Genetic variation, population structure, and mycorrhizal association of a North American orchid, Pogonia ophioglossoides (L.) Ker Gawl

Background/Question/Methods

Pogonia ophioglossoides is distributed from east-central to eastern United States and from central to eastern Canada. It is a non-rewarding species that is typically pollinated by bees, but is also characterized as self-compatible. While the U.S. Fish and Wildlife Service considers P. ophioglossoides secure, several States or Provinces have designated it as a critically imperiled, imperiled, or vulnerable species. An assessment of genetic variation within and among populations is generally necessary for understanding broad-scale gene flow patterns in plants with diverse pollination syndromes. Further, biotic factors such as obligate symbiotic relationships can limit recruitment of orchid plants and may influence their population genetic structure. The main objectives of the present study were to: (i) quantify genetic diversity within and among populations of Pogonia ophioglossoides, (ii) assess spatial distribution of genetic variation within populations; and (iii) evaluate the diversity of mycorrhizal fungi associated with P. ophioglossoides. We used three self-developed nuclear microsatellite and two chloroplast microsatellite regions to genotype approximately 311 individuals from eight populations. For mycorrhizal analyses, we used fungal ITS markers to assess the diversity of fungi associated with the orchid.

Results/Conclusions

A relatively high level of within population genetic variation (mean An = 9.08, Ho = 0.44, and He = 0.71) was detected in Pogonia ophioglossoides with the nuclear markers. Inbreeding appears to be high across the sampled populations (mean F_IS = 0.40). Genetic differentiation based on hierarchical AMOVA showed that 21% (Ф_PT = 0.21, P = 0.000) and 63% (Ф_PT = 0.63, P = 0.000) genetic diversity was distributed among populations based on nuclear and cpDNA microsatellites, respectively. Pairwise F_ST values ranged from 0.041 to 0.224 and were statistically significant. The isolation by distance estimate did not show an association between genetic differentiation and geographic distance based on both nuclear (Mantel test, r² = 0.127, P = 0.122) and cpDNA microsatellites (r² = 0.113, P = 0.065). Cluster analyses revealed that a few geographically close populations share similar genetic information. We also documented fine-scale spatial genetic structure up to 40 m distance in Texas. Gene flow across the sampled populations of P. ophioglossoides appears restricted and localized seed dispersal is suggested by the short-distance SGS. Mycorrhizal evaluations yielded nine operational taxonomic units representing three fungal families, Agaricaceae, Ceratobasidiaceae, and Tulasnellaceae. Unique fungal phylotypes were observed within the Agaricaceae and Ceratobasidiaceae.