Microsatellite-based population genetic diversity and structure of an uncommon orchid, Cypripedium kentuckiense

Background/Question/Methods

Genetic diversity is one of the key requirements to ensure continued existence of any living organism, especially under anthropogenic pressure and climate change conditions. Generally, rare species with small and isolated populations show lower level of genetic diversity than those that are more abundant and have more continuous distribution, however, this is not always the case. Cypripedium kentuckiense Reed is a perennial North American terrestrial orchid currently distributed in 10 southern states within the U.S with conservation status ranging from vulnerable to critically imperiled. This study reports the genetic diversity and structure in seven disjunct populations (217 individuals) of the species based on eight nuclear and two chloroplast microsatellite markers that were self-developed specifically for use in C. kentuckiense.

Results/Conclusions

We detected relatively low genetic diversity across the sampled populations of C. kentuckiense based on both nuclear (average An = 4.0, Ho = 0.439, He = 0.448) and chloroplast DNA microsatellites (average An = 1.57, Nh = 1.57 and H = 0.133). Although the absolute values for population differentiation were low (F_ST = 0.074; Ф_PT = 0.24), they were statistically significant. Pairwise F_ST values ranged from 0.038 to 0.123, however, isolation by distance was not detected based on the Mantel test (r² = 0.127, P = 0.122). Seventeen private alleles were documented at five nuclear microsatellites, and one private allele at one chloroplast DNA microsatellite was also observed. STRUCTURE, principal coordinate analysis, and neighbour joining tree analyses did not show any groupings according to geographic proximity or population size. Mean N_e was 205, which was higher than the number of plants recorded in each sampled population. The low genetic diversity observed in C. kentuckiense may be attributed to its historically small and isolated populations coupled with anthropogenic pressures. Results indicate that the disjunct populations of the species hold differentiated allelic pools and that each population may require protection to preserve the overall genetic diversity of the species.