Influence of Sexual Dimorphism and Dichromatism on Reproductive Success in a Rare Native Cactus

Identifying plant sexual dimorphic traits is critical in advancing our knowledge on plant-pollinator interactions. For example, dimorphism in floral colors, or sexual dichromatism, is a crucial mediator of pollinator choice on foraging decisions. We studied Cylindropuntia wolfii, an exemplary model system, with diverse flower colors and a functionally dioecious sexual system. However, it is struggling to reproduce sexually as it has a low seed set especially in naturally pollinated fruits. Thus, it is critical to this native species’ conservation to investigate its relationship with pollinators. Our goals were to: a) investigate the sexual dimorphism including the sexual dichromatism in the flowers of the cactus, and b) determine whether sexually dimorphic traits affect the pollinator attraction of both the sexes. We measured several quantitative and qualitative traits for sexual dimorphism and compared them using statistical analyses. Then we recorded the pollinator visitation rate in nature for both sexes and tracked pollinator color preference using fluorescent dyes as pollen analogues. Our study showed that male flowers of C. wolfii are bigger and brighter, and they attract more potential pollinators than females, supporting that sexual dimorphism influences pollinator visitation preference. Fluorescence dichromatism plays a role on pollinator visitation in this species, in which female flowers’ anthers fluoresce more than male flower anthers suggesting this could be female flowers’ strategy to compensate for their dark colors and small size. The results from this study showed that C. wolfii exhibits sexual dichromatism and fluorescence dichromatism, which is a novel finding in plant research.