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

豆科植物和根瘤菌之间的共生关系对人类福祉和自然生态系统都很重要，因为它提供了氮化合物，而氮化合物往往是一种关键的限制资源。 尽管对这一现象进行了世纪的深入研究，生物学家直到最近才发现某些豆类共生体属于一个称为β-变形菌的类群。 奇怪的是，这些共生体与任何先前已知的结核细菌都没有关系。 在β-变形菌门中至少有两个独立的谱系已经进化成为豆类共生体（包括伯克霍尔德氏菌属和贪铜菌属的菌株）。 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一群体中最近的创新，并且可能是由于从远亲细菌获得共生基因。 这种在远亲生物之间自然发生的横向基因转移的情况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。 本研究的目的是分析共生的β-变形菌和它们的豆科宿主的DNA序列变异，以重建这种关系的地理学和历史。 这项研究将集中在原产于美国大陆的所有含羞草植物物种的共生细菌上，以确定β-Proteobacterial共生体是否已经从热带生物群落扩散到北美的温带地区，以及该组中新豆科植物物种的进化是否与新型细菌共生体的平行进化有关。 还将进行接种实验，以测试来自较老的结核细菌谱系的共生相关基因的横向转移是否影响了β-变形菌门对各种植物宿主表现出的专门化程度。这项研究的科学价值将是提供更广泛的理解进化机制，包括横向基因转移，有助于微生物共生体的多样化。 此外，与伯克霍尔德氏菌和贪铜菌属根瘤共生体相关的物种是重要的人类或植物病原体，一些宿主β-变形菌共生体的豆类物种是全球关注的入侵杂草。 因此，这项研究也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。 该项目还将通过为初中和高中生命科学教师举办研讨会，促进研究和科学教育的整合，以改善有关科学家如何分析生物体关系和系统发育史的教学实践。

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.