Fungal endophytes are critical members of the plant microbiome, but their community dynamics throughout an entire growing season are underexplored. Additionally, most fungal endophyte research has centered on seed-reproducing hosts, while spore-reproducing plants spend a significant proportion of their lifecycle in a haploid state and may filter vertically-transmitted fungi. In order to examine annual fungal endophyte community dynamics in a spore-reproducing host, we examined endophytes in a single population of ferns, Polystichum munitum, in the Pacific Northwest. Through culture-independent sequencing, we characterized the community assembly and temporal turnover of foliar endophytes throughout a growing season. From these results, we selected endophytes with outsized representations in sequence data and performed in vitro competition assays. Finally, we inoculated sterile fern gametophytes with dominant fungi observed in the field and determined their effects on host performance. Sequencing demonstrated that ferns were colonized by a diverse community of fungal endophytes in newly-emerged tissue, but diversity decreased throughout the season leading to the preponderance of a single fungus in later sampling months. This previously-undescribed endophyte appears to form specialized associations with the fern. While competition assays on a variety of media types failed to demonstrate that the dominant fungus was competitive against other fungi isolated from the same hosts, inoculation onto sterile fern gametophytes resulted in increased host growth suggesting mutualistic effects. When considering the potential for other fungi to act antagonistically on ferns in nature, the presence of this fungus may drive host success within the study site and possibly throughout its growing range.

Catenasporaceae, endophyte competition, fern gametophyte, fungal endophyte, priority effects, temporal turnover