The effects of seed dispersal on genetic structure are much less well studied than those of pollen dispersal. Broad taxonomic comparisons have failed to demonstrate conclusively the relationship between mode of dispersal and population genetic structure. The proposed research will compare three sympatric species in the family Umbelliferae, which differ mainly in morphological adaptations for seed dispersal by attachment to animals, to isolate the effects of seed dispersal traits on genetic structure. Genetic diversity within and among 15-18 populations, ranging from <0.5 to > 100 km apart, will be evaluated from isozyme data. Two populations will be more intensively studied by mapping and genotyping several hundred individuals, as well as collecting seeds for outcrossing analyses. These spatial and genetic data, combined with observations on demography, flowering and fruiting will be used to investigate patterns of microgeographic genetic differentiation and mating systems. Field and laboratory studies will be used to estimate seed removal rates and attachment and detachment probabilities of fruits on dispersers. Correlations of genetic and spatial distances between reproductive plants and seed crops or seedlings will be used to evaluate the relative importance of pollen and seed dispersal on gene flow within the two mapped populations. This study will contribute to understanding how seed dispersal mechanisms affect population genetic structure, quantitative and qualitative difference in the apportionment of genetic variation at different spatial scales, and the relative importance of the effects of seed versus pollen dispersal on patterns of plant population differentiation. Because these species occur in "habitat islands" of relatively undisturbed forest in nature preserves, this study will provide information with which to examine the effects of habitat size and isolation on gene flow and the maintenance of genetic diversity.
