Nuclear genomic signals of the "microturbellarian" roots of platyhelminth evolutionary innovation

Flatworms number among the most diverse invertebrate phyla, and represent the most biomedically significant branch of the major bilaterian clade Spiralia, but to date, deep evolutionary relationships within this group have been studied using only a single locus (the rRNA operon), leaving the origins of many key clades unclear. Here, using a survey of genomes and transcriptomes representing all free-living flatworm orders, we provide resolution of platyhelminth interrelationships based on hundreds of nuclear protein-coding genes, exploring phylogenetic signal through concatenation as well as recently developed consensus approaches. These analyses robustly support a modern hypothesis of flatworm phylogeny, one which emphasizes the primacy of the often-overlooked 'microturbellarian' groups in understanding the major evolutionary transitions within Platyhelminthes: perhaps most notably, we propose a novel scenario for the interrelationships between free-living and vertebrate-parasitic flatworms, providing new opportunities to shed light on the origins and biological consequences of parasitism in these iconic invertebrates.

扁形动物（Flatworms）是物种多样性最为丰富的无脊椎动物门之一，同时也是主要两侧对称类群螺旋动物（Spiralia）中生物医药研究价值最高的分支。但迄今为止，针对该类群内部深层演化关系的研究仅依托单一位点（核糖体RNA操纵子，rRNA operon）开展，致使诸多关键演化支的起源仍不明晰。本研究通过对涵盖所有自由生活扁形动物目的基因组与转录组数据集进行系统性分析，基于数百个核编码蛋白基因解析了扁形动物门的类群间演化关系，并通过序列串联法以及新近开发的共识分析方法探索系统发育信号。这些分析结果稳健地支持了当前扁形动物系统发育的主流假说——该假说强调常被忽视的微涡虫（microturbellarian）类群在阐释扁形动物门内主要演化过渡事件中的核心地位；尤为关键的是，我们提出了自由生活扁形动物与脊椎动物寄生扁形动物之间演化关系的全新框架，为阐明这类标志性无脊椎动物的寄生起源及其生物学效应提供了全新的研究契机。