This year (2023 for those of you reading in the future) has had one of the worst allergy seasons in recent memory. If you are like me, you likely check the pollen count every morning to see just how bad your allergies are going to be. Here in the BeeLab, pollen is of interest for another reason: bees play a crucial role in transferring pollen amongst plants and they also use it as a source of protein. They often end up covered in the stuff as they forage for nectar, like the bee in the image below (my eyes are itching just looking at it). If we want to get a better understanding of bees and the environment they live in, having an appreciation for the function and structure of pollen will prove quite useful.
[1] A bee covered in dandelion pollen.
In terms of the function of pollen, most people probably remember from biology class that it is used by flowering plants for reproduction. Specifically, each pollen grain contains male gametes that are released upon contact with the stigma, the female reproductive structure in flowers. Based on that description of pollen, it seems like pollen is just a ball for male gametes to ride around in until they bump into something to fertilize. There can’t be too many ways to make such a ball, right? Wrong! Pollen comes in all shapes and sizes. Exactly why this occurs is still poorly understood. One guess is that certain shapes of pollen are better at reaching a stigma than others depending on how the plant they come from is pollinated (by bees, wind, birds, etc.). For example, there might be a shape of pollen that is better at sticking to a bee, like in the picture above, whereas another might be better at flying in the wind, like on the tree below. In fact, there’s a whole field of research called palynology that studies pollen structure. It has applications in natural history, anthropology, honey production, forensics, biology, and informing you of just how bad your allergies are going to be when you step outside.
[2] A cloud of pollen being blown off a tree.
The reason that pollen is so useful in all these different fields is because pollen decomposes very slowly and plants produce a lot of it. This results in pollen grains being present in lots of places, which you can use to get an idea about which plants are or used to be around these places.To give just one example, say you are an anthropologist at at archaeological site and want to determine what kind of plants people were eating and growing thousands of years ago. Most plant matter that would be at the dig site deteriorated long ago, but not the pollen! Say you find a sample of it (maybe on old clothes or in fecal matter). If you know how to classify pollen, you would be able to figure out what kind of plant it came from, giving you knowledge about what types of plants people had around them back then. With bees, the ability to classify pollen gives us information about what kind of plants they are foraging from.
So how do we classify pollen? Thanks to Morgan Carr-Markell, a postdoctoral researcher in the BeeLab, I got access to these fantastic microscope photos she took of pollen grains obtained from plants we have around us in Southern California. We’ll use a couple of them to get an idea of what pollen looks like under a microscope, the different forms it can take, and how we describe them. To describe the structure of any pollen grain, we will look for the answers to a few key questions. Is the pollen alone or in a clump with other pollen grains? What kind of features are on its surface? How many features are there on the surface? How big is it? What is the overall shape?
[3] Chamaesyce albomarginata pollen grain viewed through a microscope.
First, let’s consider the above pollen grain from Chamaesyce albomarginata, commonly known as Rattlesnake Weed. We notice that there is only one pollen grain. That is, pollen from this plant comes in groups of one. We thus call it a monad. Next, we see that this pollen grain appears to have three “arms” protruding out radially. In the language of palynology, we would call these apertures, which just refers to any feature on the surface of a pollen grain. Going further, this type of aperture is a colpus, and we would call this pollen grain tricolpate because there are three of them. From the scale, we can see that it is a little less than 25 microns in diameter. Overall, it seems like it is spherical in shape. With this as a point of comparison, let’s look at another pollen grain.
[4] Opuntia littoralis pollen grain viewed through a microscope.
This pollen grain is from Opuntia littoralis, better known as the Prickly Pear Cactus. We can see that this grain has an appearance that is fairly different from the last one. Using our vocabulary from last time, we can see that it is a monad with lots of apertures! Instead of the apertures being colpi (the plural of colpate) we see that this grain looks like it has a bunch of (roughly) circular apertures spread across its surface. Each of these circles is called a porus, plural pori. As there are many, we would classify this grain as being pantoporate (panto comes from the Greek pan- which means all). In terms of size, it looks like this grain is more than 50 microns across, over twice as large as the grain from Chamaesyce albomarginata! Finally, we note that it is also spherical in overall shape.
Although we have only scratched the surface in terms of the diversity present in pollen grains, we can already see that they come in a variety of shapes and sizes. Thanks to this diversity, we are able to use them as an indispensable tool in many scientific and industrial disciplines, like determining tomorrow’s pollen count.
Image Credits
[1] Photo by Guérin Nicolas. https://commons.wikimedia.org/wiki/File:Pollination_Bee_Dandelion_Zoom.JPG
[2] Photo by Beatriz Moisset
https://commons.wikimedia.org/wiki/File:Pollen_from_pine_tree_2.jpg
[3] Photo of pollen from California Botanical Garden RSA Herbarium Chamaesyce albomarginata specimen, accession number 817438. Prepared and photographed by Morgan Carr-Markell. https://www.cch2.org/portal/collections/individual/index.php?occid=639687&clid=0
[4] Photo of pollen from California Botanical Garden RSA Herbarium Opuntia littoralis var. austrocalifornica specimen, accession number 459613. Prepared and photographed by Morgan Carr-Markell. https://www.cch2.org/portal/collections/individual/index.php?occid=665478&clid=0
Further Reading
Handout from the University of Western Australia that goes slightly more in-depth on pollen structure and classification: https://www.uwa.edu.au/study/-/media/Faculties/Science/Docs/What-is-pollen.pdf
Open access textbook that goes very in-depth on pollen structure and classification, including instructions on how to create your own images like the ones above: https://link.springer.com/book/10.1007/978-3-319-71365-6
For more about why pollen comes in different shapes and sizes:
Jardine PE, Palazzesi L, Tellería MC, Barreda VD. Why does pollen morphology vary? Evolutionary dynamics and morphospace occupation in the largest angiosperm order (Asterales). New Phytol. 2022 May;234(3):1075-1087. doi: 10.1111/nph.18024. Epub 2022 Mar 3. PMID: 35147224.
Database of pollen grains with photos and descriptions : https://www.paldat.org/
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