A majority of orchid species is thought to require mycorrhizal fungal associations to provide nutrients for seed germination and early protocorm development. Fungi have been shown to be a key source for phosphorus, nitrogen and water in adult plants, provding evidence of the need for mycobionts throughout the plant’s life. Nevertheless, few studies have examined the diversity of the fungal community of epiphytic orchids in tropical forests and its dynamics through time.
This study aims to1) identify the fungal diversity associated with three tropical, epiphytic orchid species; 2) evaluate the dynamics of fungal composition across a two-year period. A riparian section of premontane rainforest in Tapantí National Park, Costa Rica, was selected. Five to seven individuals of Dichaea fragrantissima, Epidendrum odontochilum and Oncidium klotzchianum were sampled in July of 2015 and 2016. Root samples were cleaned, DNA was extracted and mycorrhizal identity was determined using the nuclear ribosomal ITS. Sequencing was done via standard Sanger methods and sequences were blasted against GenBank and the UNITE database. Operational Taxonomic Units (OTUs) were constructed using CD Hit configured at 0.96 similarity.
Thirty OTUs were determined. Preliminary results show that fungi in the three species belong to several clades of Basidiomycota in the orders Cantharellales, Atractiellales and Trechisporales. Mycorrhizal basidiomycetes with affinities to the Trechisporales have been found in Ericaceae, while a Trechisporales member was identified in the roots of the nonphotosynthetic orchid Wullschlaegelia aphylla, but no previous reports in a photosynthetic orchid are known. Tulasnellaceae (in Cantharellales) was the most species rich and most abundant group among the three orchids. Members of the Cantharellales and Atractiellales were found in D. fragrantissima and O. klotzschianum, while E. odontochilum had mycobionts only from Cantharellales and Trechisporales. Mycorrhizal diversity was higher in the common O. klotzschianum (20 OTUs recognized), while D. fragrantissima and E. odontochilum, considered rare species in the study site, hosted 9 and 7 OTUs respectively. These results suggest a broader mutualistic interaction in O. klotzschianum, which may confer more opportunities for establishment due to a higher likelihood of encountering a compatible fungus, and narrow associations in the other two species, which may favor growth and competitive dominance. Dichaea fragrantissima and E. odontochilum have 3 and 2 mycobionts in common with O. klotzschianum, respectively, but none between them. Within each species, less than three mycobionts were shared between years, showing high dynamism of the relationships between orchids and their mycorrhizal composition through time.