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.