Appreciation of the effects of microbial symbionts on host demography and population dynamics is growing. For vertically-transmitted, heritable symbionts it is not well understood what controls their prevalence in host populations. Frequently, heritable symbionts are observed at intermediate frequencies in nature, suggesting a tension between the cost of the interaction on the host and vertical transmission. Using a space-for-time substitution experiment over three years in the field, we examined the effects of context dependency on the prevalence of a vertically transmitted fungal endophyte within its grass host, Agrostis hyemalis. We determined the endophyte equilibrium frequency to ask if fungal endophyte frequencies are stable in nature. The context dependency treatment was composed of water addition and water control plots to parse out the effects of water on symbiont prevalence. We examined climate data and plant vital rate data, specifically vertical transmission of the endophyte, survival, and reproduction to understand the drivers of endophyte prevalence in this population.
Results/Conclusions
We found high inter-annual variability in endophyte relationship strength, which may explain the variability commonly seen in nature, effects of climatic drivers were important in some years, and non-water related benefits were also important in some years. Additional analyses of plant host demographic data to understand how symbiont prevalence is controlled are ongoing. This work integrates microbial symbionts with host demography and population dynamics to understand symbiont prevalence stability and how it is controlled within host populations.