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

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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.