Implications of Differences Between Hermaphrodite and Female Flowers for Attractiveness To Pollinators and Seed Production

IMPLICATIONS OF DIFFERENCES BETWEEN HERMAPHRODITE AND FEMALE FLOWERS FOR ATTRACTIVENESS TO POLLINATORS AND SEED PRODUCTION* by PETER G.L. KLINKHAMER, TOM J. DE JONG and RENATE A. WESSELINGH (Research Group Ecology of Plants and Herbivores, Department of Biology, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands) SUMMARY Echium vulgare is a gynodioecious species; hermaphrodite and female individuals occur in the same population. Flowers of the female individuals are smaller and produce less nectar. Female and hermaphrodite plants were equally frequently approached by bumblebees. After an approach the bumblebees visited more flowers on her- maphrodite plants. As a result, individual flowers on hermaphrodite plants received 50% more visits. In contrast, flowers on female plants produced more seeds, sug- gesting that the relation between seed set and number of visits was influenced by resource availability. Per visit bumblebees removed 16.5% of the pollen present in the anthers. In contrast to the common assertion in the literature, our data do not indicate that the number of bumblebee visits was limiting pollen transport to other plants in this situation. KEY WORDS: Echium vulgare, gynodioecy, pollination, nectar production, seed set, bumblebees, biennial. INTRODUCTION In many plant species pollen transport and seed production depend on flower visitation by insects. One of the rewards, contributing to the at- tractiveness of a flower, is nectar. Several insect species, and also hummingbirds, can assess the costs and rewards of various floral choices; they may maximize their energy gain by approaching plants that produce large amounts of nectar or by visiting more flowers on these plants. Number of pollinator visits to flowers is an important measure for the transport of pollen to other flowers (male fitness function). For the fertilization of ovules resulting in seed production (female fitness func- tion), number of visits may be less important than the quality of the