Mycorrhizal specificity in widespread and narrow-range distributed Caladenia orchid species

Mycorrhizal interactions are critical for the orchid life cycle, and some orchid species associate with a very narrow suite of fungal partners. To investigate whether mycorrhizal specificity correlates with extent of distribution in the host plant, we studied fungal symbionts in two Caladenia species-pair contrasts, each pairing one widely distributed species with a closely-related species of narrow distribution. Across four orchid species, we identified 70 fungal isolates and 121 clone sequences of the fungal genus Serendipita. The widespread C. pectinata associated with two operational taxonomic units (OTUs) and in vitro seed successfully germinated on four OTUs. The narrowly distributed C. procera associated with two OTUs and was germinated by two OTUs. Four OTUs were identified in widespread C. tentaculata compared to one OTU in the narrow distribution C. atrovespa. The suite of OTUs observed associating with adult plants was different from those successfully germinating seed of the same species, suggesting weaker specificity during germination. We found that fungi germinating narrowly distributed orchids are themselves widespread, and are therefore unlikely to limit orchid distribution at a landscape level.