Plant abundance, but not plant evolutionary history, shapes patterns of host specificity in foliar fungal endophytes

Understanding the origins and maintenance of host specificity, or why horizontally-acquired symbionts associate with some hosts but not others, remains elusive. In this study, we explored whether patterns of host specificity in foliar fungal endophytes, a guild of highly diverse fungi that occur within the photosynthetic tissues of all major plant lineages, were related to characteristics of the plant community. We comprehensively sampled all plant host species within a single community and tested the relationship between plant abundance or plant evolutionary relatedness and metrics of endophyte host specificity. We quantified host specificity with methods that considered the total endophyte community per plant host (i.e., multivariate methods) along with species-based methods (i.e., univariate metrics) that considered host specificity from the perspective of each endophyte. Univariate host specificity metrics quantified plant alpha-diversity (structural specificity), plant beta-diversity (beta-specificity), and plant phylogenetic diversity (phylogenetic specificity) per endophyte. We standardized the effect sizes of univariate host specificity metrics to randomized distributions to avoid spurious correlations between host specificity metrics and endophyte abundance. We found that more abundant plant species harbored endophytes that occupied fewer plant species (higher structural specificity) and were consistently found in the same plant species across the landscape (higher beta-specificity). There was no relationship between plant phylogenetic distance and endophyte community dissimilarity. We still found that endophyte community composition significantly varied among plant species, families, and major groups, supporting a plant identity effect. In particular, endophytes in angiosperm lineages associated with narrower phylogenetic breadths of plants (higher phylogenetic specificity) compared to endophytes within conifer and fern lineages. Overall, an effect of plant species abundance may help explain why horizontally-transmitted endophytes vary geographically within host species ranges.

Automated summary

This study explored the relationship between patterns of host specificity in foliar fungal endophytes and characteristics of the plant community. The results showed that more abundant plant species harbored fewer endophyte species and that these endophytes were consistently found in the same plant species across the landscape. There was no relationship between plant phylogenetic distance and endophyte community dissimilarity.