Format: ORAL

Nathan Muchhala

Biology Department, University of Missouri - St. Louis, USA

While the ancestral symmetry of angiosperm flowers is actinomorphic (radial symmetry), hundreds of lineages have independently evolved zygomorphic flowers (bilateral symmetry). Such shifts are associated with more specialized pollination systems, and lead to significant increases in diversification rates. But what are the actual benefits of zygomorphic flowers for a plant? One little-tested assumption is that zygomorphy maximizes pollen transfer by allowing flowers to place pollen in a more precise and repeatable way on pollinators bodies. This is supported by the fact that floral orientation tends to be more consistent, and often can be quickly readjusted, for zygomorphic flowers compared to actinomorphic ones. In this study, we performed flight cage experiments with nectar-feeding bats and artificial flowers, testing how floral symmetry and orientation affect pollen removal and transfer. For each flower, four anthers were placed either all of the way around the opening (actinomorphic) or together at one side of the opening (zygomorphic). Nearly twice as much pollen was transferred between zygomorphic flowers, but only when angled at 45, causing pollen to be consistently placed on the tops of bats heads. When flowers were positioned flat (i.e. facing upwards), bats approached from many different angles, and zygomorphic flowers performed no better than actinomorphic ones in terms of pollen transfer. Thus, results suggest that shifts to zygomorphy allow more specialized pollination systems which, with the correct floral orientation, maximize pollen transfer success.