Ectomycorrhizal Inocybe species associate with the mycoheterotrophic orchid Epipogium aphyllum but not its asexual propagules.

Background and Aims: Epipogium aphyllum is a Eurasian achlorophyllous, mycoheterotrophic forest orchid. Due to its rarity, it is often protected, and its biology is poorly known. The identity and pattern of colonization of fungal associates providing carbon to this orchid have not been studied previously. Methods: Using samples from 34 individuals from 18 populations in Japan, Russia and France, the following were investigated: (a) colonization patterns of fungal associates of E. aphyllum by microscopy; (b) their identity by PCR amplification of nuclear ribosomal ITS carried out on rhizome fragments and hyphal pelotons. Results and Conclusions: Microscopic investigations revealed that thick rhizomes were densely colonized by fungi bearing clamp-connections and dolipores, i.e. basidiomycetes. Molecular analysis identified Inocybe species as exclusive symbionts of 75% of the plants investigated and, more rarely, other basidiomycetes (Hebeloma, Xerocomus, Lactarius, Thelephora species). Additionally, ascomycetes, probably endophytes or parasites, were sometimes present. Although E. aphyllum associates with diverse species from Inocybe subgenera Mallocybe and Inocybe sensu stricto, no evidence for cryptic speciation in E. aphyllum was found. Since basidiomycetes colonizing the orchid are ectomycorrhizal, surrounding trees are probably the ultimate carbon source. Accordingly, in one population, ectomycorrhizae sampled around an individual orchid revealed the same fungus on 11.2% of tree roots investigated. Conversely, long, thin stolons bearing bulbils indicated active asexual multiplication, but these propagules were not colonized by fungi. These findings are discussed in the framework of ecology and evolution of mycoheterotrophy.