Sexual dimorphism of staminate- and pistillate-phase flowers of Saponaria officinalis (bouncing bet) affects pollinator behavior and seed set

Background/Question/Methods

Saponaria officinalisis a protandrous flowering plant whose flowers change from pink to white over an individual flower’s life.  The color change coincides with the transition from staminate phase to pistillate phase in each flower.  In 2012 we demonstrated that flowers on plants grown in full sun are pinker and contain more anthocyanin than genetically identical plants grown under 60% shade, suggesting environmental factors contribute to anthocyanin expression and flower color in this species. In 2013, we investigated whether or not the extent of floral color change is heritable and adaptive, by testing for differences in floral traits among distinct clonal genotypes, investigating whether diurnal pollinators have a color preference, and determining whether the extent of the color change affects seed production. We used clones of eight different genotypes grown in sun and 60% shade plots within a common garden.  We compared floral color, shape and size, as well as seed set of open-pollinated and hand-pollinated flowers in the sun and shade treatments among genotypes. To identify pollinator preferences for floral color and shape, we quantified insect visitation to contrasting arrays of flowers.

Results/Conclusions

Anthocyanin content varied significantly among genotypes and between treatments in male and female-phase flowers. Some genotypes exhibited greater plasticity in color change than others, suggesting a genetic component to the extent of floral color change. Diurnal pollinators displayed a clear preference for staminate-phase flowers over pistillate-phase flowers, but when we controlled for color, pollinators lacked a preference for the shape of the staminate-phase flowers. Overall, pollinators discriminated against the color pink. In both 2012 and2013, pinker flowers produced fewer seeds than pale flowers, when grown in the shade, indicating that production of floral anthocyanins may be disadvantageous to female fitness in environments where pollinators are limiting.  The adaptive function, if any, of the color change in Saponaria officinalis is not yet clear.