Ecology and Evolution of Nodule Symbiosis for Invasive Legumes

摘要：入侵豆科植物根瘤共生的生态学与进化由于有意或无意的跨区域迁移，许多豆科植物成为入侵杂草，极大地改变了世界各地植物群落的生态。豆科植物的不成比例的生态影响与它们与根瘤菌（根瘤菌）的共生密切相关。这种共生关系为豆科植物提供了充足的可用氮供应，这是一种必需的营养物质，通常是植物生长的主要限制因素。然而，这种共生关系对豆科植物入侵的影响尚不清楚。豆科植物在种子发芽后必须从其直接环境中获得根瘤菌，因为细菌不是在种子内部携带的。因此，引进的豆科植物通常会到达新的栖息地，在那里，来自它们祖先范围的适应良好的共生伙伴不存在。即使豆科植物的殖民者有足够的非特化结瘤行为，允许他们使用从一个地方的其他豆科植物分类群中获得的菌株，也没有理由期望这些菌株会最优地适应新的寄主。因此，入侵的成功可能取决于殖民植物能否随着时间的推移选择更有益的共生伙伴。到目前为止，还没有研究证明共生兼容性的进化变化是否是豆科植物范围扩大的重要因素。本项目将以高入侵性豆科植物苏格兰雀稗（Cytisus scoparius）为模型系统，回答影响豆科植物生物入侵的人口统计学和进化过程的几个基本问题。苏格兰金雀花原产于欧洲，但在其他四大洲是一种侵略性的入侵者。该项目的具体目标是：1)使用核糖体基因的DNA测序来确定美国两个不同地区入侵的苏格兰蓟植物的根瘤菌共生体的地理起源。这些细菌将与来自欧洲和北美本土的大量其他豆科植物的样本进行比较。2)测试结核细菌对入侵豆科植物的适应是否涉及编码共生性状的细菌基因的水平转移。还将对固氮共生所必需的两个细菌基因进行DNA测序，以测试不同DNA区域的家谱关系是否因细菌菌株之间遗传物质的转移而改变；3)利用田间接种试验确定豆科殖民地的人口统计学表现是否受到相容性根瘤菌稀缺的限制。将比较植物在有和没有补充根瘤细菌的自然环境中的生长情况；4)通过比较来自植物祖先地区（欧洲）的细菌和来自新殖民的北美地区的细菌接种植物时的生长情况，评估豆科植物入侵者可用的根瘤菌是否质量低劣。人类对自然生态系统最严重的环境影响之一是导致物种入侵新的地理区域的活动。通过提供微生物共生体在豆科植物范围扩展中的作用，该项目将有助于更好地了解生物入侵，并为管理各种入侵豆科杂草的具体计划提供相关的概念和方法。

Abstract: Ecology and evolution of nodule symbiosis for invasive legumes As a result of both deliberate and accidental transport across regions, many species of legumes have become invasive weeds that have greatly altered the ecology of plant communities throughout the world. The disproportionate ecological impact of legumes is strongly related to their symbiosis with root nodule bacteria (rhizobia). This symbiosis provides legumes with an abundant supply of usable nitrogen, an essential nutrient that is often a primary limiting factor for plant growth. However, the influence of this symbiosis on invasion by legumes is poorly understood. Legumes must acquire rhizobia from their immediate environment following seed germination, because bacteria are not carried internally within seeds. As a result, introduced legumes will often arrive at new habitats where well-adapted mutualist partners from their ancestral range are nonexistent. Even if legume colonists have sufficiently unspecialized nodulation behavior to allow them to use bacterial strains obtained from other legume taxa indigenous to a site, there is no reason to expect that such strains will be optimally adapted to the novel host. Invasion success may thus depend on whether colonist plants can select for more beneficial symbiotic partners over time. To date, there has been no research to test whether evolutionary changes in symbiotic compatibility are an important factor in legume range expansion. This project will use the highly invasive legume Scotch broom (Cytisus scoparius) as a model system to answer several fundamental questions about demographic and evolutionary processes affecting biological invasion by legumes. Scotch broom is native to Europe, but is an aggressive invader on four other continents. The specific objectives of the project are: 1) to use DNA sequencing of ribosomal genes to identify the geographic origin of rhizobial symbionts for invasive Scotch broom plants in two separate regions of the U.S. These bacteria will be compared to samples from a broad range of other legumes species native to both Europe and North America. 2) to test whether adaptation of nodule bacteria to invading legumes has involved horizontal transfer of bacterial genes encoding symbiotic traits. DNA sequencing will also be performed on two bacterial genes essential to the nitrogen-fixation symbiosis, to test whether the genealogical relationships of different DNA regions have been altered by transfer of genetic material between bacterial strains; 3) to use field inoculation experiments to determine whether the demographic performance of legume colonists is limited by a scarcity of compatible rhizobia. The growth of plants in natural environments with and without exposure to supplemental root-nodule bacteria will be compared; and 4) to evaluate whether the rhizobia available to legume invaders are of inferior quality, by comparing how plants grow when inoculated with bacteria from the plant's ancestral range (Europe) vs. bacteria from newly colonized North American sites. One of the most serious types of human environmental impact on natural ecosystems involves activities that have caused species to invade new geographic regions. By providing insights about the role of microbial symbionts in legume range expansion, this project will contribute to a better understanding of biological invasions, and will also generate concepts and methods relevant to specific programs for managing a variety of invasive legume weeds.