Mating System and Pollen-Mediated Gene Flow in Bartsia alpina

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Abstract
A northern Swedish population of Bartsia alpina L. (Scrophulariaceae), an arctic-alpine, bumblebee-pollinated, perennial herb, was found to have a mixed mating system. However, deposition of self pollen did not occur without pollinator intervention, and outcrossing was further promoted by pollen carry-over, adaptive post-pollination floral changes, and by selective resource allocation to seeds resulting from cross-pollination. Multiple paternity apparently dominated in natural pollinations. In the study population, we estimated pollen-mediated gene flow by simulating pollinator flight patterns according to predictions of optimal foraging bahaviour in bumblebees. Due to changes in flower density affecting pollinator flight distances, neighbourhood size varied 100-fold during the summer. Pollinations occurring early and late in the season enhanced gene flow substantially. We therefore suggest that comparisons of estimates of neighbourhood size between species or populations cannot be made without considering temporal variation in gene flow.