Evolutionary significance of discrete functional adaptations to pollinators in generalist flowers: a case study of three species ofRanunculus s.l.(Ranunculaceae) with distinct petal nectary scales

Morphological adaptations between flowers and pollinators are thought to reflect natural selection by excluding ineffective pollinators and increasing pollination efficiency. Previous studies have reported a variety of tube or spur flower shapes that influence the functional fit between pollinator and plant. However, little is known about the ‘open’ flower types, which like those of Ranunuculus s.l. are interpreted as ‘generalist flowers’. However, petals in Ranunuculus s.l. have nectary scales which vary in shape, and to determine whether their diversity reflects adaptation to different pollinator traits, we compared Ranunculus japonicus with flap-like nectary scales to Halerpestes ruthenica and Batrachium bungei with cup-like and ring-like nectary scales. Visitation rate, visit duration and pollinator efficiency of each group were compared to distinguish true pollinators from visitors. Regression analysis was also used to identify potential correlation between different petal nectary scale shapes and morphological traits of proboscides of pollinators. Our results reveal that flap-like nectary scales exclude hoverflies from accessing nectar and increase both the length of visits by small bees and pollen transfer efficiency. In contrast, cup- and ring-like nectary scales partially allowed nectar access to hoverflies. To our knowledge, this study demonstrates for the first time that pollinator mouthparts are the main driver of nectary shape rather than body and size.