Bacterial Population Structure: Evaluating Gene Flow in the Symbionts of Deep-Sea Mussels \(Genus Bathymodiolus\)

最近的证据表明，在空间和化学碎片化的生境中，微生物种群并不是普遍分布的，而是具有地理结构的。异质环境限制了种群间的基因流动，促进了遗传分化、局部适应和物种形成。受地形特征、深海洋流和喷口流体化学变化的影响，热液喷口呈斑片状分布。虽然化学合成细菌及其无脊椎宿主之间的关联为深海热液喷口的大型动物生产提供了基础，但对于共生体的遗传变异分布以及细菌种群结构如何影响喷口的生态相互作用和共生进化几乎一无所知。本研究项目将通过检测沿东太平洋上升（EPR）、中大西洋海脊（MAR）和劳弧后盆地（Lau）的遗传结构，分别对居住在快速、缓慢和不连续扩散中心的贻贝的硫自养和甲烷营养内共生种群的基因流动模型进行测试。深海贻贝几乎在所有的热液喷口都有发现，它们很可能每一代都从环境中获得共生体，这使它们成为评估细菌内共生体地理结构的模型系统。将研究3个山脊的共生生物，每个山脊5个喷口场，以及每个喷口场2-4个地点的贻贝。对于这两种类型的共生体，每个位点将从20-30个贻贝个体中测序4个DNA标记（分别为甲烷化和硫自养生物的16s rRNA， ITS， ftsZ和pmoA或soxY）。数据分析将采用种群遗传、聚结和系统地理方法。影响细菌内共生体种群结构的机制对深海热液喷口微生物的生物地理学、多样性以及生命的起源、进化和生态学具有重要意义。这项研究将产生第一个高分辨率的基因流在环境传播的内共生种群的检查。热液喷口场之间的比较将提供有关历史生物地理学和细菌物种传播障碍的有价值的信息。这些发现将对深海洋流如何影响化学自养群落的扩散和进化提供更广泛的推断。此外，由于热液喷口与早期地球和地外环境类似，本研究的结果将对真核细胞器的起源以及极端生物群系中微生物的生态学和进化产生影响。带注释的基因序列将存储在GenBank中，并通过一个网站（www.endosymbiont.org）向公众开放，研究人员使用该网站向科学界和公众社区提供有关内共生的信息。研究人员还为公众举办科学主题的研讨会，包括微生物共生和多样性，化学合成和地球上的生命等。本科生、研究生和博士后将有充分的机会参与这些基因序列的分析，无论是通过他们的研究还是通过纳入大学课程。

Recent evidence suggests that microbial populations in spatially and chemically fragmented habitats are not distributed ubiquitously, but rather exhibit geographic structure. Heterogeneous environments restrict gene flow among populations, which promotes genetic differentiation, local adaptation, and speciation. Hydrothermal vents are distributed in a patchy array due to topographic features, deep ocean currents, and variations in vent fluid chemistry. Though associations between chemosynthetic bacteria and their invertebrate hosts provide the basis for macrofaunal production at deep-sea hydrothermal vents, almost nothing is known about the distribution of genetic variation in the symbionts and how population structure of bacteria affects ecological interactions and the evolution of symbioses at vents. This research project will test models of gene flow for populations of thioautotrophic and methanotrophic endosymbionts of mussels inhabiting fast-, slow-, and discontinuous spreading centers by examining genetic structure along the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR) and within the Lau Back-arc Basin (Lau), respectively. Bathymodiolus mussels are found at nearly all hydrothermal vent sites and most likely acquire their symbionts from the environment in each generation, making them model systems for evaluating geographic structure of bacterial endosymbionts. Symbionts from 3 ridges, 5 vent fields per ridge, and mussels from 2-4 sites per vent field will be studied. For both types of symbiont, 4 DNA markers (16s rRNA, ITS, ftsZ, and either pmoA or soxY for the methanotroph and thioautotroph, respectively) will be sequenced from 20-30 mussel individuals per site. Population genetic, coalescent, and phylogeographic approaches will be employed in the data analyses. Mechanisms that affect the population structure of bacterial endosymbionts have important implications for microbial biogeography, diversity, and the origin, evolution, and ecology of life at deep-sea hydrothermal vents. This research will yield the first high-resolution examination of gene flow among environmentally transmitted endosymbiont populations. Comparisons between hydrothermal vent fields will provide valuable information on the historical biogeography and barriers to dispersal for bacterial species. These findings will allow broader inferences regarding how deep ocean currents influence dispersal and the evolution of chemoautotrophic communities. Further, because hydrothermal vents serve as analogs to early Earth as well as extraterrestrial environments, results from this research will have implications for the origin of eukaryotic cell organelles and the ecology and evolution of microbes in extreme biomes. Annotated gene sequences will be deposited in GenBank and made publicly accessible through a website (www.endosymbiont.org) that the investigators use to present information on endosymbiosis to the scientific and public community. The investigators also give seminars designed for the general public on scientific topics, including microbial symbiosis and diversity, chemosynthesis and life on this planet and others. Undergraduates, graduate students, and postdoctoral fellows will have ample opportunity to participate in analyses of these gene sequences, both through their research and through inclusion in university courses.