All plants are colonized by fungal endophytes, many of which have been shown to play a critical role in host fitness. However, the ecological factors driving colonization, community composition, and downstream host-symbiont interactions of this group are poorly understood. Furthermore, there is little knowledge on fungal endophytes that colonize ferns—the second largest group of vascular plants. To better understand how early stages of symbiosis influence endophyte community composition over time, we examined the initial colonization of fungal endophytes in newly emerged leaf tissue of a temperate fern, Polystichum munitum (western sword fern) and also analyzed changes in community composition, monthly over 10 months. We sampled leaflet tissue from 20 plants in a natural population of P. munitum in the Oregon Coast Range upon initial emergence in the spring and every month thereafter. Leaflets were surface sterilized, total DNA was extracted, and barcoded amplicons of fungal DNA were sequenced on the Illumina MiSeq platform in an effort to examine the importance of priority effects and document the temporal turnover in this vastly understudied group of endosymbionts.
Results/Conclusions
A diverse assemblage of fungal endophytes colonizes newly emerged fern tissue in a patchy manner, but after 30 days, the community composition shifts to favor a single taxon. Taxonomic assignment of this fungus indicates that Flagellospora fusarioides (Nectriaceae) is the best match at 87% sequence similarity. Species accumulation curves reveal increasing OTU richness in endophyte communities until the summer months, when F. fusarioides becomes the most dominant in the fern population; this trend continues through the winter. These results call into question the importance of priority effects in endophyte communities. Since the study site resides in a dense understory of an established forest, there is likely an abundance of fungal propagules that promote colonization of fern hosts. If the host tissue is assumed to be relatively homogenous across the study site, a competition/colonization trade-off seems to be occurring, whereby F. fusarioides may counteract the effects of dispersal from competitors. Such a sharp shift in community composition favoring a single fungal taxon implies a greater importance of competition over dispersal in endophyte communities. Finally, these results call into question the common practice of relying on single sampling events to characterize the community composition of this dynamic group of microbes.