Forest Biology Lab.

Flower size dimorphism is a common secondary sexual character in flowering plants, but the causes of this dimorphism are not well understood. Both flower number and size may affect pollinator selectivity, for example, small flowers with large number of blooming flowers may attract pollinators as well as large flowers with few synchronically blooming flowers. Such pattern of differential flowering has been little studied.
We explored the causes and consequences of pollinator response to female and male floral display in a monoecious aquatic plant (Sagittaria trifolia L.), to assess the role of pollinator-mediated selection for floral dimorphism. In this species, male flowers are larger than female flowers, but female flowers have a more compressed flowering period than males, creating larger female floral displays compared to male flower displays.
Patterns of pollinator movement were observed in replicate arrays, in which plants were manipulated to display 3, 6, 9 or 12 female or male flowers per plant. Overall, pollinators preferred to visit male displays than female displays of the same size. Larger female displays generally attracted more visits by different pollinators than small displays, but male displays did not show increased attractiveness with display sizes. Daily pollen receipt and pollen remaining correlated negatively. The predicted relation that pollen removal, rather than pollen receipt, is limited by pollinator visitation was observed. The results suggest lack of selection on large male displays may affect the evolution of flower size dimorphism. Keywords: Alismataceae, flower size dimorphism, floral display size, monoecious, pollen receipt, pollen removal, pollinator choice, pollinator-mediated selection.