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

Nearly all plants and animals harbor microbes that live on or in them as “symbionts”. These include fungi that inhabit crop plants, bacteria that inhabit mosquitoes, and microbes that live inside the human gut. Harnessing these symbioses may advance the ability to solve problems in agriculture, wildlife disease, and human health.

Before these applications can be fully developed, environmental biologists first require a better understanding of when and how microbial symbionts influence the health of their host. This project is testing the hypothesis that the effects of symbionts on their host fluctuate from year to year, being beneficial in bad years (when hosts need assistance) but neutral or costly in good years (when hosts are okay on their own).

As a consequence, hosts with microbes may be more stable compared to hosts without microbes, and this may be an important but overlooked benefit of harboring microbes. The scientific team is exploring this idea with a unique long-term study of grasses and their fungal microbes in Texas and Indiana. Fungal endophytes are widespread microbial symbionts in grasses, including forage grasses that are important to ranchers and turf grasses used by landscapers, and this research will benefit these groups. The project will train undergraduate students and conduct outreach activities to high schools.

The project’s core data derive from a unique symbiont-removal experiment in which populations of cool-season grasses were established either symbiotically with seed-transmitted Epichloë endophytes or with symbionts eliminated through heat treatment. Replicated across seven host species and now running for 15-years with thousands of individuals, the experiment’s longitudinal demographic data reveal the fitness impacts of fungal symbionts on their host plants and how these impacts fluctuate in response to fluctuations in the environment.

The research team is using these data to build stochastic demographic models that address two novel hypotheses, rooted in population biology theory for fluctuating environments and testable only with long-term data. First, through context-dependent fitness effects (symbionts are more beneficial in more stressful years), microbial symbionts may reduce inter-annual variability in host demography.

By buffering hosts against harsh conditions, symbionts may also limit genetic drift and promote higher genetic diversity in host populations. Second, unique responses of symbiotic and symbiont-free hosts to environmental fluctuations can generate niche opportunities in time via the storage effect, possibly promoting stable mixtures of symbiotic and symbiont-free hosts.

By continuing this study for the next five years (providing a total of 20-years), the project will reveal for the first time how endophytes may help host plants cope with year-to-year fluctuations in climate.

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.