The influence of clonal structure and pollinator movement on mating success of self-incompatible Dicentra canadensis

Background/Question/Methods   Clonal growth in terrestrial plants is likely to increase the number of genetically identical individuals in the adjacent area due to limited dispersal ability of clonal propagules. This may reduce mate availability in self-incompatible plants, if their pollinators tend to visit adjacent plants. We investigated the spatial distribution of compatible mates, pollinator foraging distances, and pollen carryover to determine the effect of pollinator-mediated mating pattern on reproductive success in Dicentra canadensis, a self-incompatible clonal herb pollinated by bumble bees (Bombus). We estimated the frequency of ovule fertilization in haphazardly paired individuals that are nearby (< 3 m away) or are 20 or 100 m apart to determine mate availability at these distances. During peak flowering (> 80% flowers open), we recorded the distance between consecutive plants visited by foraging bees and the number of flowers probed per plant. A supplemental pollination experiment using long-distance (> 100 m) pollen donors was also conducted to test whether seed set is limited by inadequate pollination under natural conditions.

Results/Conclusions   Pollen supplementation resulted in higher fruit set than natural pollination alone (0.95 vs. 0.55 fruits/flower), suggesting pollen limitation. The frequency of mating success increased with outcross distance: 19% of nearest neighbor pairs were compatible mates, versus 31% and 61% respectively in pairs that were 20 m and 100 m apart. Pollen removal data indicated 71% of flowers were visited at least once by the fifth day of peak flowering, thus pollinator visitation was not a major limitation. However, pollinators tended to forage among nearby plants. The average distance between consecutively visited plants was 1.0 m (SD 1.1), and 2.2 flowers (SD 1.1) were visited per plant. Pollinators visited 3 plants (SD 1.0) during an average 6-flower foraging bout (mean 2.8 m, SD 2.5, N=185 bouts) and 97% of the observed bouts were less than 10 m long. Our results suggested that pollinators of D. canadensis regularly foraged among nearby plants, which were often incompatible mates. Despite frequent pollinator visitation, inadequate pollen exchange between compatible mates may limit the reproductive success of self-incompatible plants. Investigations in progress will document how foraging behaviors may vary as flower density and nectar reward change throughout the flowering season, and its relation to overall reproductive success of the plant.