Mycorrhizal interactions play an important role in the assembly and stability of plant communities. In the case of orchids, all species depend on mycorrhizal fungi for seed germination and early seedling establishment. However, it is currently debated whether individual orchid species depend on specific or on a broad spectrum of orchid mycorrhizal fungi (OMF). In addition, the diversity and function of a diverse group of fungi, broadly considered as root-associated fungi (RAF), is poorly known.

We assessed the structure of RAF communities associated with sympatric epiphytic orchids in several geographic areas around Ecuador. To characterize RAF communities associated with each orchid species, the complete ITS-5.8S or the sole ITS2 region was sequenced by Sanger Sequencing-cloning or Illumina MiSeq technology. Fungal OTUs were delimited using 97% of sequence similarity.

Orchids hosted a high OTUs richness which included mycorrhizal and root associates without a defined role. Mycorrhizal fungi consisted of a polyphyletic group where Serendipitaceae and Tulasnellaceae were the richest groups followed by Ceratobasidiaceae and Atractiellales. Meanwhile, RAF involved a diverse array of fungi. Our results indicate that studied orchids have site-adjusted mycorrhizal communities structured around keystone mycorrhizal species. Interaction with multiple mycorrhizal fungi could favor orchid site occurrence and coexistence of many closely related orchid species. Further studies are necessary to elucidate the identity of fungal species and the roles of RAF communities in orchid life.