Is the Distribution of Two Rare Orchis Sister Species Limited by Their Main Mycobiont?

Jacopo Calevo,Samuele Voyron,Mariangela Girlanda,Enrico Ercole

doi:10.3390/d12070262

Jacopo Calevo, Samuele Voyron + Show 2 more

Open Access

https://doi.org/10.3390/d12070262

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Abstract

As orchids rely on their mycorrhizal fungi for nutrient supply, their spatial range is dependent on the distribution of orchid mycorrhizal (OM) fungi. We addressed possible correlations between mycorrhizal specificity and the geographic distribution of orchids and OM fungi in three populations of the rare sister species Orchis patens and O. canariensis. Metabarcoding of the fungal ITS2 region indicated that, although adult plants of either species were colonized by several ceratobasidioid, tulasnelloid, sebacinoid and serendipitoid fungi, the mycobiont spectra were dominated by Tulasnella helicospora (which occurred in 100% of examined plants with high read numbers), which is a globally distributed fungus. In vitro assays with a T. helicospora isolate obtained from O. patens indicated the effectiveness of this OM fungus at germinating seeds of its native host. At a local scale, higher read numbers for T. helicospora were found in soil samples collected underneath O. patens roots than at locations unoccupied by the orchid. Although these findings suggest that the geographical pattern of the main fungal symbiont does not limit the distribution of O. patens and O. canariensis at this scale, the actual causal link between orchid and OM fungal occurrence/abundance still needs to be better understood.

Highlights

Biotic interactions are usually only measured on local scales, so they are often seen as not playing an important role in determining the larger, broad-scale geographical distributions of taxa, which have traditionally been assumed as being determined mainly by abiotic variables, such as climatic and edaphic factors [1]
These sequences, which matched GenBank sequences of tulasnelloid fungi, exhibited a pairwise percentage identity of 97.6% and clustered together (RAxML analysis, data not shown), indicating that both the Orchis patens and Orchis canariensis analyzed plants were associated to the same tulasnelloid mycobiont
The ceratobasidioid, sebacinoid and serendipitoid fungi found in O. patens and O. canariensis were closely related either to fungi amplified from the roots of other orchid species, the roots of non-orchid plants or soil (Supplementary Information Figures S1 and S2)

Summary

Introduction

Biotic interactions are usually only measured on local (population) scales, so they are often seen as not playing an important role in determining the larger, broad-scale geographical distributions of taxa, which have traditionally been assumed as being determined mainly by abiotic variables, such as climatic and edaphic factors [1]. Orchids have associations with root-associating (orchid mycorrhizal, OM) fungi which they depend on to germinate from seed to a degree unparalleled in the plant kingdom [6]. Dependence on OM fungi is extreme in orchids, which rely on their mycobionts for seed germination and seedling establishment, and in most cases, they maintain associations with these fungi into adulthood as well [7]. OM associations exhibit a broad range of specificities and levels of dependency on fungi [8,9,10]. Specificity appears to be generally narrower in seedlings than in adult plants [12,13], and seedling mycobionts are usually a subset of the fungi colonizing

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Discussion

Conclusion