BUMBLEBEE PREFERENCE FOR SYMMETRICAL FLOWERS

Fluctuating asymmetry, which represents small random deviations from o therwise bilateral symmetry, is a measure of the phenotypic quality of individuals indicating the ability of controlled development under gi ven environmental and genetic conditions. I tested whether floral symm etry reliably reflects phenotypic quality measured in terms of pollina tor rewards and whether pollinators respond to floral symmetry in a se ries of observations and experiments on Epilobium angustifolium (Onagr aceae). Lower petal asymmetry was negatively related to mean lower pet al length, whereas asymmetry in leaf width was positively related to m ean leaf width. Flowers visited by bumblebees were larger add more sym metrical than the nearest neighboring flower. This relationship betwee n pollinator preference for large and symmetrical flowers was demonstr ated to be causal in experiments in which the lower petals were manipu lated symmetrically or symmetrically. Nectar production was larger in symmetrical flowers, and this may explain the bumblebee preference for flower symmetry. Floral symmetry therefore reliably reflects nectar p roduction and hence enhances pollen transport. Extensive embryo aborti on has been documented in E. angustifolium and other outcrossing plant species. Floral fluctuating asymmetry, which reflects general develop mental homeostasis, may explain such development selection in these pl ants.