Ecological and evolutionary drivers of host-symbiont associations in the phyllosphere

共生生活方式是生态系统微生物多样性的重要组成部分。对于微生物依赖宿主生存的共生体(即专性共生体)而言，与宿主的关系如何塑造微生物生态和进化已经被研究得最广泛。这些研究强调了宿主协会在推动共生体专门化方面的重要性，有时会导致共生体与宿主的共同多样化。在共生体经常有机会与自由生活的栖息地和不同的寄主物种相互作用的系统中，专门化是否是宿主联合的常见结果，这是微生物共生研究的一个重要前沿。在这项提议的背景下，我结合田间观察实验和实验室中的操纵实验来解决驱动植物叶细菌寄主范围的机制。这些调查针对三个短期目标。对于第一个目标，我将确定不同类型的植物特性在推动其细菌的寄主专门化或通用化方面的影响。通过采样超过50种菊科植物的细菌群落，我将具体评估叶片细菌与多个寄主物种的联系(即泛化)是受寄主之间资源供应的差异(例如叶片碳水化合物含量)的影响最大，还是受寄主防御化合物(例如酚类化合物)的差异的限制最大。对于第二个目标，我将测试细菌与多个寄主物种的关联是由表型可塑性驱动的，还是由可在寄主之间跟踪的一种类型资源的专门化驱动的。我的实验室将从目标一中描述的植物物种中培养细菌菌株，并进行表型分析，以确定它们可以代谢的碳的特性和多样性对其寄主范围的影响。对于第三个目标，我将开发一种新的微生物研究系统--植物标本--来表征植物叶片细菌群落组成的长期时间周转，作为对气候变化和城市化的反应。对来自与标本馆标本相同地点的活标本上的微生物进行测序，将能够比较它们随时间的组成。这项工作将涉及旧细菌DNA工作方案的开发。总体而言，我提供了高度创新和多方面的方法，以提高我们对植物-微生物共生的出现和持续的理解，对于预测这些关系在不断变化的环境中的命运具有重要的后果。

Symbiotic lifestyles are important components of microbial diversity across ecosystems. How associations with hosts are shaping microbial ecology and evolution has yet been studied most extensively for symbioses in which microbes depend on their host for survival (i.e. obligate symbioses). These studies have emphasized the importance of host association in driving the specialization of symbionts, sometimes leading to the co-diversification of symbionts with their hosts. Whether specialization is a common outcome of host association in systems where symbionts have frequent opportunities to interact with free-living habitats and diverse host species is an important frontier in the study of microbial symbioses. In the context of this proposal, I combine field observational experiments and manipulative experiments in the laboratory to address the mechanisms that drive host range in plant leaf bacteria. These investigations target three short-term objectives. For the first objective, I will determine the effect of different types of plant traits in driving the host specialization or generalization of their bacteria. By sampling bacterial communities from over 50 species of Asteraceae, I will specifically assess whether the association of leaf bacteria with multiple host species (i.e. generalization) is most constrained by variation in resource provision among hosts (e.g. leaf carbohydrate content) or by variation in the host defence compounds (e.g. phenolics). For the second objective, I will test if bacterial association with multiple host species is driven by phenotypic plasticity or by specialization on one type of resource that can be tracked among hosts. My lab will cultivate bacterial strains from plant species characterized in objective one and perform phenotypic assays to determine the influence of the identity and diversity of carbons they can metabolize on their host range. For the third objective, I will develop a novel microbial study system - herbarium specimens - to characterize long-term temporal turnover in bacterial community composition of plant leaves as a response to climate change and urbanization. Sequencing of microbes on live specimens from the same locations as the herbarium specimen will enable comparison of their composition through time. This work will involve the development of protocols for work with old bacterial DNA. Overall, I provide highly innovative and multi-faceted approaches to improve our understanding of the emergence and persistence of plant-microbe symbioses, with important consequences for predicting the fate of these relationships in changing environments.