Interplay between Endophyte Prevalence, Effects and Transmission: Insights from a Natural Grass Population

Plant performance response to eight different types of symbiosis

Almost all plant species interact with one or more symbioses somewhere within their distribution range. Bringing together plant trait data and growth responses to symbioses spanning 552 plant species, we provide for the first time on a large scale (597 studies) a quantitative synthesis on plant performance differences between eight major types of symbiosis, including mycorrhizas, N-fixing bacteria, fungal endophytes and ant-plant interactions. Frequency distributions of plant growth responses varied considerably between different types of symbiosis, in terms of both mean effect and 'risk', defined here as percentage of experiments reporting a negative effect of symbiosis on plants. Contrary to expectation, plant traits were poor predictors of growth response across and within all eight symbiotic associations. Our analysis showed no systematic additive effect when a host plant engaged in two functionally different symbioses. This synthesis suggests that plant species' ecological strategies have little effect in determining the influence of a symbiosis on host plant growth. Reliable quantification of differences in plant performance across symbioses will prove valuable for developing general hypotheses on how species become engaged in mutualisms without a guarantee of net returns.

Testing the roles of vertical transmission and drought stress in the prevalence of heritable fungal endophytes in annual grass populations

Beneficial inherited symbionts are expected to reach high prevalence in host populations, yet many are observed at intermediate prevalence. Theory predicts that a balance of fitness benefits and efficiency of vertical transmission may interact to stabilize intermediate prevalence. We established populations of grass hosts (Lolium multiflorum) that varied in prevalence of a heritable fungal endophyte (Epichloё occultans), allowing us to infer long-term equilibria by tracking change in prevalence over one generation. We manipulated an environmental stressor (elevated precipitation), which we hypothesized would reduce the fitness benefits of symbiosis, and altered the efficiency of vertical transmission by replacing endophyte-positive seeds with endophyte-free seeds. Endophytes and elevated precipitation both increased host fitness, but symbiont effects were not stronger in the drier treatment, suggesting that benefits of symbiosis were unrelated to drought tolerance. Reduced transmission suppressed the inferred equilibrium prevalence from 42.6% to 11.7%. However, elevated precipitation did not modify prevalence, consistent with the result that it did not modify fitness benefits. Our results demonstrate that failed transmission can influence the prevalence of heritable microbes and that intermediate prevalence can be a stable equilibrium due to forces that allow symbionts to increase (fitness benefits) but prevent them from reaching fixation (failed transmission).

Variation in the Prevalence and Transmission of Heritable Symbionts Across Host Populations in Heterogeneous Environments

Heritable microbes are abundant in nature and influential to their hosts and the communities in which they reside. However, drivers of variability in the prevalence of heritable symbionts and their rates of transmission are poorly resolved, particularly across host populations experiencing variable biotic and abiotic environments. To fill these gaps, we surveyed 25 populations of two native grasses (Elymus virginicus and Elymus canadensis) across the southern Great Plains (USA). Both grass species host heritable endophytic fungi (genus Epichloё) and can hybridize where their ranges overlap. From a subset of hosts, we characterized endophyte genotype using genetic loci that link to bioactive alkaloid production. First, we found mean vertical transmission rates and population-level prevalence were positively correlated, specifically for E. virginicus. However, both endophyte prevalence and transmission varied substantially across populations and did not strongly correlate with abiotic variables, with one exception: endophyte prevalence decreased as drought stress decreased for E. virginicus hosts. Second, we evaluated the potential influence of biotic factors and found that, after accounting for climate, endophyte genotype explained significant variation in symbiont inheritance. We also contrasted populations where host species co-occurred in sympatry vs. allopatry. Sympatry could potentially increase interspecific hybridization, but this variable did not associate with patterns of symbiont prevalence or transmission success. Our results reveal substantial variability in symbiont prevalence and transmission across host populations and identify symbiont genotype, and to a lesser extent, the abiotic environment as sources of this variation.

The Role of Host Demographic Storage in the Ecological Dynamics of Heritable Symbionts

Heritable symbioses are widespread and ecologically important. Many host organisms have complex life cycles that include diverse opportunities for symbionts to affect their host and be lost during development. Yet, existing theory takes a simplified view of host demography. Here, we generalize symbiosis theory to understand how demographic "storage" in the form of dormant or prereproductive life stages can modify symbiosis dynamics. Using grass-endophyte symbioses as context, we developed models to contrast the role of the seed bank (a storage stage) against the reproductive stage in symbiont persistence and prevalence. We find that the seed bank is as important as or more important than the reproductive stage in driving symbiont dynamics, as long as passage through the seed bank is obligate. Flexible entry to the seed bank substantially weakens its influence on symbiont persistence but can modify prevalence in counterintuitive ways. Our models identify a role for legacy effects, where hosts that lose symbionts retain their demographic influence. The retention of benefits via legacy effects can reduce symbiont prevalence and even cause prevalence to decline with increasing benefits to hosts because symbiont-free hosts carry those benefits. Our results resolve connections between individual-level host-symbiont interactions and population-level patterns, providing guidance for empirical studies.