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-在地球上它不能生活的地方。研究范围限制是如何产生的很重要，因为它有助于人们了解在哪里可能或不可能发现害虫或疾病媒介等物种，以及物种是否能够随着环境的变化而迁移。人们普遍认为，当一个物种接近分布的边缘时，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，许多植物和动物体内都有丰富的微生物，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，微生物可能会改变物种分布的限制。拟议中的研究将使用与微观真菌共生的植物来验证这一假设。在其他地方，这种真菌有助于植物科普干旱胁迫。研究人员将从一些植物中去除真菌共生体，并将它们留在其他植物中，然后将这两种植物类型放入实验地块，这些地块跨越植物物种分布的潮湿核心到干燥边缘。他们将对三种对牧场生产力很重要的草种做这个实验。 该实验的数据将用于建立数学模型，揭示真菌微生物如何影响宿主植物在其范围边缘附近和远离其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-微生物共生体在气候变化中的重要性。真菌共生体是通过种子从成年植物传来的。因此，研究人员将访问植物标本馆，这是储存植物收藏的博物馆，从过去100年来储存的草种子中提取真菌，以测试植物-真菌相互作用如何随着时间的推移而变化。该项目旨在建立植物微生物组研究和物种分布研究之间的联系，这在历史上很少有交叉。了解物种地理分布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，物种之间的相互作用，加上非生物强迫，可能会在塑造地理分布发挥关键作用。然而，目前的理论和数据绝大多数都集中在消费和竞争等对抗性相互作用上。该项目将测试宿主生物及其微生物共生体之间的互利共生对当前和未来气候情景下双方地理分布的影响。使用一个通用的模型系统与一个发达的工具包（冷季草和垂直传播的真菌内生菌），该项目将结合联合收割机地理分布的共生体去除实验，人口范围建模，气候变化预测，收集为基础的调查，共生体的遗传分析，以衡量，第一次，主机共生共生的地理足迹。实验将集中在北美东部的三种草种上，这些草种沿着美国中南部剧烈的干旱梯度达到了它们的西部极限，而美国中南部的干旱梯度正在气候变化的影响下发生变化。首先，研究小组将测试共生对宿主范围限制影响的相互竞争的假设，这取决于共生体如何影响宿主对从范围核心到范围边缘的非生物（干旱）和生物（食草）应激的反应。用实验数据建立的机制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主机范围限制。其次，研究小组将量化环境依赖的适应性效应如何扩大到影响共生体流行的倾向，这决定了共生体的地理分布。最后，研究小组将通过从植物标本馆标本中取样9种宿主物种来追求更大的空间、时间和分类覆盖范围。植物标本馆的工作将重建地理和时间趋势的流行和健身的内生菌在美国中部的影响，以测试主机共生体的相互作用如何应对快速的环境变化。总的来说，这项工作将促进对范围限制起源的机械理解，并提高预测物种范围将如何应对未来环境变化的能力-这是人口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，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

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.