Darwin devoted an entire book to style and stamen polymorphisms, exemplifying the importance of pollen movement efficiency as a selective agent on floral form.1 However, after its publication, his interest was piqued by a description of floral handedness2 or enantiostyly.3 Todd2 described how left- and right-handed Solanum rostratum flowers have styles deflected to the left and right, respectively. Darwin4 wrote to Todd for seeds so that he could "…have the pleasure of seeing the flowers and experimenting on them," but he died just days later on 19 April 1882. More than a century elapsed 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 in Wachendorfia paniculata, which has one stamen on the same side of the style and two deflected in the opposite direction. We found that handedness leads to outcrossing because left- and right-handed morphs place most of their pollen on different sides of the pollinators. However, the partial separation of stamens and style also results in two-dimensional pollen quality mosaics on each side of carpenter bee pollinators, generating hotspots and coldspots of outcrossed pollen. Similar mosaics were not found on honeybee pollinators. Outcrossed pollen receipt was much higher than expected because stigmatic positions are fine-tuned to match the outcross pollen hotspots on carpenter bees. Exploitation of these pollen mosaics enables plants to increase the probability of between-morph (i.e., disassortative), outcross pollen movement beyond the expectations of enantiostyly.
Keywords: Wachendorfia; enantiostyly; female choice; floral handedness; mate choice; outcrossing; pollen fate; pollen quality mosaic; pollen tracking; quantum dots.
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