Pollinator response to female and male floral display in a monoecious species and its implications for the evolution of floral dimorphism

Abstract

Summary• Pollinator-mediated selection has been hypothesized as one cause of size dimorphism between female and male flowers. Flower number, ignored in studies of floral dimorphism, may interact with flower size to affect pollinator selectivity.• In the present study, we explored pollinator response, and estimated pollen receipt and removal, in experimental populations of monoecious Sagittaria trifolia , in which plants were manipulated to display three, six, nine or 12 female or male flowers per plant. In this species, female flowers are smaller but have a more compressed flowering period than males, creating larger female floral displays.• Overall, pollinators preferred to visit male rather than female displays of the same size. Both first visit per foraging bout and visitation rates to female displays increased with display size. However, large male displays did not show increased attractiveness to pollinators. A predicted relationship that pollen removal, rather than pollen receipt, is limited by pollinator visitation was confirmed in the experimental populations.• The results suggest that the lack of selection on large male displays may affect the evolution of floral dimorphism in this species.

Effects of Population Size on Synchronous Display of Female and Male Flowers and Reproductive Output in Two Monoecious Sagittaria Species

“…Plants reproduce both sexually and clonally by producing shoot tubers. Unisexual, nectar-producing flowers are visited by a wide spectrum of pollinators including bees, flies, beetles, butterflies and wasps, and individual flowers last a single day [10]. Female flowers are smaller than males.…”

“…Pollinator-meditated selection should result in larger male floral displays and hence higher ratio of male to female flowers, because the number of male flowers influences pollen production and attraction to pollinators [6]–[9]. On the other hand, sexual selection should favor extended duration of male function, thus increasing the number and variety of mating partners [10]–[15]. Studies investigating flower dimorphism and floral display with unisexual flowers (monoecy or dioecy) often report male function extends the duration of their floral displays through producing or opening only a fraction of their flowers each day compared to female function [10], [11], [16].…”

“…On the other hand, sexual selection should favor extended duration of male function, thus increasing the number and variety of mating partners [10]–[15]. Studies investigating flower dimorphism and floral display with unisexual flowers (monoecy or dioecy) often report male function extends the duration of their floral displays through producing or opening only a fraction of their flowers each day compared to female function [10], [11], [16]. Hence, the functional significance of floral display is not the total number of flowers produced by an inflorescence or ramet but rather the number of flowers in anthesis each day [11].…”

“…Sagittaria (Alismataceae), a worldwide genus of emergent aquatics with monoecious and dioecious sexual systems [10], [16], is a good system to study how interactions among population size, flowering synchrony, and ratio of male to female flowers per day affect reproductive success. Individual plants of the monoecious Sagittaria species can produce several sequential racemes.…”

“…Female flowers occur on basal whorls of the inflorescence and male flowers on upper whorls. It is infrequent for flowering periods of female and male function to overlap within an inflorescence [10], [16]–[18]; thus, monoecious Sagittaria are protogynous at the inflorescence level. The mating system of Sagittaria species is dominated by outcrossing mechanisms [19], [20].…”