Latitudinal variation in mycorrhizal diversity associated with a European orchid

Aim: Biodiversity is known to be unevenly distributed along latitudinal gradients. While this pattern has been observed for many different organisms, it is unclear whether the distributions of ecologically important belowground mutualists, such as orchid mycorrhizal fungi (OMF), also vary according to latitude. Location: Europe. Taxon: Spiranthes spiralis (L. Chevall) Orchidaceae. Methods: We sampled 37 populations over a >3,000 km latitudinal gradient of the European orchid Spiranthes spiralis to test whether the diversity and community composition of OMF are influenced by latitude. For this, we sequenced the broad spectrum ITS3/4OF region to identify OMF and quantified environmental variables that may influence their occurrence. To assess whether seed germination was affected by suitable OMF in different regions, we performed a reciprocal translocation germination experiment at the northern, central and southern part of its range. Results: In total, 75 OMF taxa were associated with S. spiralis, the majority belonging to the Ceratobasidiaceae and Thelephoraceae. Both taxonomic and phylogenetic OMF diversity decreased with increasing latitude. Three Ceratobasidiaceae taxa were widespread, but the number of sequences of two of these taxa decreased with increasing latitude. Reciprocal translocation germination experiments revealed that S. spiralis seeds germinated in each region regardless of seed provenance, but germination rates were highest in the southern part of its range where mycorrhizal diversity was highest. Main conclusions: OMF diversity decreased with increasing latitude, indicating that communities of OMF may be shaped by latitudinal gradients. These results highlight the need to understand the factors that determine the distribution of belowground mutualists, which are essential to predict how plant distributions will shift under -future environmental change.