Platanthera leucophaea (Orchidaceae) is a federally threatened species that is adapted to mesic prairies east of the Mississippi River. Populations have been declining rapidly due to the encroachment of competing weedy species. Studies were initiated to examine differences in reproductive success and genetic variability in populations of variable size. Seed viability was assessed among selfed, hand-outcrossed, and naturally pollinated flowers in a large population (greater than 5,000 flowering individuals) and a smaller more typical population (fewer than 75 flowering individuals). In the large population, seeds and capsules produced from selfing weighed nearly half as much as those resulting from hand-outcrossing or natural pollinations. Additionally, selfed capsules contained relatively few healthy seeds while hand-outcrossed and naturally pollinated capsules had significantly higher levels of viable embryos. In contrast, seed viability in the smaller population did not differ significantly among any of the pollination treatments. These preliminary data suggest that the large population is adapted to outcrossing and experiences inbreeding depression as a result of selfing while the small population has a history of inbreeding. Theory predicts that inbred populations should contain less genetic variability than outcrossed populations. However, populations of varying size were found to have comparable levels of variability at allozyme and RAPD loci. Additionally, most of the variation resides within populations, suggesting high levels of interpopulation gene flow or recent divergence from a common ancestral population. Potential implications for the management of P. leucophaea populations are considered.

Key words: conservation biology, genetic diveristy, Orchidaceae, Platanthera leucophaea