The impact of life form on the architecture of orchid mycorrhizal networks in tropical forest

Abstract

Understanding the processes that determine the architecture of interaction networks represents a major challenge in ecology and evolutionary biology. One of the most important interactions involving plants is the interaction between plants and mycorrhizal fungi. While there is a mounting body of research that has studied the architecture of plant–fungus interaction networks, less is known about the potential factors that drive network architecture. In this study, we described the architecture of the network of interactions between mycorrhizal fungi and 44 orchid species that represented different life forms and co‐occurred in tropical forest and assessed the relative importance of ecological, evolutionary and co‐evolutionary mechanisms determining network architecture. We found 87 different fungal operational taxonomic units (OTUs), most of which were members of the Tulasnellaceae. Most orchid species associated with multiple fungi simultaneously, indicating that extreme host selectivity was rare. However, an increasing specificity towards Tulasnellaceae fungal associates from terrestrial to epiphytic and lithophytic orchids was observed. The network of interactions showed an association pattern that was significantly modular (M = 0.7389, Mrandom = 0.6998) and nested (NODF = 5.53, p < 0.05). Terrestrial orchids had almost no links to modules containing epiphytic or lithophytic orchids, while modules containing epiphytic orchids also contained lithophytic orchids. Within each life form several modules were observed, suggesting that the processes that organize orchid–fungus interactions are independent of life form. The overall phylogenetic signal for both partners in the interaction network was very weak. Overall, these results indicate that tropical orchids associate with a wide number of mycorrhizal fungi and that ecological rather than phylogenetic constraints determine network architecture.

Generalized mycorrhizal interactions and fungal enemy release drive range expansion of orchids in southern Florida

“…Differences between epiphytic and terrestrial rhizosphere are also considered to be a primary factor influencing mycorrhizal associations (Xing et al 2019). Unfortunately, a strictly epiphytic comparison could not be made here, because there are no other purely epiphytic Cyrtopodium species that are naturalized in southern Florida.…”

“…For terrestrial orchids, some of the same OTUs were detected over geographically distant sites (i.e., SE and SW Florida), and in different habitat types. Although there has been a recent surge in studies examining the relationship between habitat type variation and mycorrhizal associations, overall patterns remain unclear (Jacquemyn et al 2016, 2017, Xing et al 2019).…”

Similar mycorrhizal fungal communities associated with epiphytic and lithophytic orchids of Coelogyne corymbosa

“…A previous study has reported OMF have highly modular architecture, reflecting an ecological barrier between epiphytic and terrestrial subnetworks on a paleotropic island ( Martos et al., 2012 ). Other studies have also reported OMF specificities (mostly Sebacinales and Tulasnellaceae), differing between terrestrial, epiphytic, and lithophytic habitats ( Oja et al., 2015 ; Těšitelová et al., 2015 ; Xing et al., 2015 , 2019 ). Because studies on the relationships among OMF compositions and orchid life forms were all conducted in tropical areas, it remains unclear if individual epiphytic and lithophytic orchids harbor different OMF at higher elevations.…”

“…Coelogyne orchids can be both epiphytic and lithophytic at the same site and are always colonized by mycorrhizal fungi. Some Coelogyne species have therefore been studied to assess correlations of OMF composition and life form ( Xing et al., 2015 , 2019 ). Many Coelogyne species are also of high ornamental value, and wild populations of this genus have been threatened by over-collection and habitat destruction.…”

“…Studies on the OMF of Coelogyne orchids have been mainly conducted on two tropical species— Coelogyne viscosa Rchb. f. and Coelogyne ovalis Lindl., and have been based on Sanger sequencing of randomly picked clones from amplicon libraries of the ITS-rDNA region ( Xing et al., 2015 , 2019 ). The OMF Operational Taxonomic Units (OTUs) have been mostly identified as fungi of Sebacinales and Tulasnellaceae, with only a few OTUs designated as Ceratobasidiaceae.…”