WALLACE, LISA E. Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, OH 43210. - The significance of population size on reproductive success and genetic variability in the Eastern Prairie White Fringed Orchid, Platanthera leucophaea.
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