GENETIC CONSEQUENCES OF OUTCROSSING IN THE CLEISTOGAMOUS ANNUAL, IMPATIENS CAPENSIS. I. POPULATION-GENETIC STRUCTURE.

We examined the genetic structure of 11 populations of Impatiens capensis, a cleistogamous annual herb, using starch gel electrophoresis. We sampled both cleistogamous (CL) and chasmogamous (CH) progeny (if present) from maternal parents in each population to infer maternal genotypes and to estimate the extent and pattern of inbreeding within and among populations. Only eight of 31 loci were polymorphic, with one to six (mean = 3.1) loci varying within each population. Mean heterozygosity per individual is quite low (mean = 3.9%) and comparable to highly self-fertilized species. Gene flow is low, and genetic distances do not parallel geographical distances, suggesting a population structure similar to Wright's Island model with drift among the populations. Fixation indexes within populations (f̂ or F ) span the largest range yet reported for a plant species (0.26 to 0.94, mean = 0.57). Further inbreeding results from population substructuring (θ^ or FST=0.46), resulting in a total average inbreeding coefficient (F̂ or F ) of 0.77. Despite these high overall levels of inbreeding, chasmogamy significantly reduces fixation, which may account for the observed greater fitness of CH progeny.