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.

Almost all orchids rely on mycorrhizal fungus to support their seed germination. To date, the effect of active components in mycorrhizal fungus on orchid seed germination largely remains unknown. In this study, we aimed to investigate the impact of active components found in mycorrhizal fungus on orchid seed germination. Specifically, we focused on a terrestrial orchid Gymnadenia conopsea and its host-specific seed germination supporting fungus Ceratobasidium GS2. In total, several steroids (1–7) were isolated from this fungus. Notably, compounds 1, 2, 4, and 5 exhibited significant enhancements in protocorm volume. Moreover, compounds 1–6 demonstrated strong promotion of protocorm differentiation. These findings suggest that steroids may play a crucial role in the symbiotic germination of G. conopsea seeds. Future studies should continue to explore the specific mechanisms through which these steroids exert their effects, contributing to our understanding of orchid biology and mycorrhizal interaction.

Global biodiversity loss and mass extinction of species have raised serious global concerns, especially in fragile ecosystems. Karst forest, a well-known fragile ecosystem, is served as a natural laboratory for biogeographical, ecological, evolutionary, and taxonomic research. However, species richness and distribution patterns in karst forests largely remain unclear. Orchidaceae has been regarded as the "flagship" group of plant diversity. In this study, we use Wanfengshan Nature Reserve in southeast Yunnan, China, as a model to determine the conservation status of orchids in the karst forest by conducting systematic field surveys in the past three years from 2019 to 2022. In total, we identified 78 orchid species in 35 genera in Wanfengshan Nature Reserve. The dominant species were Bulbophyllum andersonii, Eria coronaria, Nervilia mackinnonii and Paphiopedilum micranthum. Multiple life forms were also observed, including epiphyte (14.29%), facultative epiphyte (18.18%), lithophyte (23.38%), and terrestrial (44.15%). Orchid species richness showed a bimodal pattern varied with altitude, the higher richness was recorded at 1,500–1,600 and 1,200–1,300 ​m, while the minimum was below 1,100 ​m. Each independent hill is rich in orchid species including 'endemic' species. A total of 25 (33.8%) orchid species were considered threatened by the Redlist of Chinese Vascular Plants (Qin et al., 2017), including 3 CR species, 5 ​EN species and 17 VU species. We call for more research addressing the mechanisms of ecological adaption, mycorrhizal interactions, and pollination of orchids in karst forests, and effective ways for recovery and conservation of orchids.