Strange Floral Attractors: Pollinator Attraction and the Evolution of Plant Sexual Systems

Individual plants of hummingbird-pollinated Besleria triflora display two flower morphs: staminate flowers, which have shortened styles and do not produce fruit, and hermaphrodite flowers. Experiments with B. triflora indicate that pollinator attraction can drive the evolution of a dimorphic plant sexual system. In field-manipulated plants, visitation increased at large floral displays; however, pollen receipt increased only when staminate flowers were used to enlarge the display. Laboratory experiments showed that staminate flowers do not remove pollen from visiting pollinators, effectively concentrating outcross pollen onto stigmas of fertile flowers. A dimorphic sexual system is favored because the morphology of staminate flowers enhances their role in pollinator attraction.