Interplay between abiotic conditions and mycorrhizal abundance determines differentiation and potential adaptation in a Mediterranean orchid

BACKGROUND AND AIMS: Disentangling the relative influence of abiotic and biotic factors on plant population differentiation remains a major challenge in evolutionary ecology yet is essential for understanding the drivers of local adaptation. Orchids are suitable for addressing this question since they often occur in patchily distributed populations, and all orchids depend on orchid mycorrhizal (OrM) fungi for seed germination and establishment. Hence, local abiotic conditions together with OrM fungi may influence population differentiation and adaptation. METHODS: Based on 316,952 polymorphic SNPs sampled from 21 populations across the central Mediterranean range of Orchis italica Poir., we performed a suite of analyses to test how population differentiation and potential adaptation are influenced by the interplay between abiotic factors and OrM fungi. Outlier detection and functional annotation were performed on a subset of SNPs aligned to the Ophrys sphegodes Mill. reference genome. RESULTS: We found strong differentiation at the regional level, while loci under selection were associated with temperature, precipitation regime, soil texture, and the abundance of specific OrM fungal taxa. Functional annotation revealed that outlier SNPs were enriched for genes involved in transcription and RNA processing, signal transduction, stress response and metabolic processes, including in the presence of OrM fungi. CONCLUSIONS: Abiotic conditions and OrM fungi jointly determine differentiation in O. italica. Identifying the selective pressures underlying differentiation and adaptive variation is critical for understanding plant responses to ongoing environmental change.