Independent, specialized invasions of ectomycorrhizal mutualism by two nonphotosynthetic orchids

Abstract

We have investigated the mycorrhizal associations of two nonphotosynthetic orchids from distant tribes within the Orchidaceae. The two orchids were found to associate exclusively with two distinct clades of ectomycorrhizal basidiomycetous fungi over wide geographic ranges. Yet both orchids retained the internal mycorrhizal structure typical of photosynthetic orchids that do not associate with ectomycorrhizal fungi. Restriction fragment length polymorphism and sequence analysis of two ribosomal regions along with fungal isolation provided congruent, independent evidence for the identities of the fungal symbionts. All 14 fungal entities that were associated with the orchid Cephalanthera austinae belonged to a clade within the Thelephoraceae, and all 18 fungal entities 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 fungi associated internally with Cephalanthera also form typical external ectomycorrhizae and that (ii) ectomycorrhizae formed by other Basidiomycetes were abundant where the orchid grows but these fungi did not associate with the orchid. This is the 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 mutualisms, are susceptible to cheating. The extreme specificity in these orchids relative to other ectomycorrhizal plants agrees with trends seen in more conventional parasites.

Symbiotic germination and development of the myco‐heterotrophic orchid Neottia nidus‐avis in nature and its requirement for locally distributed Sebacina spp.

“…The molecular data concerning the identities of the two N. nidus-avis fungi suggest that this orchid, similar to other MHP orchids including Cephalanthera austinae (Taylor & Bruns, 1997), Rhizanthella gardneri ( Warcup, 1985) and several Corallorhiza species (Taylor & Bruns, 1999a;McKendrick et al, 2000a), associates with a narrow taxonomic group of fungi. The finding that N. nidus-avis 1 was the predominant fungal associate of both seedlings and adults in the UK study sites agrees with a number of previous studies showing that adults and seedlings of some orchids target the same fungi (Masuhara & Katsuya, 1994;McKendrick et al, 2000a).…”

“…The extract was purified using GeneClean (BIO 101, Inc., Carlsbad, CA, USA) glass milk as described in Taylor & Bruns (1997). The fungal ITS region of the nuclear ribosomal repeat was amplified with the general fungal primer ITS1F (Gardes & Bruns, 1993) combined with the universal primer Expansion of embryo to a length ≥ 300 µm and breadth ≥ 200 µm together with formation of pelotons 2…”

Specialized cheating of the ectomycorrhizal symbiosis by an epiparasitic liverwort

“…Although photosynthetic plants are generalists in their compatibility with fungal partners, the epiparasites examined so far display exceptional specificity towards narrow groups of closely related fungi (Cullings et al 1996;Taylor & Bruns 1997;. Because Cryptothallus may be the only lineage of non-vascular plants to have achieved cheating, it is of considerable interest to identify its fungal symbionts and to determine the pathways and sources of its carbon supplies.…”

“…Unfortunately, reliance on culturability can introduce biases if the target fungi are difficult to isolate. In fact, recalcitrant fungi are often the sole mycorrhizal associates of non-photosynthetic mycorrhizal plants (Taylor & Bruns 1997;Bidartondo & Bruns 2001;.…”

Symbiotic germination and development of myco‐heterotrophic plants in nature: transfer of carbon from ectomycorrhizal Salix repens and Betula pendula to the orchid Corallorhiza trifida through shared hyphal connections

“…Experimental confirmation that C. trifida is associated with fungi which form ectomycorrhizas was first provided by Zelmer & Currah (1995), who isolated a basidiomycete from pelotons and used it to synthesize ectomycorrhiza on seedlings of Pinus contorta. More recent studies comparing DNA sequences of fungi in coralroot orchids in North America with those of ectomycorrhizal fungi on adjacent tree roots have confirmed that these orchids are indeed associated with ectomycorrhizal fungi (Taylor & Bruns, 1997).…”