The host bias of three epiphytic Aeridinae orchid species is reflected, but not explained, by mycorrhizal fungal associations

 Premise of the study: The three co-occurring epiphytic orchid species, Sarcochilus hillii , Plectorrhiza tridentata , and Sarcochilus parvifl orus vary in host specifi city; all are found predominantly on the tree Backhousia myrtifolia but some also associate with a broad range of species. Despite this specialization, no fiadvantage has been detected for adult orchid plants growing on the preferred host. Therefore, we predicted that the host specialization of these orchid species is a consequence of a bias toward particular orchid mycorrhizal fungi, which are in turn biased toward particular woody plant species.  Methods: To test this hypothesis, we sampled representatives of each orchid species on B. myrtifolia and other host species across sites. Rhizoctonia -like fungi were isolated from orchid roots and identifi ed using molecular markers.  Key results: Three groups of fungi were identifi ed, and the orchid species varied in their specifi city for these. All fungal groups were found on the host B. myrtifolia ; yet at all sites, only one orchid species, S. hillii , associated with all three groups.  Conclusions: Our results demonstrate that these orchid species did vary in their mycorrhizal specifi city; however, the distribution of their mycorrhizal associates did not directly explain their host associations. Rather, we propose that the mycorrhizal relationship of these orchid species is complex and have suggested future avenues of research.