入侵物种。借此契机，我们对现有自由生活扁形动物与选定的寄生扁形动物开展了比较线粒体基因组学分析，以期揭示生命周期通过突变与自然选择对核苷酸组成的潜在影响。出乎意料的是，我们未在寄生扁形动物的线粒体基因组中发现任何选择松弛的分子特征；与之相反，所研究的4个三肠类自由生活线粒体基因组中有3个呈现更高的A+T含量与选择松弛水平。此外，本研究还提供了全新的珍贵分子数据，可用于开发未来用于真涡虫科与地扁虫科系统发育研究的分子标记。