SELECTION ON FLORAL DESIGN INPOLEMONIUM BRANDEGEEI(POLEMONIACEAE): FEMALE AND MALE FITNESS UNDER HAWKMOTH POLLINATION

Abstract

Plant-pollinator interactions promote the evolution of floral traits that attract pollinators and facilitate efficient pollen transfer. The spatial separation of sex organs, herkogamy, is believed to limit sexual interference in hermaphrodite flowers. Reverse herkogamy (stigma recessed below anthers) and long, narrow corolla tubes are expected to promote efficiency in male function under hawkmoth pollination. We tested this prediction by measuring selection in six experimental arrays of Polemonium brandegeei, a species that displays continuous variation in herkogamy, resulting in a range of recessed to exserted stigmas. Under glasshouse conditions, we measured pollen removal and deposition, and estimated selection gradients (β) through female fitness (seeds set) and male fitness (siring success based on six polymorphic microsatellite loci). Siring success was higher in plants with more nectar sugar and narrow corolla tubes. However, selection through female function for reverse herkogamy was considerably stronger than was selection through male function. Hawkmoths were initially attracted to larger flowers, but overall preferred plants with reverse herkogamy. Greater pollen deposition and seed set also occurred in reverse herkogamous plants. Thus, reverse herkogamy may be maintained by hawkmoths through female rather than male function. Further, our results suggest that pollinator attraction may play a considerable role in enhancing female function.

SELECTION ONPOLEMONIUM BRANDEGEEI(POLEMONIACEAE) FLOWERS UNDER HUMMINGBIRD POLLINATION: IN OPPOSITION, PARALLEL, OR INDEPENDENT OF SELECTION BY HAWKMOTHS?

“…Here we describe selection on floral traits by hummingbirds in experimental populations, hereafter “arrays.” We then compare these results to our previous study examining selection by hawkmoths on P. brandegeei floral design, where we documented selection for reverse herkogamy through female function, and selection for narrow corolla tubes and high nectar sugar concentration through male function (Kulbaba and Worley ). Measurement of hummingbird foraging behavior, pollen deposition, and removal, in addition to the number of seeds set and sired by each plant, allowed us to compare various indices of reproductive success.…”

“…To measure selection by hummingbirds on the floral traits of P. brandegeei , we used a similar approach to our previous study that measured selection by hawkmoths (Kulbaba and Worley ). We presented arrays of plants to the ruby‐throated hummingbird ( Archilochus colubris ), and measured pollen movement (pollen removal and deposition), and selection gradients on floral traits through female (seeds set) and male (seeds sired) fitness.…”

“…The exsertion of sex organs beyond the opening of the corolla tube further reinforces contact between sex organs and the hummingbird. In contrast, plants pollinated primarily by hawkmoths often display flowers with narrow corolla tubes (Nilsson ; Alexandersson and Johnson ; Brunet ; Kulbaba and Worley ). Furthermore, plants pollinated by hawkmoths may also display stigmas recessed below the anthers.…”

Sexual Dimorphism of Staminate- and Pistillate-Phase Flowers of Saponaria officinalis (Bouncing Bet) Affects Pollinator Behavior and Seed Set

“…Floral traits that increase pollinator attraction should evolve under selection for increased male fitness more so than for selection for increased female fitness. However, selection can also act on pollinator attraction through female function if pollen is limiting [18]–[22]. In S. officinalis , staminate flowers have wider corollas than pistillate flowers (Fig.…”

“…Empirical studies of selection on male and female fitness in plants with hermaphroditic flowers have shown that this assumption is not always correct. In particular, when seed set is pollen limited, selection acts on floral traits through female function as well [18]–[22]. If increased pollinator visitation increases both male and female fitness, selection may result in a common phenotype for both pistillate and staminate phase flowers.…”