Habitat-driven variation in mycorrhizal communities in the terrestrial orchid genus Dactylorhiza

Abstract

Orchid species are critically dependent on mycorrhizal fungi for completion of their life cycle, particularly during the early stages of their development when nutritional resources are scarce. As such, orchid mycorrhizal fungi play an important role in the population dynamics, abundance, and spatial distribution of orchid species. However, less is known about the ecology and distribution of orchid mycorrhizal fungi. In this study, we used 454 amplicon pyrosequencing to investigate ecological and geographic variation in mycorrhizal associations in fourteen species of the orchid genus Dactylorhiza. More specifically, we tested the hypothesis that variation in orchid mycorrhizal communities resulted primarily from differences in habitat conditions where the species were growing. The results showed that all investigated Dactylorhiza species associated with a large number of fungal OTUs, the majority belonging to the Tulasnellaceae, Ceratobasidiaceae and Sebacinales. Mycorrhizal specificity was low, but significant variation in mycorrhizal community composition was observed between species inhabiting different ecological habitats. Although several fungi had a broad geographic distribution, Species Indicator Analysis revealed some fungi that were characteristic for specific habitats. Overall, these results indicate that orchid mycorrhizal fungi may have a broad geographic distribution, but that their occurrence is bounded by specific habitat conditions.

Figure 1. Diversity of putative orchid mycorrhizal fungi detected in the roots of fourteen Dactylorhiza species sampled in 35 populations across Europe.

(a) The number of OTUs belonging to different orchid families/genera. (b) Pie chart displaying the frequency distribution of sequences belonging to the different families/genera.

Figure 2. Partitioning of mycorrhizal communities detected in fourteen different Dactylorhiza species sampled in 35 populations across Europe.

(a) Venn diagram showing the number of OTUs that are shared between diploid (2x), triploid (3x) and tetraploid (4x) Dactylorhiza species. (b) Nonmetric multidimensional scaling (NMDS) plot of mycorrhizal fungi. Each point denotes a different Dactylorhiza species. Different colors denote the habitats from which the species were sampled.

Figure 3. Matrix representation of the studied orchid mycorrhizal network encompassing fourteen Dactylorhiza species (rows) and putative orchid mycorrhizal operational taxonomic units (OTUs) (columns).

Different colors represent different modules. Red cells are species links gluing the six modules together into a coherent network, and non-red cells are links within modules.