Pollination of Blumenbachia amana (Loasaceae): Flower morphology and partitioned pollen presentation guarantee a private reward to a specialist pollinator

Abstract

Blumenbachia amana possesses the characteristic 'tilt-revolver flowers' and the pollen partitioning mechanism found exclusively in Loasoideae. Stamens are sheltered in naviculate petals that alternate with five nectar scales, each one associated with two free staminodes. In the staminate phase, stamens move successively towards the centre of the flower and present pollen. Bees of oligolectic Actenosigynes mantiqueirensis (Colletidae) are the unique effective pollinators. Clinching the nectar scale-staminode complex, they bend the scales outward one by one with their head to take up nectar, thereby triggering the movement of a stamen, which arrives in the centre of the flower ~3 min after the stimulus. Females returned to previously visited flowers after short time intervals by establishing pollen foraging routes in flower patches that were maintained for days. In flowers experimentally hand-stimulated in the nectar scales to simulate pollinator visits, twice as many stamens moved, and the longevity of bee-visited flowers decreased by shortening the staminate phase. Partitioned pollen presentation in B. amana leads to a unique pollen foraging behaviour characterized by absolute flower fidelity by this bee in this specialized plant-pollinator relationship.