The role of vertical and horizontal transmission in the assembly of seed fungal endophyte communities in wheat and wheat wild relatives

Plants acquire fungal endophytes either from the environment or from their progenitors. These transmission modes are central in shaping the community as they affect species composition and balance. We studied fungal endophyte communities (FEC) and their seed-to-seed transmission in three Triticeae plant species: bread wheat (Triticum aestivum), wild emmer wheat (Triticum turgidum dicoccoides) and wild barley (Hordeum spontaneum). The FECs in the three plant species contained similar fungal taxa, however they were overall different. The most prevalent class of fungi was Dothideomycetes, which was dominated by the taxon Alternaria infectoria. In field collected plants, the number of taxa in the seeds was less than half the number in stems, with close to 90% of the taxa found in seeds also found in stems. Growing the same plant species in a controlled environment infection greatly affected their FEC composition; the FECs in the stems and seeds of these plants were richer and more diverse than in the original seeds, they were not dominated by a single taxon, and FECs in the new seeds had a similar richness and diversity to the stem FECs, with only 40% overlap. The controlled environment experiment confirmed vertical transmission of certain species, but also showed that external infection of the seeds is the main source for specific taxa, including A. infectoria. Collectively, our results show that many taxa can reach the seeds through the internal pathway, albeit in different abundance, and both internal and external sources significantly affect the composition of seed FECs.