Diverse suites of mycorrhizal fungi vary among populations of the rare Neotropical lady’s slipper orchid, Phragmipedium longifolium

Background/Question/Methods

Symbiotic mycorrhizal fungi are ubiquitous in plant communities. Understanding the composition of mycorrhizal communities could advance the conservation of rare or exploited plants. All orchid species have obligate symbioses with mycorrhizal fungi because orchid seeds cannot germinate without associating with an appropriate mycorrhizal fungus. In this study, we assess the identity of mycorrhizal fungi associating with the rare Neotropical lady’s slipper orchid, Phragmipedium longifolium, in northwest Costa Rica. We sampled root tissue from five plants in each of three populations that occur in distinct habitats, including a recent volcanic lava flow, pasture used for livestock, and a maturing second growth forest. Based on the extensive literature describing mycorrhizal associates of lady’s slipper orchids, we expected that P. longifolium associates with a narrow suite of orchid-mycorrhizal fungi from the family Tulasnellaceae. Since specialization of orchid species on particular mycorrhizal fungi tends to be evolutionarily conserved, we further expected that P. longifolium associates with the same type of habitat-generalist fungus in all populations. We used genetic barcodes to identify mycorrhizal fungi associating with P. longifolium and statistically compare fungal assemblages from each population using rarefaction of operational taxonomic units and the UniFrac phylogenetic analysis tool.

Results/Conclusions

We detected broad fungal diversity in association with P. longifolium, including common and unexpected types of orchid-mycorrhizas. Consistent with expectations, the Tulasnellaceae occur in all populations and comprise the most common type of mycorrhiza. Contrary to expectations, the phylogeny of the Tulasnellaceae reveals five separate tulasnelloid clades and UniFrac shows that assemblages of these clades statistically differ between the lava flow and forest population. Plants in the pasture associated with other possible orchid-mycorrhizas from the rust lineage Atractiellales and the jelly-fungi lineage Auriculariales. Plants in the forest associated with different possible orchid-mycorrhizas from the mushroom-forming lineage Agaricales. Overall, rarefaction shows that the lava flow population has less mycorrhizal diversity than other populations and UniFrac shows that the total mycorrhizal assemblage of each orchid population is phylogenetically unique. Thus, sampling fungi from multiple populations and habitats allowed detection of greater mycorrhizal diversity than sampling any single population. We suggest that unexpectedly high mycorrhizal diversity and different types of mycorrhizal associations among P. longifolium populations could be due to 1) opportunistic association with locally abundant fungi, 2) change in available fungal communities during ecological succession, and/or 3) evolutionary specialization of orchid populations on specific fungi.