Ecological dynamics of vertically transmitted symbionts in hosts with complex life histories

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Many plants and animals harbor microbial symbionts, i.e., microbes that live on or in individuals of a host species, which are transmitted from host parent to offspring. However, there is striking variation in the frequency of microbial symbiosis across host species and even among populations of the same species. In some cases, a microbial symbiont species is present in all individuals of a host species, leading to tight integration of the ecology and evolution of host and symbiont. In other cases, such as fungal symbionts living within plants, the symbiont occurs at intermediate frequencies, with symbiotic and symbiont-free individuals co-occurring within host populations. This project will address the question of why there is wide variation in the frequency of host-symbiont associations. Given the strong evolutionary and ecological impacts of microbial symbionts, answering this question is a major challenge in biology. This project will test the hypothesis that the life history traits of the host species can explain much of the observed variation in symbiosis. Theory suggests that the population dynamics of symbionts are determined by how they affect the fitness of their host and the efficiency of their transmission among individual hosts. Host species with more complex life histories (longer lifespan, repeated reproduction, different stages of development) have more opportunities to be affected by symbionts and to lose them. This project will develop new mathematical models to examine the influence of host life history traits on symbiont population dynamics, and use field experiments to test model predictions. Experimental work will focus on the well-studied model system of grasses and their fungal symbionts. Symbiont dynamics in annual host grasses will be compared to dynamics in perennial host grasses that have more complex life histories. The integration of theoretical and field-based research is expected to provide a transformative approach to understanding and predicting the dynamics of symbionts.A theoretical understanding of the dynamics of symbionts that are passed from parents to their offspring has broad application in conservation biology, ecosystem management and epidemiology. For example, global climate change has disrupted key symbioses, with warmer waters causing coral bleaching and drier climates altering the frequency and outcomes of animal and plant symbioses. This research will generate a framework for evaluating the responses of host-symbiont interactions to ongoing global change. In addition, this project will train students in using mathematical approaches to addressing fundamental questions in microbial ecology and evolution. Finally, the research team will develop hands-on projects for high school classrooms to illustrate concepts in the ecology and evolution of symbiosis.