In the mycorrhizal symbiosis, plants exchange photosynthates for mineral nutrients acquired by fungi from the soil. This mutualistic arrangement has been subverted by hundreds of mycorrhizal plant species that lack the ability to photosynthesize. The most numerous examples of this behaviour are found in the largest plant family, the Orchidaceae. Although non-photosynthetic orchid species are known to be highly specialized exploiters of the ectomycorrhizal symbiosis, photosynthetic orchids are thought to use free-living saprophytic or pathogenic fungal lineages. However, we present evidence that putatively photosynthetic orchids from five species that grow in the understorey of forests (i) form mycorrhizas with ectomycorrhizal fungi of forest trees and (ii) have stable-isotope signatures indicating distinctive pathways for nitrogen and carbon acquisition approaching those of non-photosynthetic orchids that associate with ectomycorrhizal fungi of forest trees. These findings represent a major shift in our understanding of both orchid ecology and evolution because they explain how orchids can thrive in low-irradiance niches and they show that a shift to exploiting ectomycorrhizal fungi precedes viable losses of photosynthetic ability in orchid lineages.

Summary• Some green orchids obtain carbon (C) from their mycorrhizal fungi and photosynthesis. This mixotrophy may represent an evolutionary step towards mycoheterotrophic plants fully feeding on fungal C. Here, we report on nonphotosynthetic individuals (albinos) of the green Cephalanthera damasonium that likely represent another evolutionary step.• Albino and green individuals from a French population were compared for morphology and fertility, photosynthetic abilities, fungal partners (using microscopy and molecular tools), and nutrient sources (as characterized by 15 N and 13 C abundances).• Albinos did not differ significantly from green individuals in morphology and fertility, but tended to be smaller. They harboured similar fungi, with Thelephoraceae and Cortinariaceae as mycorrhizal partners and few rhizoctonias. Albinos were nonphotosynthetic, fully mycoheterotrophic. Green individuals carried out photosynthesis at compensation point and received almost 50% of their C from fungi. Orchid fungi also colonized surrounding tree roots, likely to be the ultimate C source.• Transition to mycoheterotrophy may require several simultaneous adaptations; albinos, by lacking some of them, may have reduced ecological success. This may limit the appearance of cheaters in mycorrhizal networks.