The mating system and pollination ecology of swamp milkweed (Asclepias incarnata L.)

I investigated the mating system and interfertility of both subspecies of Asclepias incamata using hand-pollinations and horizontal starch-gel electrophoresis. Plants of ssp. incamata were about 1/6 as likely to mature hand self-pollinated fruits as were ssp. pulchra plants. The two subspecies were interfertile, but hybrid plants showed reduced pollen fertility relative to parental plants. Reduced pollen fertility in hybrids, along with geographical barriers, may help to maintain subspecific differentiation. Outcrossing rate estimates were high in natural populations, and plants varied significantly with respect to outcrossed male-fertility, but not self-fertility.; Family-level outcrossing rate estimates varied widely in three populations of A. incarnata. Neighborhood floral density was positively correlated with one outcrossing estimate in one population, but showed no relationship with either of two other estimates of outcrossing for any population. Monte-Carlo simulations revealed that statistical variation associated with the three estimation procedures can be large enough to explain all of the variation in outcrossing estimates. These results also show that spurious correlations with neighborhood floral density could arise, on average, 7% of the time by chance.; I compared insect species that visited flowers of A. incarnata with respect to pollen loads, visit frequency, and pollen removal and deposition throughout the flowering season for two years. I also independently measured natural variation in pollination rates and fruit-set. Hymenoptera carried more pollinia than did Lepidoptera, visitation varied substantially within and between flowering seasons, and insects differed significantly in their ability to transfer pollen between virgin flowers. Total pollinator effectiveness explained {dollar}>{dollar}90% of the variance in pollination rates of naturally-pollinated flowers. Temporal variation in pollinator quality is likely to maintain a generalized pollination syndrome for A. incarnata, which seems appropriate given the dynamic, early-successional habitats in which it occurs. (Abstract shortened by UMI.)...