The tiny-leaved orchid Disperis neilgherrensis primarily obtains carbon from decaying litter via saprotrophic Ceratobasidium.

While most green orchids establish associations with non-ectomycorrhizal rhizoctonias belonging to Ceratobasidiaceae, Tulasnellaceae, and Serendipitaceae, fully mycoheterotrophic orchids-excluding albino mutants-primarily depend on either ectomycorrhizal fungi or saprotrophic non-rhizoctonia fungi. This suggests that non-ectomycorrhizal rhizoctonias may be unable to meet the carbon demands of adult orchids that exhibit a high degree of mycoheterotrophy. To understand the physiological ecology of Disperis neilgherrensis, an orchid species with reduced leaves growing in decaying litter from non-ectomycorrhizal trees, we employed molecular and stable isotope analyses to identify its mycorrhizal partners and ultimate nutritional sources at two populations on Ishigaki Island, Japan. Molecular barcoding techniques revealed that D. neilgherrensis forms exclusive associations with non-ectomycorrhizal Ceratobasidiaceae fungi. The Disperis specimens exhibited δC and δN isotopic values similar to those found in fully mycoheterotrophic orchids that exploit litter-decaying fungi. Furthermore, the pelotons of D. neilgherrensis showed significantly elevated δC values similar to saprotrophic non-rhizoctonia fungi. Our findings indicate that D. neilgherrensis primarily obtains its carbon from decaying litter through a specialized relationship with non-ECM Ceratobasidiaceae. Given that saprotrophic Ceratobasidiaceae facilitate nearly fully mycoheterotrophic growth in D. neilgherrensis, at least under warm and humid conditions, it is plausible that other (nearly) fully mycoheterotrophic tropical orchids also meet their carbon requirements through associations with saprotrophic rhizoctonias.