Context-dependent variability in the population prevalence and individual fitness effects of plant-fungal symbiosis

1. Heritable symbionts, found within a diverse array of flora and fauna, are often observed at intermediate prevalence within host populations, despite expectations that positive fitness feedbacks should drive beneficial symbionts to fixation. Intermediate prevalence may reflect neutral dynamics of symbionts with weak fitness effects, transient dynamics of symbionts trending toward fixation (or elimination), or a stable intermediate outcome determined by the balance of fitness effects and failed symbiont transmission. Theory suggests these outcomes should depend on symbiont-conferred effects and vertical transmission efficiency, which may both depend on environmental context.

2. We tested these alternatives by tracking temporal change in population-level symbiont prevalence in a three-year field experiment with grasses and vertically transmitted fungal endophytes under the context of manipulated precipitation. Precipitation can mediate endophyte effects on host grasses, and is a rele...