Unraveling the evolutionary history of leeches: phylogenomics and the evolution of bloodfeeding

理解行为、生态和形态的进化转变，以及生物多样性的起源和分布，是生物学的基础。然而，每一种假设都以一个强大而可靠的系统发育假设为前提，为进化事件提供方向和时间，并揭示驱动进化分歧的潜在过程（模式）。系统发生学如果加以仔细研究，就会为一个群体的系统分类提供科学依据，并可能为过去的进化事件提供信息，这就是为什么系统发生学成为研究大量生物体进化机制的重要工具。尽管水蛭具有世界性，数量丰富，进化意义和医学用途，但水蛭亚纲（水蛭）没有系统发育假说，这使得有关该群体进化的推论不必要地费力。除了为水蛭的进化提供一个总体框架外，系统发育是解决水蛭进化中两个突出问题的重要组成部分。(i)基因组组成是什么，即在不同的分类群中表达了什么基因，是否丢失了任何基因，是否重新排列了任何基因（目前只存在一个基因组，这掩盖了在远亲分类群之间进行比较分析的可能性）。此外，（ii）通过破译在中风患者愈合和皮瓣和手指再植手术中经常使用的抗凝因子的多样性、选择和进化历史，以及似乎在水蛭生长和摄食策略中发挥重要作用的细菌共生体的功能和系统发育历史，可以获得哪些关于这些医学上重要动物的吸血进化的知识。我的研究计划将通过对水蛭全基因组（目标1）、约100个末端的靶捕获序列（目标2）、10个物种的唾液转录组（目标3）和5个细胞内细菌共生体的全细菌基因组（目标4）进行测序，利用大规模测序工作来阐明这些问题，并推断出一个坚实的亚类全系统发育。为此，将在三大洲（北美[主要是加拿大]、南美和亚洲）进行实地考察，并将尖端测序技术与复杂的下游生物信息学相结合。总的来说，这些项目将为水蛭吸血的演变提供一个新鲜和现代的观点。

Understanding evolutionary transitions in behaviour, ecology and morphology, as well as the origin and distribution of biological diversity, is fundamental in biology. However, each of these presupposes a robust and reliable phylogenetic hypothesis to give direction and timing to evolutionary events and to reveal the underlying processes (mode) driving evolutionary divergence. When implemented with care, phylogenies provide the scientific basis for the systematics of a group and may inform on past evolutionary events, which is why they have become vital tools for studying the mechanisms of evolution for a wide array of organisms. Despite of their cosmopolitan nature, abundance, evolutionary significance and medical utility, no phylogenetic hypothesis exists for the subclass Hirudinea (leeches), which makes inferences regarding the evolution of the group unnecessarily strenuous. Beyond providing a general framework for the evolution across leeches, the phylogeny is an important component in addressing two outstanding questions that are conspicuous regarding the evolution of leeches. (i) What is the genomic makeup, i.e., what genes are expressed, have any been lost and have any been rearranged across different taxa (only a single genome currently exists, which belies the possibility of comparative analyses between distantly related taxa). Further, (ii) what knowledge can be gained regarding the evolution of bloodfeeding in these medically important animals by deciphering both the diversity, selection and evolutionary histories of the anticoagulation factors that are frequently used in the healing of stroke patients and following flap and digit replantation surgery, as well as the function and phylogenetic histories of the bacterial symbionts that seem to play a vital role in leech growth and feeding strategies. My research programme will use large-scale sequencing efforts to shed light on these questions and to infer a solid subclass-wide phylogeny, by sequencing a full leech genome (Objective 1), target-capture sequences for ~100 terminals (Objective 2), salivary transcriptomes for 10 species (Objective 3) and full bacterial genomes for five intracellular bacterial symbionts (Objective 4). To this end, fieldwork will be conducted on three continents (North America [mainly Canada], South America and Asia), and cutting-edge sequencing technologies will be coupled with sophisticated downstream bioinformatics. Taken together, these projects will provide a fresh and modern view of the evolution of bloodfeeding in leeches.