Evidence for mycorrhizal races in a cheating orchid

Abstract

Disruptive selection on habitat or host-specificity has contributed to the diversification of several animal groups, especially plant-feeding insects. Photosynthetic plants typically associate with a broad range of mycorrhizal fungi, while non-photosynthetic plants that capture energy from mycorrhizal fungi ('mycoheterotrophs') are often specialized towards particular taxa. Sister myco-heterotroph species are often specialized towards different fungal taxa, suggesting rapid evolutionary shifts in specificity. Within-species variation in specificity has not been explored. Here, we tested whether genetic variation for mycorrhizal specificity occurs within the myco-heterotrophic orchid Corallorhiza maculata. Variation across three single-nucleotide polymorphisms revealed six multilocus genotypes across 122 orchids from 30 sites. These orchids were associated with 22 different fungal species distributed across the Russulaceae (ectomycorrhizal basidiomycetes) according to internal-transcribed-spacer sequence analysis. The fungi associated with four out of the six orchid genotypes fell predominantly within distinct subclades of the Russulaceae. This result was supported by Monte Carlo simulation and analyses of molecular variance of fungal sequence diversity. Different orchid genotypes were often found growing in close proximity, but maintained their distinct fungal associations. Similar patterns are characteristic of insect populations diversifying onto multiple hosts. We suggest that diversification and specialization of mycorrhizal associations have contributed to the rapid radiation of the Orchidaceae.

Further advances in orchid mycorrhizal research

“…Conservation approaches for these species are closely dependent on determination of appropriate ex situ methods of growth so that more seed and /or seedlings can be used to stabilise natural populations. As these orchid species depend on ECM fungi for their nutrition (Taylor et al 2004;Selosse et al 2004; Dearnaley and Le Brocque 2006 but see Yagame et al 2007 for a recent review of MH orchids that can be cultivated with non ECM fungi) ex situ growth will require establishing tripartite symbiotic interactions with tree seedlings, ECM fungi and orchids under controlled conditions. Warcup (1985Warcup ( , 1988Warcup ( , 1991, McKendrick et al (2000) and Bougoure (pers.…”

“…Helaxectris spp., Epipactis spp., Dipodium spp. (Taylor et al 2004;Selosse et al 2004;Dearnaley and Le Brocque 2006) and further study (eg. mycobiont identification; stable C and N isotope ratios, CO 2 exchanges) is required of rare chlorophyllous species to confirm physiological status (ie.…”

Interactions with mycorrhizal fungi in two closely related hybridizing orchid species

“…Early molecular studies have revealed that terrestrial orchids interact with a wide range of mycorrhizal fungi. Some orchids are extreme specialists and are associated to a single fungal species (MCCORMICK et al 2004, TAYLOR et al 2004, DEARLANEY 2007, SHEFFERSON et al 2008, while others are generalists, hosting many different fungi (MCCORMICK et al 2006, BONNAR-DEAUX et al 2007, IRWIN et al 2007). …”

Evidence of a myco-heterotroph in the plant family Ericaceae that lacks mycorrhizal specificity

“…Raf. where a particular genotype will only associate with specific subclades in the Russulaceae Taylor et al 2004). The few known exceptions to this pattern of specificity are albino mutants of normally green orchids in the genera Cephalanthera and Epipactis that were found to associate with the same suite of ectomycorrhizal (EM) fungi as green individuals (Selosse et al 2004;Julou et al 2005;Abadie et al 2006).…”