Polystichum munitum ferns host highly-specialized endophyte communities across four unique habitat types

Fungal endophytes are ubiquitous plant endosymbionts with important effects on host fitness and plant distributions, but the patterns of endophyte community composition across spatial gradients are poorly described. Using a temperate fern host system (Western swordfern; Polystichum munitum), we explored the extent of spatial turnover and patterns of host specificity of endophyte communities in four unique habitat types spanning 150 km at a single point in time. Specifically, we sampled foliar tissue from individual fern hosts and the two closest neighboring plants belonging to different species and determined endophyte community composition through culture-independent methods on the Illumina platform. We observed a significant degree of spatial community turnover across the habitat types. Interestingly, spatial turnover was not as pronounced in the fern hosts compared to neighboring plant species. Instead, ferns harbored more similar endophyte communities to one another across the range than to the nearest sympatric plants. Additionally, the fern endophyte communities were not nested subsets of the local or regional endophyte pool, leading us to conclude that a high degree of host specificity is occurring. These results document a striking example of specialized symbioses between a plant host and its fungal endophytes. Whether such a high degree of specificity is found in other plant systems, more broadly, deserves clarification.