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

豆科植物和根瘤细菌之间的共生对人类福祉和自然生态系统都很重要，因为它提供的氮化合物往往是关键的限制资源。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类共生体属于一种被称为β-变形杆菌的群体。奇怪的是，这些共生体与之前已知的任何根瘤细菌都没有关系。在β-变形杆菌中，至少有两个不同的谱系已经进化成豆类共生体(包括伯克霍尔德氏菌和铜绿假单胞菌属的菌株)。然而，这一特征仅限于这些属中的少数物种，并且在整个β-变形杆菌中是高度非典型的。因此，共生能力的进化是这一组中最近的一项创新，很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。本项目的目的是分析共生的β-变形杆菌及其豆科寄主的DNA序列变异，以重建这种关系的生物地理和历史。这项研究将重点研究原产于美国大陆的所有种类含羞草植物的共生细菌，以确定β-Protebacterial共生体是否已经从热带生物群扩散到北美的温带地区，以及这一组中新豆科植物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，以测试从较老的根瘤细菌谱系横向转移共生相关基因是否影响了β-变形杆菌对不同植物宿主表现出的专化量。这项研究的科学价值将是提供对包括横向基因转移在内的有助于微生物共生多样性的进化机制的更广泛的理解。此外，与伯克霍尔德氏菌和铜绿假单胞菌共生的物种都是重要的人类或植物病原体，一些寄主β-变形杆菌共生体的豆科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以及主要生态转变(如从寄生到互惠)的频率，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，促进研究与科学教育的融合，以改进有关科学家如何分析有机体关系和系统发育历史的教学实践。

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.