Heteranthery in Clarkia: pollen performance of dimorphic anthers contradicts expectations

Premise of the StudyWild plant species that require the services of pollen-feeding insects for reliable pollination may evolve features that attract and reward their mutualistic partners. Heterantherous species have been proposed to exhibit a division of labor whereby feeding anthers (which produce pollen that may be consumed by an insect) are distinguished from reproductive anthers (which produce pollen more likely to contribute to reproduction). In some heterantherous species, including Clarkia unguiculata (Onagraceae), these two anther types differ with respect to stamen length, anther size, pollen production, and pollen color. MethodsThe primary goal of this study was to test one component of the division of labor hypothesis by comparing the performance of the pollen produced by each type of anther in C. unguiculata. To achieve this goal, under greenhouse conditions, we hand pollinated and assessed pollen performance (using epifluorescence microscopy) within similar to 228 flowers. Key ResultsThe pollen produced by the two anther types differed significantly with respect to both stigma and style penetration. The inner series of anthers produce pollen with higher performance than the outer series of longer, dark red anthers. ConclusionsThese findings contradict previous descriptions of the genus, reporting that the inner diminutive series of anthers in Clarkia produce abortive and nonfunctional pollen. We outline the future research required to demonstrate the ecological function of heteranthery in this iconic wildflower group.