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Tracking pollen with quantum dots

20 November 2019

A low-cost technique traces the interactions of flowers and their pollinators.

Carpenter bee with quantum dots.
The wing of a carpenter bee fluoresces from quantum dots attached to pollen. Credit: Corneile Minnaar

Ecosystems and modern agriculture hinge on the movement of pollen, yet the process is not very efficient. Only about 10% of pollen from any given flower finds its way to another flower of the same species, allowing it to reproduce. That inefficiency makes pollen tracking an important area of research. But current tracking methods, such as dyeing pollen grains or tagging them with radioactive elements, either alter the pollen or pose potential hazards to the environment.

To find out what happens to pollen once it leaves its flower, Corneile Minnaar, a pollination biologist at Stellenbosch University in South Africa, turned to quantum physics. He attached semiconductor nanocrystals to individual pollen grains and then viewed their fluorescent glow with a 3D-printed box of his own design that can fit under a dissection microscope. Using only that equipment, Minnaar and his colleagues have shown how one flower species is in the process of evolving into two subtypes.

“This technique changes the way we are able to study pollen movement,” says Minnaar’s colleague Bruce Anderson, a principal investigator in Stellenbosch’s biological interactions lab.

A zoologist by training, Minnaar became interested in quantum mechanics when he read a 2004 Nature Biotechnology paper describing how to track cancer cells in mice with quantum dots, nanometer-sized clumps of semiconductor atoms that fluoresce when illuminated with UV light. “It sounded as though you could attach quantum dots to pretty much anything,” he says, including grains of pollen.

Pollen with quantum dots.
Pollen tagged with quantum dots fluoresces in the stigma of an Oxalis flower. Credit: Corneile Minnaar

Over the course of two years, Minnaar found a way to tag individual pollen grains with quantum dots. Free of toxic heavy metals, the dots he chose were 5–10 nm and composed of a core of copper, indium, selenium, and sulfur and a shell of zinc and sulfur. By adding a lipophilic bonding agent to the dots, he managed to attach them to the sticky outer layer of the pollen, called the pollenkitt. The choice of dot determined the specific optical frequency of its fluorescence when exposed to UV.

However, viewing the grains and their faint glow was an issue. Minnaar did not have thousands of dollars to spend on a fluorescence detection microscope. So he designed a box that could be 3D printed for about $340 and could fit under any dissection microscope.

The box, with dimensions of about 15 cm by 20 cm, blocks out external light and has LEDs that flood the area inside with UV radiation. “The UV-flooded sample can then be viewed through a window that is made from a UV- and violet-blocking filter—only visible light above 450 nanometers gets through the window,” he says. “This allows the user to spot any quantum-dot-labelled pollen without damaging their eyes.”

In a paper published earlier this year in Methods in Ecology and Evolution, Minnaar described his technique and included the design plans for the fluorescence box. His former supervisor, Anderson, quickly made use of the quantum-dot method to study the long-tubed iris Lapeirousia anceps.

Lapeirousia anceps and its pollinator.
(a) Some of the individuals in a population of the iris Lapeirousia anceps have unusually short tubes. (b) The flowers are pollinated by the long-tongued fly Moegistorhynchus longirostris. Credit: C. Minnaar, M. L. de Jager, B. Anderson, New Phytologist 224, 1160 (2019)

Plants and their pollinators often evolve together. For example, the nectar tube of the star orchid in Madagascar is 30 cm—the same length as the tongue of its pollinator, Darwin’s moth. In 2003 Anderson discovered a population of L. anceps irises with both long and short tubes. Yet he could find only long-tongued pollinators, the fly Moegistorhynchus longirostris.

Adaptive divergence, in which populations of the same species evolve differences, is an important process in the formation of new species. Populations of flowers that evolve different tube lengths in response to different pollinators is a good example. But speciation also requires a mechanism that stops divergent populations from interbreeding, which would cause them to lose their differences and collapse back into the same species. Anderson wondered how the subtypes could avoid interbreeding when both relied on the same pollinator.

Anderson, Minnaar, and Stellenbosch colleague Marinus de Jager tagged the pollen from the two different groups of flowers with quantum dots that glowed with different colors, then offered it to the long-tongued fly. They found that the two subtypes were depositing pollen on different areas of the fly’s tongue. When the fly fed on the long-tubed flowers, it deposited pollen from other long-tubed flowers. Similarly, short-tubed flowers received pollen from short-tubed flowers. The scientists published their findings in May in New Phytologist.

Box for dissection microscope.
A box designed by Corneile Minnaar that can be 3D printed for $340 allows researchers to observe the faint fluorescence of quantum dots under a standard dissection microscope. Credit: Corneile Minnaar

David Twell, a plant biologist at the University of Leicester in the UK, says the quantum-dot method has “significant potential” to expand the precision of studies into pollen transport dynamics. And he’s impressed by the relatively low cost of the experimental setup.

Minnaar purposely kept the cost of the viewing box low. Although he could have made money from his innovation, “this method was designed out of necessity for a lab that doesn’t have a lot of money,” he says. “To take it and commercialize it, preventing access for other labs that don’t have a lot of money, seems unethical to me.”

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