Month: February 2024

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Another Year of Pollination: Viscin Threads

While we’re on the subject of pollen-gluing mechanisms, there is another material apart from pollenkitt that a limited number of flowering plant families use to link their pollen grains together. It functions, much like pollenkitt, by aiding in the attachment of pollen to visiting animals. However, unlike pollenkitt, it isn’t sticky, oily, or viscous, and is instead more like a series of threads. Viscin threads to be exact.

One of the major differences between pollenkitt and viscin threads is their composition. The lipid-rich coating that surrounds pollen grains, which we call pollenkitt, is derived from breakdown materials of an inner layer of the anther. It is added to pollen grains after they are formed and before the anther dehisces. Viscin threads are made up of sporopollenin, the same biopolymer that exine (the outer wall of a pollen grain) is composed of. Viscin threads have points of attachment on an outer layer of the exine called the ektexine. Unlike pollenkitt, viscin threads don’t add new color to pollen grains, nor do they contain scent compounds. Their thickness, length, abundance, and texture are dependent on the species of plant they are found on, much like pollenkitt varies in form and composition depending on species.

pollen strands of tufted evening primrose (Oenothera caespitosa)

Viscin threads evolved independently in three distantly related plant families. These include Onagraceae (the evening primrose family), Ericaceae (the heath family), and a subfamily in the pea family known as Caesalpinioideae (the peacock flower subfamily). Viscin threads are found in many, but not all, of the species in these three families. Some species in other plant families have what appear to be viscin threads but are actually ropy strands of pollenkitt, as they are composed of pollenkitt and not sporopollenin. Because they are made up of the same durable material as exine, viscin threads can be preserved in the fossil record. A paper published in Grana (1996) looked at the morphology of pollen grains with viscin threads from the Tertiary Period and concluded that “this advanced pollination syndrome using viscin threads as a pollen connecting agent” dates back to at least the Eocene and perhaps much earlier.

While pollenkitt’s stickiness adheres pollen grains together, viscin threads are more of a tangling device. Single pollen grains or pollen grain groupings called tetrads become tangled up together and then become entangled with a visiting insect, bird, or bat and carried away to a nearby flower. Disentanglement from the pollinator ideally happens when the threads are brushed against the sticky surface of a stigma. The viscin threads themselves vary by species and family. Micheal Hesse, in a paper published in Grana (1981), describes the threads in Onagraceae as “long, numerous, thin, and sculptured” with “knobs, furrows, etc.,” while those in Ericaceae are thin and smooth and those in Caesalpinioideae are thick and smooth.

smooth azalea, pink form (Rhododendron arborescens)

The length and size of tangled pollen masses also differ by species and can offer clues as to which pollinators visit which flowers. Research published in New Phytologist (2019) looked at the size of pollen thread tangles (PTT) in 13 different species of Rhododendron. They also noted which pollinators visited each species and how often they visited. The researchers found that species presenting pollen in small but abundant PTT were visited by bees, and those with large but few PTT were visited by birds and Lepidoptera (butterflies and moths). Bees also visited the flowers more frequently than birds and Lepidoptera. Bees collect and consume pollen. Between visits to anthers, they spend time grooming themselves, removing pollen clusters from their bodies and packing them into corbiculae (i.e. pollen baskets) for later*. Birds and Lepidoptera don’t groom pollen from their bodies and don’t collect it. In the authors terms, this “suggests pollinator-mediated selection on pollen packaging strategies.” Since flowers pollinated by bees lose much of their pollen in the process, they present it in smaller packages, and since flowers pollinated by birds and Lepidoptera are visited less frequently, their pollen packages are larger.

This is an example of the pollen presentation theory, and is something we will revisit as the Year of Pollination continues.

*This applies specifically to bee species that have corbiculae, and many bee species do not.

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Another Year of Pollination: Pollenkitt

Pollination in flowering plants is the process of moving pollen grains, which carry sperm cells, from the anthers to the stigmas of either the same flower or a separate flower. If things go well from there, sperm cells will be transported via pollen tubes into the ovaries where fertilization with egg cells can take place and seeds can form. Pollen grain development occurs within the anthers, and by the time the anthers dehisce – or split open – they are ready for transport.

In order to protect the enclosed sperm cells and aid in their movement, pollen grains consist of a series of layers that, among other things, help ensure safe travel. Two major layers are an internal layer called intine, composed largely of cellulose, and an external layer called exine, composed mainly of sporopollenin (a highly durable and complex biopolymer). In many flowering plants, especially those that rely on animals to help carry their pollen, an additional outer layer called pollenkitt is added to the pollen grains before anthers dehisce.

three different pollen grains (image credit: wikimedia commons/Asja Radja)

Pollenkitt is an oily, viscous, hydrophobic layer composed of lipids, carotenoids, flavonoids, proteins, and carbohydrates derived from the breakdown of an internal layer of the anther called the tapetum. Pollenkitt forms a sticky layer around the pollen grains and can add color to the pollen other than the typical yellow. The thickness of the pollenkitt and its composition is species specific. In fact, the look, size, and shape of pollen grains themselves are unique to each species and can even be used to help identify plants. Pollenkitt is found in almost all families of flowering plants and is particularly prevalent in species that are animal-pollinated. One exception is the mustard family (Brassicaceae), whose pollen grains are coated in a substance known as tryphine, which functions similar to pollenkitt but whose formation and composition differ enough to be considered separately.

dandelion pollen (image credit: wikimedia commons/Captainpixel)

The sticky nature of pollenkitt has numerous functions. For one, it helps pollen grains remain on anthers until an animal comes along to remove them. It also holds pollen grains together in clumps, helps pollen grains stick to insect (and other animal) pollinators during transport, and helps adhere them to stigmas when deposited. A paper published in Flora (2005) lists twenty possible functions for pollenkitt, many of which have been confirmed in certain species and some of which are hypothetical. In addition to functions having to do with pollen movement and placement, pollenkitt may also provide protection from water loss, UV radiation, and fungal and bacterial invasions. In species where pollen is offered as food to pollinating insects, pollenkitt is a more easily digestible food source than the pollen grain itself. Thanks to carotenoids, pollenkitt can make pollen more colorful, which may help attract pollinating insects, or, depending on the color, can also hide pollen from insect visitors.

Another important function of pollenkitt is to give pollen a scent. Odors can help encourage insect visitors or deter them, so depending on the situation, scented pollenkitt may be attracting pollinators or discouraging pollen consumers. In a study published in American Journal of Botany (1988), Heidi Dobson analyzed the chemical composition of 69 different species of flowering plants. She isolated numerous scent compounds in pollenkitt and suggested that “some of the chemicals in pollenkitt may … serve as identification cues to pollen-foraging bees.” Most of the species she analyzed were pollinated by bees (which consume pollen), but the few that were mainly pollinated by hummingbirds and butterflies tended to have fewer scent compounds. Since birds and butterflies are there for the nectar and not the pollen, it would make sense that the pollen of these plant species wouldn’t need to carry a scent.

bee collecting pollen (image credit: wikimedia commons)

In flowers that are wind-pollinated, the pollenkitt layer is either very thin or absent altogether. In this case, pollen grains need to be easily released from the anther and are better off when they aren’t sticking to other pollen grains. That way, they are free to be carried off in the breeze to nearby flowers. Some plant species are amphiphilous, meaning they can be both animal-pollinated and wind-pollinated, and according to the authors of the paper published in Flora (2005), pollenkitt layers in these species exhibit intermediate characteristics of both types of pollen grains, generally with thinner, less-sticky pollenkitt and more pollenkitt found within the cavities of the exine.

It’s clear that this unique pollen-glueing substance plays a critical role in the pollination process for many plant species. Considering that each species of plant has its own story to tell, there is still more to learn about the forms and functions that pollenkitt takes.

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This is the first in a series of posts in 2024 in which, once again, I am exploring the world of pollinators and pollination. You can read more about this effort in last month’s Year in Review post.