PREMISE: Orchids depend primarily on mycorrhizal fungi to obtain nutrients throughout their life cycle. Epiphytic orchids account for 69% of orchid diversity. The unstable availability of water and nutrients in their arboreal habitats often results in severe water and nutrient stresses. Consequently, mycorrhizal associations may be important for the survival of epiphytic orchids, but our understanding thereof remains limited. Here, we investigated the mycorrhizal community in a single epiphytic orchid species, using more samples than in any previous study.

METHODS: We assessed the mycorrhizal communities of Thrixspermum japonicum, one of the most common epiphytic orchids in the temperate region of Japan. In total, 144 individuals were collected from 28 host tree species at 20 sites across 1300 km. The mycorrhizal fungi were identified based on nuclear ribosomal DNA internal transcribed spacer sequences and assigned operational taxonomic units (OTUs) based on 97% sequence similarity.

RESULTS: We obtained 24 OTUs, of which 9 and 15 belonged to the Ceratobasidiaceae and Tulasnellaceae, respectively. These OTUs are widely distributed throughout the phylogenetic trees of the two fungal families. However, a single Ceratobasidiaceae OTU accounted for 49.7% of all fungal sequences and was predominant in samples from 15 host tree species and 12 sites.

CONCLUSIONS: Our results imply that despite having a broad range of mycorrhizal partners, T. japonicum was predominantly associated with a single fungal taxon at most of the sites among the host-tree species investigated. These findings contribute to elucidating mycorrhizal symbiosis in epiphytic ha

The nuclear ribosomal DNA sequences from the three data sets were aligned using ClustalW. Phylogenetic analysis was performed using MEGA7. The phylogenetic tree was obtained using the maximum likelihood method based on the Kimura two-parameter model. To estimate the relative robustness of the branches, the bootstrap method was used with 1000 replicates.