Mycorrhizal specificity in orchids: a multidimensional filtering framework.

Understanding how specificity emerges in mutualistic symbioses remains a central challenge in evolutionary ecology. In orchids, mycorrhizal specificity ranges from strict specialization to broad generalism. Although many orchids show low physiological specificity in vitro, natural populations associate with much narrower subsets of partners, indicating that realized specificity reflects constraints rather than compatibility alone. To provide a unified explanation and testable predictions for this discrepancy, we propose a multidimensional filtering framework that distinguishes physiological compatibility, representing the fundamental niche, from ecological filters that determine realized associations. Physiological compatibility, mediated by signal recognition and immune modulation, establishes the fundamental prerequisites for symbiosis. However, the transition to realized specificity is constrained by four hierarchical ecological filters. Phylogeny and biogeography first delineate the potential regional lineage pool, while local environmental heterogeneity restricts fungal availability. Subsequently, host-specific nutritional demands drive active partner selection, and interspecific competition enforces niche differentiation. This synthesis clarifies the observed discrepancy between fundamental and realized mycorrhizal niches and provides a conceptual foundation that may inform conservation strategies for endangered orchids through targeted ecological matching.