Orchid mycorrhizal communities associated with Orchis italica are shaped by ecological factors and geographical gradients

AimThe influence of mutualists on plant distributions is only beginning to be understood. Orchids depend on orchid mycorrhizal (OrM) fungi to germinate, yet the distribution of OrM and how they vary according to both abiotic and biotic factors is unclear. We investigated the abundance and diversity of OrM communities associated with the Mediterranean orchid Orchis italica and quantified how they vary according to both geographical and ecological factors. LocationMediterranean Basin. TaxonOrchis italica Poir. (Orchidaceae) MethodsWe used metabarcoding of the ITS2 region to identify OrM fungi associated with adult individuals in 23 populations of O. italica across latitudinal and longitudinal gradients in the Mediterranean region. We used both multivariate analyses and Joint Species Distribution Models (JSDMs) based on geographical, climate, and soil variables to test how both common OrM fungi and their communities vary according to geographical and ecological factors. ResultsEighty OrM OTUs were found associating with O. italica. However, five Tulasnellaceae OTUs and one Ceratobasidiaceae OTU were found in every population. Abundance of these taxa, as measured by number of reads, increased from west to east and decreased from south to north, indicating OrM abundance may be determined by geographical gradients. OrM community composition varied according to precipitation, annual mean temperature, and soil phosphorous content. JSDMs revealed there were both positive and negative co-occurrences among these ubiquitous OrM. Main ConclusionsDespite associating with many OrM across its range, only six OrM were widespread, indicating that O. italica may be an apparent generalist in its association with OrM. Abundance of these OrM is determined by geographical gradients and ecological factors. This highlights the importance of quantifying the distribution of belowground mutualists in understanding the limits to plant distributions.