Physiographic links in the mycorrhizal host specialization of a rare orchid, Cypripedium acaule

Background/Question/Methods

The mycorrhiza is a requirement for the germination and establishment of orchids in nature. Most conservation efforts ignore mycorrhizal host specialization when designing management and rehabilitation strategies for rare orchids. Without an adequate understanding of how variability in host selection and the evolutionary consequences of interactions shape these communities, current management strategies for rare orchids could ultimately lead to diminishing abundance and distribution. Here, we analyzed the physiographic dimensions of host specialization in the rare orchid Cypripedium acaule and determined patterns in mycorrhizal host specialization associated with soil ecotones. We sampled C. acaule populations across four different physiographic provinces in Georgia and Tennessee: Piedmont, Blue Ridge, Valley Ridge, and Appalachian Plateaus. Fungal identification was performed using BLAST analysis of ITS and mtLSU loci from fungi in the sampled mycorrhizal roots. We collected soil samples at the site of each population in order to test for available carbon, nitrogen, calcium, water, pH and soil microbial community. Analysis of the soil samples was performed using canonical correspondence analysis.

Results/Conclusions

Our results suggest that C. acaule has a broad mycorrhizal host breadth in comparison to most Cypripedium species, even associating with ectomycorrhizal fungi. The dominant fungi are members of the Tulasnellaceae family, most notably, T. cystidiophora, but Russula spp. and Thelephora spp. also associated with C. acaule. BLAST analyses were typically within three percent sequence similarity on these identifications. Initial analysis suggests a dependence of C. acaule on low pH soils, although soil analysis continues.  Habitat and soil resources mitigate the interaction between C. acaule and its mycorrhizal host. We conclude that the maintenance of mycorrhizal associations across large geographic ranges has a meaningful impact on conservation. Efficient and effective solutions to the problems facing populations of rare orchids cannot be developed using only part of the formula.