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

Abstract

The sequential separation of male and female function in flowers of dichogamous species allows for the evolution of differing morphologies that maximize fitness through seed siring and seed set. We examined staminate- and pistillate-phase flowers of protandrous Saponaria officinalis for dimorphism in floral traits and their effects on pollinator attraction and seed set. Pistillate-phase flowers have larger petals, greater mass, and are pinker in color, but due to a shape change, pistillate-phase flowers have smaller corolla diameters than staminate-phase flowers. There was no difference in nectar volume or sugar content one day after anthesis, and minimal evidence for UV nectar guide patterns in staminate- and pistillate-phase flowers. When presented with choice arrays, pollinators discriminated against pistillate-phase flowers based on their pink color. Finally, in an experimental garden, in 2012 there was a negative correlation between seed set of an open-pollinated, emasculated flower and pinkness (as measured by reflectance spectrometry) of a pistillate-phase flower on the same plant in plots covered with shade cloth. In 2013, clones of genotypes chosen from the 2012 plants that produced pinker flowers had lower seed set than those from genotypes with paler flowers. Lower seed set of pink genotypes was found in open-pollinated and hand-pollinated flowers, indicating the lower seed set might be due to other differences between pink and pale genotypes in addition to pollinator discrimination against pink flowers. In conclusion, staminate- and pistillate-phase flowers of S. officinalis are dimorphic in shape and color. Pollinators discriminate among flowers based on these differences, and individuals whose pistillate-phase flowers are most different in color from their staminate-phase flowers make fewer seeds. We suggest morphological studies of the two sex phases in dichogamous, hermaphroditic species can contribute to understanding the evolution of sexual dimorphism in plants without the confounding effects of genetic differences between separate male and female individuals.

Sex‐specific floral attraction traits in a sequentially hermaphroditic species

“…In one of the only other studies to date to examine sexual dimorphism in a sequentially hermaphroditic plant species, Davis et al () found that female‐phase flowers of Saponaria officinalis (Caryophyllaceae) have larger petals and are “pinker” in color than male‐phase flowers. However, due to a change in floral presentation, female‐phase flowers have smaller corolla diameters than male‐phase flowers and attract significantly fewer pollinators.…”

“…However, due to a change in floral presentation, female‐phase flowers have smaller corolla diameters than male‐phase flowers and attract significantly fewer pollinators. Pollinators of S. officinalis prefer whiter (less pink) flowers independent of the presence of stamens, nectar volume, and flower size (Davis et al, ). A strong pollinator preference for the floral phenotype associated with male sexual function in S. officinalis supports the hypothesis that sexual dimorphism may result in features that promote male fitness more than female fitness in a bisexual flower (at least in S. officinalis ).…”

“…We did not find evidence for selection for more attractive male‐phase flowers in C. unguiculata (using the suite of visual floral traits examined here). Davis et al () found that pollinators preferred whiter flowers (compared with pink ones) and that male‐phase flowers were significantly whiter than female‐phase flowers. In this study, we also identified a pollinator preference for a specific color phenotype; however, it was a composite phenotype.…”

“…This is among the first studies to examine sexual dimorphism between the functional sex stages of a bisexual flower (Davis et al, 2014;Jabbari et al, 2013). Here, in each of three functional sex stages of a common annual forb, we quantified a suite of floral traits that are visually distinguishable to pollinating insects and are known to influence pollinator attraction (in other species) (Glaettli & Barrett, 2008;Jones, 1996a;Leonard & Papaj, 2011;Rae & Vamosi, 2013).…”

“…In one of the only other studies to date to examine sexual dimorphism in a sequentially hermaphroditic plant species, Davis et al (2014) found that female-phase flowers of Saponaria officinalis…”