EFFECTIVE POLLEN DISPERSAL IN A NATURAL-POPULATION OF ASCLEPIAS-EXALTATA - THE INFLUENCE OF POLLINATOR BEHAVIOR, GENETIC SIMILARITY, AND MATING SUCCESS

We investigate Pollen-mediated gene dispersal within a natural population of poke milkweed, Asclepias exaltata. We use paternity exclusion analysis with seven polymorphic isozymes to quantify pollen dispersal among plants in the population. Realized gene dispersal via pollen is much greater than potential gene flow inferred from pollinator flight distances between plants but is similar to a random distribution based on all interplant distances. Extensive pollen carryover due to the unique pollination system of the plants and the behavior of their large butterfly pollinators may explain the extensive pollen movement in the population. There was no evidence of an "optimal outcrossing distance" in this population. Matings between genetically similar plants resulted in reduced seed-set and seed viability within fruits, but genetic similarity between mating pairs was independent of the spatial distance separating them. Thus, extensive gene dispersal via pollen creates a large neighborhood size in milkweeds that may prevent genetic structure in small local populations, a prerequisite for optimal outcrossing distances to exist.