Vol.:(0123456789) 1 3 Oecologia https://doi.org/10.1007/s00442-019-04388-y PLANT-MICROBE-ANIMAL INTERACTIONS – ORIGINAL RESEARCH Biotic filtering of endophytic fungal communities in Bromus tectorum Kevin D. Ricks 1,2  · Roger T. Koide 1 Received: 10 March 2019 / Accepted: 19 March 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The assembly of horizontally transmitted endophytic fungi within plant tissues may be affected by “biotic filtering”. In other words, only particular endophytic fungal taxa from the available inoculum pool may be able to colonize a given plant species. We tested that hypothesis in Bromus tectorum, an important invasive species in the arid, western United States. We collected seed from Bromus tectorum and sources of inoculum for endophytic fungi including soil and various kinds of plant litter at a field site in central Utah. We characterized, using Illumina sequencing, the endophytic fungal communities in the various inoculum sources, inoculated Bromus tectorum seedlings under gnotobiotic conditions with the various sources, and then characterized the communities of endophytic fungi that assembled in their roots and leaves. Different inoculum sources containing significantly different endophytic fungal communities produced complex communities of endophytic fungi in leaves and roots of Bromus tectorum. In leaves, the communities assembling from the various inoculum sources were not significantly different from each other and, in roots, they were only slightly different from each other, mainly due to variation in a single fungal OTU, Coprinopsis brunneofibrillosa. Consequently, there was significantly more variation in the structure of the communities of endophytic fungi among the inoculum sources than in the resultant endophytic fungal communities in the leaves and roots of Bromus tectorum. These results are consistent with biotic filtering playing a significant role in endophytic fungal community assembly. Keywords Community assembly · Horizontal transmission · Inoculum · Plant–microbe interactions · Symbiosis Introduction Endophytic fungi are an important group of plant symbionts, which have been found in all plant species investigated thus far (Arnold et al. 2000; Rodriguez et al. 2009). Plants can be colonized by complex communities of endophytic fungi composed of hundreds of species (Arnold 2007; Rodriguez et al. 2009; Arnold and Lutzoni 2013). Because individual endophytic fungal taxa range from mutualists (Redman 2002; Rodriguez and Redman 2008; Rodriguez et al. 2009), to latent pathogens (Saikkonen et al. 1998; Delaye et al. 2013) and latent saprotrophs (Promputtha et al. 2007; Szink et al. 2016), the structure of the endophytic fungal com- munity is likely to influence host performance. Identifying factors that affect endophytic fungal community assembly, therefore, may help us understand an important controller of plant fitness. In particular we ask whether assembled endo- phytic fungal communities simply reflect the structure of the inoculum community (the result of a neutral process), or whether they are influenced by biotic filtering and are thus partially independent of the inoculum community structure. Some endophytic fungi are transmitted vertically from propagules carried by the seed, but many are acquired hori- zontally (Schardl et al. 2004) via propagules present in the environment (Christian et al. 2015). In a given plant commu- nity, there are several sources of inoculum for horizontally transmitted endophytic fungi including soil and litter from various plant species (Kaneko and Kakishima 2001; Chris- tian et al. 2015), each of which host fungal communities Communicated by Corné Pieterse. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00442-019-04388-y) contains supplementary material, which is available to authorized users. * Kevin D. Ricks ricks3@illinois.edu 1 Department of Biology, Brigham Young University, Provo, UT 84663, USA 2 Present Address: Program in Ecology, Evolution and Conservation Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA