LTREB: Collaborative Research: Host-microbe symbiosis through the lens of stochastic demography

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Nearly all plants and animals harbor microbes such as bacteria that live on or in them. Understanding the influence of these microscopic organisms on their host species may advance our ability to solve problems in agriculture, wildlife disease, and human health. Before these applications can be fully realized, biologists first require a better understanding of when and how microbes influence the fitness of their host. This project will test the hypothesis that the effects of microbes on their hosts fluctuate from year to year, being beneficial in stressful years (when hosts need assistance) but neutral or costly in favorable years. As a consequence, hosts with microbes may experience reduced year-to-year fluctuation in fitness compared to hosts without microbes, which may be an important benefit of harboring microbes. This hypothesis will be tested by extending data collection of a long-term field experiment with grass species that harbor fungal microbes, collecting new data on the genetics of the grasses in that experiment, an applying a statistical modeling approach. Fungal microbes that live inside plants are a widespread in grasses, including forage grasses that are important to livestock, and this research could have important agricultural applications. Additionally, this project will provide research training for students at the high school, undergraduate, and graduate levels.The hypothesis of reduced demographic variance in symbiotic hosts leads to two novel predictions that this project is designed to test. First, by buffering hosts against harsh conditions, symbionts may limit genetic drift and promote higher genetic diversity in host populations. Second, hosts with and without symbionts may exhibit different patterns of temporal fluctuation in their demographic rates, thus promoting the storage effect, a mechanism that can stabilize competitive coexistence of symbiotic and symbiont-free hosts, and potentially explain the mixtures of host types that are widely observed in a diversity of plant and animal symbioses. The scientific team will test these predictions using a long-term field experiment with sites in Texas and Indiana, in which grass populations were established with or without fungal endophytes. Building on nine years of demographic data and leveraging a hierarchical Bayesian analytical framework, the team will quantify and compare the effects of symbiosis on host fitness via reductions in variance versus effects on mean performance, and test the contributions of the storage effect to the coexistence of hosts with and without symbionts.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.