The geographic footprint of host-symbiont mutualism

地球上的每一个物种都有其地理分布的限制--在地球上不能生活的地方。研究范围界限是如何产生的很重要，因为它有助于人们了解哪里可能或不可能发现害虫或病媒等物种，以及物种是否能够随着环境的变化而迁徙。人们普遍认为，在接近物种分布的边缘时，来自恶劣环境的压力最终会导致死亡率超过出生率，从而使种群无法自我维持。这将生成范围边缘。然而，许多动植物体内蕴藏着丰富的微生物，这些微生物(统称为微生物组)可能有助于缓解压力。这导致了一种假设，即通过帮助宿主应对压力，微生物可能会改变一个物种的分布限制。这项拟议的研究将使用与微生物真菌共生的植物来验证这一假设。在其他地方，这种真菌被认为可以帮助植物应对干旱压力。研究人员将从一些植物中移除真菌共生体，并将它们留在另一些植物中，然后将这两种植物类型放入试验田，这些试验田横跨植物物种分布的湿核心到干燥边缘。他们将对三种对牧场生产力至关重要的草种进行这项实验。这项实验的数据将被用来建立数学模型，揭示真菌微生物如何影响寄主植物中靠近和远离范围边缘的出生率和死亡率的平衡。这将有助于我们理解植物-微生物共生在气候变化中的重要性。真菌共生体从成年植物通过种子传递。因此，研究人员将访问草药，这是储存植物收藏品的博物馆，从过去100年来储存的牧草种子中提取真菌，以测试植物与真菌的相互作用如何随着时间的推移而变化。该项目的目的是在植物微生物群研究和物种分布研究之间建立联系，这两个研究在历史上几乎没有什么相互作用。了解对物种地理分布产生限制的过程是生态学中的一个经典问题，面对快速的环境变化，这一问题变得紧迫起来。人们广泛认识到，物种相互作用，加上非生物强迫，可能在塑造地理分布方面发挥关键作用。然而，目前的理论和数据压倒性地集中在消费和竞争等对立互动上。该项目将测试寄主生物体和它们的微生物共生体之间的互惠关系对当前和未来气候情景下合作伙伴的地理分布的影响。利用一个通用的模型系统和完善的工具包(冷季草及其垂直传播的真菌内生菌)，该项目将结合地理上分布的共生体去除实验、人口范围建模、气候变化预测、基于集合的调查和共生体的遗传图谱，首次测量宿主-共生体互惠关系的地理足迹。实验将集中在北美东部的三种草，它们符合美国中南部戏剧性的干旱梯度的西部极限，该梯度在气候变化的影响下正在发生变化。首先，研究小组将测试关于共生对宿主活动范围限制的影响的相互竞争的假说，这取决于共生体如何从范围核心到范围边缘影响宿主对非生物(干旱)和生物(草食)胁迫的反应。用实验数据建立的机械范围模型将量化共生体如何通过这两种类型的保护来修改宿主范围限制。其次，该团队将量化上下文相关的健康效应如何扩大到影响共生菌流行率的临床，这决定了共生菌的地理分布。最后，研究小组将通过从标本馆标本中采样9个寄主物种来寻求更大的空间、时间和分类学覆盖范围。植物标本馆的工作将重建美国中部内生菌流行和健康影响的地理和时间趋势，以测试宿主-共生体的相互作用如何对快速的环境变化做出反应。总体而言，这项工作将促进对范围限制起源的机械性理解，并增强预测物种范围将如何应对未来环境变化的能力-这是种群和社区生态学中最紧迫的优先事项之一。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Every species on Earth has limits to its geographic distribution – places on the planet where it cannot live. Studying how range limits arise is important because it helps people understand where species such as pests or disease vectors are likely or unlikely to be found, and whether species might be able to migrate as environments change. It is generally accepted that, approaching the edges of a species’ distribution, stress from harshness of the environment eventually causes the death rate to exceed the birth rate, so that populations cannot sustain themselves. This generates a range edge. However, many plants and animals harbor a rich array of microbes that live inside of themselves, and these microbes (collectively called the microbiome) may help to ameliorate stress. This leads to the hypothesis that, by helping hosts deal with stress, microbes may alter the limits of a species’ distribution. The proposed research will test this hypothesis using plants that live symbiotically with microscopic fungi. The fungi are known elsewhere to help the plants cope with drought stress. The researchers will remove fungal symbionts from some plants and leave them in others, then place both plant types out into experimental plots that span the wet core to the dry edge of the plant species’ distributions. They will do this experiment for three grass species important to rangeland productivity. Data from this experiment will be used to build mathematical models that reveal how the fungal microbes affect the balance of birth and death rates in the host plants near and away from their range edges. This will help us understand the importance of plant-microbe symbioses as climate changes. Fungal symbionts are passed from adult plants through seeds. The researchers will thus visit herbaria, which are museums of stored plant collections, to extract fungi from seeds of stored grasses over the last 100 years to test how the plant-fungal interactions have changed through time. The project is designed to build connections between the study of plant microbiomes and the study of species’ distributions, which historically have had little cross-talk. Understanding the processes that generate limits on species’ geographic distributions is a classic problem in ecology that takes on urgency in the face of rapid environmental change. There is wide recognition that species interactions, together with abiotic forcing, may play a key role in shaping geographic distributions. However, current theory and data overwhelmingly focus on antagonistic interactions like consumption and competition. This project will test the influence of mutualism between host organisms and their microbial symbionts on the geographic distributions of both partners under current and future climate scenarios. Using a generalizable model system with a well-developed toolkit (cool-season grasses and their vertically-transmitted fungal endophytes), this project will combine geographically distributed symbiont removal experiments, demographic range modeling, climate change forecasting, collections-based surveys, and genetic profiling of symbionts to measure, for the first time, the geographic footprint of host-symbiont mutualism. Experiments will focus on three eastern North American grass species that meet their western limits along the dramatic aridity gradient of the south-central US, which is shifting under climate change. First, the research team will test competing hypotheses for the effect of symbiosis on host range limits, which depends on how symbionts influence host responses to abiotic (aridity) and biotic (herbivory) stress from range-core to range-edge. Mechanistic range models built with experimental data will quantify how symbionts modify host range limits via these two types of protection. Second, the team will quantify how context-dependent fitness effects scale up to influence clines in symbiont prevalence, which determine the symbiont geographic distribution. Finally, the research team will pursue greater spatial, temporal, and taxonomic coverage by sampling nine host species from herbarium specimens. Herbarium work will reconstruct geographic and temporal trends in the prevalence and fitness effects of endophytes across the central US to test how host-symbiont interactions have responded to rapid environmental change. Collectively, this work will advance mechanistic understanding of the origins of range limits and enhance the ability to forecast how species’ ranges will respond to future environmental change – among the most urgent priorities in population and community ecology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.