Stamen dimorphism, bee visitation, and pollen removal in three species of Agalinis (Orobanchaceae)

Abstract

Stamen dimorphism of buzz-pollinated flowers frequently involves heteranthery, where vibrated, poricidal anthers produce less pollen of lower viability. These anthers often allow for a progressive dispensing of pollen as predicted by the pollen presentation hypothesis. Buzz-pollinated, didynamous stamens of Agalinis lack poricidal anthers but possess staminal trichomes. We quantified pollen characteristics, bee visitation, and foraging behavior (sternotribic vs. nototribic) for three buzz-pollinated species of Agalinis. Single visits to Agalinis auriculata (Michx.) S.F.Blake were performed to evaluate pollen dispensing associated with different foraging behaviors. For all three species, the abaxial stamens, those vibrated by bees, possessed greater total pollen of larger size and viability than smaller adaxial stamens, thus not being heterantherous. Most bee visits (81%) were inverted whether vibrating stamens or not. Single visits to flowers of A. auriculata removed between 75%–80% of the available pollen in contrast to a more progressive dispensing of pollen, likely a consequence of filament trichomes allowing vibrating bees a better grasp of stamens. The flower form of Agalinis appears to encourage inverted foraging, which has not led to a division of labor for the dimorphic stamens typical of heterantherous species. The annual habit and short-lived flowers in these species likely select for rapid pollen loss.