Using quantum dots as pollen labels to track the fates of individual pollen grains

Despite a long history of significant advances in understanding natural selection and evolution, the field of plant reproductive biology has largely studied plant mating without directly tracking pollen movement due to a lack of suitable pollen‐tracking methods. Here, we develop and test a novel pollen‐tracking technique using quantum dots as pollen‐grain labels. Quantum dots are semiconductor nanocrystals that are so small, they behave like atoms. When exposed to UV light, they emit extremely bright light in both visible and infrared wavelengths. We tested the suitability of non‐toxic CuInSe x S 2 −  x /ZnS (core/shell) quantum dots with oleic‐acid ligands as pollen‐grain labels. Using a micropipette, we dispensed quantum dots dissolved in hexane in minute volumes (0.15–0.5 μl) directly onto dehisced anthers of four different plant species from four different families ( Wachendorfia paniculata [Haemodoraceae], Sparaxis villosa [Iridaceae], Arctotheca calendula [Asteraceae], Oxalis purpurea [Oxalidaceae]). After application, the hexane solvent evaporated immediately, leaving behind quantum dots that remained attached to pollen grains of the four different plant species even after agitation in a polar solvent. This suggests a lipophilic interaction between oleic‐acid ligands on quantum dots, and pollenkitt surrounding pollen grains. We also showed that most pollen grains within anthers of the same four plant species were labelled with quantum dots after applying a volume of quantum‐dot solution sufficient to cover an individual anther. To test whether quantum‐dot pollen labels influenced pollen transport, we conducted pollen transfer trials (one donor, 10 sequential recipients) on S. villosa using captively reared honey bees to ensure bees were free of external pollen prior to experiments. We found no difference in pollen transport to recipients from donor flowers with labelled or unlabelled pollen grains. We demonstrate that quantum dots can be used as pollen labels allowing subsequent tracking of pollen fates. This method is relatively inexpensive (<$500 for equipment and ca. $0.02 per labelled anther thereafter) and can be simply and directly applied to anthers of most flowers in the lab and field. The ability to track pollen grain movement in situ, may help to address a historically neglected aspect of plant reproductive ecology and evolution.