INDEPENDENT, SPECIALIZED INVASIONS OF ECTOMYCORRHIZAL MUTUALISM BY 2 NONPHOTOSYNTHETIC ORCHIDS

We have investigated the mycorrhizal associations of two nonphotosynth etic orchids from distant tribes within the Orchidaceae. The two orchi ds were found to associate exclusively with two distinct clades of ect omycorrhizal basidiomycetous fungi over wide geographic ranges. Yet bo th orchids retained the internal mycorrhizal structure typical of phot osynthetic orchids that do not associate with ectomycorrhizal fungi. R estriction fragment length polymorphism and sequence analysis of two r ibosomal regions along with fungal isolation provided congruent, indep endent evidence for the identities of the fungal symbionts. All 14 fun gal entities that were associated with the orchid Cephalanthera austin ae belonged to a clade within the Thelephoraceae, and all 18 fungal en tities that were associated with the orchid Corallorhiza maculata fell within the Russulaceae. Restriction fragment length polymorphism and single-strand conformational polymorphism analysis of ectomycorrhizal tree roots collected adjacent to Cephalanthera showed that (i) the fun gi associated internally with Cephalanthera also form typical external ectomycorrhizae and that (ii) ectomycorrhizae formed by other Basidio mycetes were abundant where the orchid grows but these fungi did not a ssociate with the orchid, This is tale first proof of ectomycorrhizal epiparasitism in nature by an orchid, We argue that these orchids are cheaters because they do not provide fixed carbon to associated fungi, This view suggests that mycorrhizae, like other ancient mutualism, ar e susceptible to cheating, The extreme specificity in these orchids re lative to other ectomycorrhizal plants agrees with trends seen in more conventional parasites.