Reproductive Biology of Apiaceae. II. Cryptic Specialization and Floral Evolution in Thaspium and Zizia

The classification of specialized floral syndromes has imposed a bias in the interpretation of pollination systems which may be either more generalized, or more specialized, than we have universally acknowledged. An analysis of floral biology in two umbellifer genera, Thaspium and Zizia, was undertaken in order to determine the extent to which cryptic floral or inflorescence variations determine pollination specialization despite a broad visitor spectrum and open reward system. Separate analyses were made of the primary attractants, nectar and pollen, and the secondary attractants, floral color and floral and inflorescence structure in conjunction with analyses of pollinator movements, stigmatic pollen loads and fruit set. All data support the conclusion that cryptic variation in floral and inflorescence characters enhance specialization for pollination by solitary bees or syrphid flies. In addition, evidence is presented for the importance of the oligolectic relationship between taxa of Thaspium and Zizia and the solitary bee, Andrena ziziae. The need for more experimental work both to further define the oligolectic relationship and to understand how floral and inflorescence color and structure affect insect movements is especially revealed by this study. The degree of pollination specialization in Thaspium and Zizia is not uncommon in Apiaceae and has important implications for floral evolution in this family and other plant groups with pollination systems categorized as “promiscuous.”