Direct tracking of pollen with quantum dots reveals surprising uniformity in dispersal distance across 11 populations of an annual plant.

PREMISE: Pollen movement is a crucial component of dispersal in seed plants. Although pollen dispersal is well studied, methodological constraints have made it challenging to directly track pollen flow within multiple populations across landscapes. We labeled pollen with quantum dots, a new technique that overcomes past limitations, to evaluate the spatial scale of pollen dispersal and its relationship with conspecific density within 11 populations of Clarkia xantiana subsp. xantiana, a bee-pollinated annual plant. METHODS: We used experimental arrays in two years to track pollen movement across distances of 5-35 m within nine populations and across distances of 10-70 m within two additional populations. We tested for distance decay of pollen dispersal, whether conspecific density modulated dispersal distance, and whether dispersal kernels varied among populations across an environmentally complex landscape. RESULTS: Labeled pollen receipt did not decline with distance over 35 m within eight of nine populations or over 70 m within either of two populations. Pollen receipt increased with conspecific density. Overall, dispersal kernels were consistent across populations. CONCLUSIONS: The surprising uniformity in dispersal distance within different populations was likely influenced by low precipitation and plant density in our study years. This suggests that spatiotemporal variation in the abiotic environment substantially influences the extent of gene flow within and among populations.

Pollen layering and male-male competition: Quantum dots demonstrate that pollen grains compete for space on pollinators.

PREMISE: Almost nothing is known about what happens to pollen grains once they attach to pollinators, although some have postulated that pollen from different donors may form complex, two- or three-dimensional landscapes (e.g., layers or mosaics) that can facilitate male-male competition. For example, pollen that is already on pollinators may preclude the deposition of subsequent pollen grains. METHODS: Using quantum dots to mark the pollen of individual flowers, we explored the possibilities of layering and preclusion in a fly-pollinated iris, Moraea lurida. RESULTS AND CONCLUSIONS: The proportion of labeled pollen from the last flower visited diminished in sequential pollen samples taken from the top to the bottom of the pollen load, representing the first empirical evidence for pollen layering. However, the consequences in terms of pollen preclusion were equivocal: Although the pre-existing pollen load size was not a good predictor of new pollen receipt, labeled pollen loads from the last flower visited were significantly smaller than pollen loads from the previous flower visited. Thus, pollen from the previous flower may preclude pollen placement from a subsequently visited flower, and pollen from different flowers may compete for space on pollinators.

A combination of pollen mosaics on pollinators and floral handedness facilitates the increase of outcross pollen movement.

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.