The macroevolution of broad interactions: specificity and symbiont community are phylogenetically determined in lady’s slipper orchids (Cypripedium spp.)

Background/Question/Methods

Ecological interactions are a ubiquitous force behind much of the diversification of life. Understanding the factors that promote or limit species interactions is of fundamental importance for any efforts to predict ecological responses to changing global and local environments.  Of particular concern given the extremely high rate of environmental change most ecosystems are currently experiencing, is the extent to which ecological interactions are shaped over millions of years by evolutionary and co-evolutionary processes.  We developed a comprehensive phylogeny of the lady’s slipper orchid genus, Cypripedium, and used it to test whether the breadth and identities of communities symbiotic mycorrhizal fungi evolved in ways suggestive of specialization. Sampling involved collecting both leaf and mycorrhizal root tissue from ~600 plants from >200 populations of ~40 species worldwide. We tested for phylogenetic signal to the specificity of the interaction using the phylogenetic diversity (PD) of the mycorrhizal fungi from a well-resolved reference phylogeny of the Kingdom Fungi. We then tested for the macroevolutionary importance of the taxonomic identities and phylogenetic relationships of the mycorrhizal fungi  by assessing the phylogenetic community dissimilarity (PCD) among fungi associating with each Cypripedium taxon, using the fungal phylogeny as reference.

Results/Conclusions

Specificity, taxonomic identity, and phylogenetic relationships among mycorrhizal fungi associating with each Cypripedium species were conserved across the Cypripedium phylogeny, as indicated by significant bootstrapped Blomberg’s K and a significantly positive regression slope to plant taxon PD vs. fungal community PCD. However, a slight upward tick in the slope of the latter regression in sister species suggests the possibility of competitive exclusion in communities of mycorrhizal fungi. Because our Cypripedium phylogeny is nearly complete and we bootstrapped Blomberg’s K, our results are not biased by Type I error from incompletely resolved trees and inherent bias in Blomberg’s K. We suggest that mycorrhizal fungi fill niches among their associating plant hosts that are conserved among both mycorrhizal fungi and the plants. Further, given that these plants are sometimes seen as generalists, our results suggest that specialization constrains the evolution of interactions even with many associates.