The role of epiphytism in architecture and evolutionary constraint within mycorrhizal networks of tropical orchids

Abstract

Characterizing the architecture of bipartite networks is increasingly used as a framework to study biotic interactions within their ecological context and to assess the extent to which evolutionary constraint shape them. Orchid mycorrhizal symbioses are particularly interesting as they are viewed as more beneficial for plants than for fungi, a situation expected to result in an asymmetry of biological constraint. This study addressed the architecture and phylogenetic constraint in these associations in tropical context. We identified a bipartite network including 73 orchid species and 95 taxonomic units of mycorrhizal fungi across the natural habitats of Reunion Island. Unlike some recent evidence for nestedness in mycorrhizal symbioses, we found a highly modular architecture that largely reflected an ecological barrier between epiphytic and terrestrial subnetworks. By testing for phylogenetic signal, the overall signal was stronger for both partners in the epiphytic subnetwork. Moreover, in the subnetwork of epiphytic angraecoid orchids, the signal in orchid phylogeny was stronger than the signal in fungal phylogeny. Epiphytic associations are therefore more conservative and may co-evolve more than terrestrial ones. We suggest that such tighter phylogenetic specialization may have been driven by stressful life conditions in the epiphytic niches. In addition to paralleling recent insights into mycorrhizal networks, this study furthermore provides support for epiphytism as a major factor affecting ecological assemblage and evolutionary constraint in tropical mycorrhizal symbioses.

Interactions of fungi with other organisms

“…Not unexpectedly, the AM symbiosis revealed nested networks (MontesinosNavarro et al 2012). The same was found for ORM networks (Jacquemyn et al 2011b;Martos et al 2012), a particularly interesting result since the mutualistic nature of this symbiosis remains controversial. Further investigations on mycorrhizal networks as bipartite networks are awaited, especially to study responses of the network to disturbance or environmental changes, since they may be crucial parameters for ecosystems resilience.…”

Enigmatic Sebacinales

“…In A. laxiflora and S. vomeracea sebacinoid ORMs were detected, in the latter one also by Weiß et al (2011). In La Réunion, Sebacinales represented 9.5 % of the ORM fungi on the 28 terrestrial orchid species investigated by Martos et al (2012).…”

“…Besides tulasnelloid fungi, Group B Sebacinales were found in ORMs of epiphytic orchids in an Andean cloud forest (Suárez et al 2006), and at La Réunion, Sebacinales represented 23 % of the symbionts in the 45 epiphytic species investigated by Martos et al (2012). Epiphytic orchids from Ecuadorian mountain rainforests have Group B Sebacinales mycobionts , forming mycorrhizae that were proven ultrastructurally in Stelis spp.…”

Influence of Habitat and Climate Variables on Arbuscular Mycorrhizal Fungus Community Distribution, as Revealed by a Case Study of Facultative Plant Epiphytism under Semiarid Conditions

“…However, later reports found abundant mycorrhizal 7) showing a significant mycorrhizal presence in epiphytic life forms from tropical forests. The mycorrhizal status of epiphytic vascular plants has only been widely studied and discussed in the family Orchidaceae (22)(23)(24)(25)(26), but orchids generally form mycorrhizas with Basidiomycota (27)(28)(29), and no clear evidence exists for arbuscular mycorrhiza colonizing them. In humid tropical forests, trees show plenty of vascular epiphytes on their branches and trunks, but little is known about vectors for AMF dispersion in these habitats.…”