MIP: Symbiotic Niche Invasion by Beta-Rhizobia in North America

豆科植物和根瘤细菌之间的共生关系对人类福祉和自然生态系统至关重要，因为它提供氮化合物，而氮化合物通常是关键的限制性资源。尽管对这一现象进行了一个多世纪的深入研究，生物学家直到最近才发现某些豆科共生体属于一种被称为β变形菌门的群体。奇怪的是，这些共生体与任何已知的结核细菌都没有关系。至少有两个独立的变形菌门已经进化成豆科共生体（包括伯克霍尔德菌属和铜杆菌属的菌株）。然而，这种特性仅限于这些属中的少数物种，并且在整个β -变形菌门中是高度非典型的。因此，共生能力的进化是这个群体最近的创新，很可能是由于从远亲细菌那里获得了共生基因。这种在远亲生物之间自然发生的横向基因转移有可能极大地影响自然环境中细菌的生态关系和进化。然而，生物学家对这些过程的了解有限。本项目的目的是分析共生β -变形菌及其豆科宿主的DNA序列变异，以重建这种关系的生物地理学和历史。该研究将集中于来自美国大陆原产含羞草植物的所有物种的共生细菌，以确定β -变形菌共生细菌是否已经从热带生物群系扩散到北美温带地区，以及该群体中新的豆科植物物种的进化是否与新型细菌共生细菌的平行进化有关。接种实验还将进行，以测试来自较老的结核菌系的共生相关基因的横向转移是否影响了β -变形杆菌对不同植物宿主的专门化程度。这项研究的科学价值将是提供对进化机制的更广泛的理解，包括有助于微生物共生多样化的横向基因转移。此外，与伯克霍尔德菌和铜瘤菌共生体相关的物种是重要的人类或植物病原体，而一些携带β -变形菌共生体的豆科植物物种是全球关注的入侵杂草。因此，这项研究还将有助于更好地了解这些细菌的基本生态学和主要生态转变的频率（例如，从寄生到共生），以便更好地评估它们对环境和人类健康的影响。该事业还将通过为初中和高中生命科学教师举办研讨会，改善科学家如何分析生物关系和系统发生历史的教学实践，促进研究和科学教育的融合。

The symbiosis between legume plants and root-nodule bacteria is important to human welfare and to natural ecosystems, because it provides nitrogen compounds that are often a key limiting resource. Despite more than a century of intensive study of the phenomenon, biologists did not discover until recently that certain legume symbionts belong to a group known as the beta-Proteobacteria. Curiously, these symbionts are not related to any previously known nodule bacteria. At least two separate lineages in the beta-Proteobacteria have evolved to become legume symbionts (including strains of the genera Burkholderia and Cupriavidus). However, this trait is restricted to only a few of the species in these genera, and is highly atypical in the beta-Proteobacteria as a whole. Thus, the evolution of symbiotic ability is a recent innovation in this group, and was likely due to acquisition of symbiotic genes from distantly related bacteria. Such cases of naturally occurring lateral gene transfer between distantly related organisms have the potential to greatly affect the ecological relationships and evolution of bacteria in natural environments. However, biologists have only a limited understanding of these processes. The aim of this project is to analyze DNA sequence variation of both symbiotic beta-Proteobacteria and their legume hosts to reconstruct the biogeography and history of this relationship. The research will focus on symbiotic bacteria from all species of Mimosa plants native to the continental United States in order to determine whether beta-Proteobacterial symbionts have spread out of tropical biomes into temperate regions of North America, and whether the evolution of new legume species in this group has been associated with parallel evolution of new types of bacterial symbionts. Inoculation experiments will also be performed to test whether lateral transfer of symbiosis-related genes from older lineages of nodule bacteria has influenced the amount of specialization exhibited by beta-Proteobacteria toward various plant hosts. The scientific value of this research will be to provide a broader understanding of the evolutionary mechanisms, including lateral gene transfer, that contribute to diversification of microbial symbioses. In addition, species related to both Burkholderia and Cupriavidus nodule symbionts are important human or plant pathogens, and some of the legume species that host beta-Proteobacterial symbionts are invasive weeds of global concern. Therefore, this research will also help to provide a better understanding of the basic ecology of these bacteria and the frequency of major ecological transitions (e.g., from parasitism to mutualism), in order to better evaluate their impact on the environment and on human health. The project will also foster integration of research and science education by holding workshops for middle school and high school life science teachers to improve teaching practices concerning how scientists analyze organism relationships and phylogenetic history.