Handed Flowers Pick High Quality Mates from Pollinators

Abstract

Darwin devoted an entire book to stigma and anther polymorphisms, which exemplified the importance of pollen movement efficiency as a selective agent on floral form. However, five years after its publication, his interest was piqued by a description of floral handedness or enantiostyly. Todd described how left handed Solanum rostratum flowers have stigmas deflected to the left and anthers deflected to the right, while the opposite is true for right-handed flowers. In his final scientific correspondence, Darwin wrote to Todd for seeds so that he could … have the pleasure of seeing the flowers and experimenting on them. Darwin died days later on 19 April 1882. More than a century would elapse before the first experiments demonstrated that handedness leads to high rates of outcrossing [5, 6]. By attaching quantum dots to pollen grains, we tracked pollen movement, revealing the mechanisms behind high outcrossing rates in a plant with partial handedness (a majority of anthers deflected to one side). Handedness leads to outcrossing, because left- and right-handed morphs place most of their pollen on different sides of the pollinators. However, we also found that partial handedness results in pollen quality mosaics on each side of pollinators, comprising hotspots and coldspots of outcrossed pollen. Outcrossed pollen receipt was much higher than expected because stigmatic positions are fine-tuned to match the outcross pollen hotspots. Exploitation of these pollen quality mosaics suggests that like animals, female choice in plants can occur before or during the act of mating.