A common epiphytic orchid forms narrow, specific mycorrhizal associations

Background/Question/Methods

Despite its central importance in ecology, we still lack mechanistic understandings of the spatial distributions of most plant species. The Orchidaceae is one of the most species-rich plant families comprising approximately 10% of all angiosperm flora, and 70% of all orchids occur as epiphytes in tropical forests. All orchids are initially mycoheterotrophic (i.e., parasitic on fungi), requiring fungi for germination (protocorm development) of the dust-like seeds and their recruitment. These mycorrhizal associations can be highly specific or can exhibit low specificity but they persist throughout the life of an orchid. Our central objective was to determine whether fungal association in widely separated populations of a common epiphytic orchid species is host orchid species- or site-dependent. We determined the identity of mycorrhizal fungi of the twig orchid Trizeuxis falcata, and of other orchid species occurring on the same host tree branches, from sites across Costa Rica. Orchid mycorrhizal fungi from within the orchid roots were identified by sequencing the nuclear ribosomal ITS region. Primers designed for orchid fungi (ITS1-OF and ITS4-OF) were used for amplification and sequencing. Fungal identity analyses were conducted by utilizing www.borealfungi.uaf.edu, and phylogenetic analyses also were conducted to determine the relationships among all fungal sequences.

Results/Conclusions

Despite the fact that Trizeuxis falcata is distributed across central and southern Costa Rica and can grow on several different species of host trees (coffee, citrus, guava, etc.), it appears to associate specifically with a single narrow clade of fungi within the genus Ceratobasidium (Ceratobasidiaceae, Basidiomycota) without exception. The fungi of T. falcata also represent previously un-sequenced species (i.e. novel OTUs) in the Ceratobasidiaceae. Other orchid species sampled from the same host tree branches where T. falcata plants were collected yielded fungi belonging to Thanatephorus and Ceratobasidiaceae. Some of these associated orchid species that were sampled at multiple sites along with T. falcata were colonized by fungi representing two different fungal families. This result indicates that while its neighboring orchids may utilize a generalist approach in forming mycorrhizal associations, T. falcata is narrowly specific in its choice of fungi. Such niche-partitioning would be important in facilitating co-existence of multiple epiphytic orchids on single host trees. Our results also indicate that orchid-fungal specificity, or lack there-of, likely influences distributions of individual epiphytic orchid species in the hyper-diverse neotropical forests.