What is Pollination?


A bee pollinating a white flower

Augochlorella sp. sweat bee on Yarrow. Image credit: Heather Broccard-Bell

Author: Heather Broccard-Bell, Ph.D., Honey Bee Health Researcher. 


WHAT IS POLLINATION?
 

Happy Pollinator Week, everyone! 

At NOD, we typically focus on one specific pollinator: the Western Honey Bee (Apis mellifera). Last year, I wrote about how the amazing communication system of the honey bee makes this species a super pollinator. 

This year, I want to cover a topic that seems to be curiously absent from most discussions about pollinators: what is pollination?  

Most people know that pollination is important for food production, but a healthy number would struggle if pressed to explain exactly how or why that’s the case. So, let’s start at the beginning. 

IT’S LITERALLY THE BIRDS AND THE BEES 

Pollination is an important part of the life cycle of plants that produce seeds. Like animals, plants generate new plants using sexual reproduction – the combining of egg and sperm cells, known as fertilization. Pollination is what leads to fertilization. As in animals, fertilization is all about creating offspring having a mix of genes from each parent. I talked about the importance of this type of mixing in my recent post about inbreeding and varroa mites.

Fertilization in animals generally happens when male animals come into direct contact with female animals. Animal sperm cells come equipped with a tail that makes them able to move around to get to eggs. Males just need to deliver sperm in the general vicinity of an egg, and the sperm will take care of the rest. 

But what do you do if you’re a plant and you can’t move around? If you live somewhere like a swamp, having motile sperm like animals can still work – and in fact, a few plant species do use this approach. But far in the distant past, plants began to grow in drier and drier places. To do so, they had to solve the problem of how to reproduce with little or no water. Motile sperm with no way of delivering sperm to eggs was of no help!

ATTACK OF THE CLONES 

Plants have been wildly successful at colonizing even the most far-flung regions of the planet, in no small part because of their remarkable ability to do something most animals can’t: clone themselves. No need to worry about the problem of fertilization if propagation from a small part of the original is all that’s needed! 

However, as it is with varroa, so it is for plants. The trade-off for relatively easy reproduction is a whole population with identical weaknesses. One well-known example in agriculture is the Gros Michel variety of banana, the predominant type of dessert banana grown until the 1950s. 

ABSOLUTELY BANANAS 

Bananas have been cultivated by humans for some 10,000 years. Like many crops, the bananas we eat today bear very little resemblance to the wild plants from which they are derived. The act of choosing only to grow bananas from seeds of plants with the characteristics we liked over thousands of years fundamentally changed banana plants. For example, wild bananas have many large seeds (see Figure 1), whereas cultivated bananas have tiny seeds, if they have seeds at all. In fact, we’ve so successfully selected against banana seeds that today it is virtually impossible to grow bananas from seed. All modern commercial banana plants are clones. 

FIGURE 1: Cross section of a wild-type banana with many large seeds. Image credit: Warut Roonguthai, Wikimedia Commons. 

Up until the 1950s, the Gros Michel, grown on vast plantations in South America, accounted for most of dessert banana production globally. However, Panama Disease, a fungal infection to which Gros Michel was particularly susceptible, began to wipe out the crop in the early 20th century. Since all Gros Michel banana plants were clones, they were all equally susceptible to Panama Disease. Thus, by the 1950s, large-scale production of the Gros Michel had become untenable, and the cultivar was replaced by the now-familiar Cavendish variety. Although they have not gone extinct, today most of us have never tasted a Gros Michel banana [1]. 

And this is where we come back to pollinators and pollination. 

POLLINATION 101 

Pollination occurs when pollen grains produced by male flower parts, collectively called the stamen, contacts female flower parts, collectively called the pistil (see Figure 2). Many flowers contain both stamen and pistils. Although some plants can self-pollinate, a repeating theme of biology is that genetic mixing is usually preferred. Consequently, many plants have adaptations that prevent self-pollination, such as pistils and stamen parts reaching maturity at different times.

FIGURE 2: Diagram of a mature flower. Image credit: Mariana Ruiz, Wikimedia Commons.

A pollen grain is a complex structure, consisting of an outer coating wrapped around at least one each of two types of cells: tube cells and generative cells. When a pollen grain contacts the stigma of a pistil, a cascade of events that eventually leads to fertilization and the development of a seed is initiated. But how does pollen make its way from stamen to pistil? 

GETTING FROM A TO B 

The simplest way to get pollen from one plant to another is to let the wind carry it. Although this is a low-effort method, it is not very precise. Nonetheless, many plants – from conifers to almost all species of grass (including most cereal crops) are still largely wind-pollinated. 

Some groups of plants take pollination to the next level, recruiting animals to help them out. First, as all beekeepers know, pollen itself is a great source of protein, attractive to many animals on its own. Many plants also produce substances, like nectar, to attract even more animals to their flowers. As animals, like insects, birds, and bats fly through the air, their bodies accumulate electrical charge. Interestingly, pollen grains are also electrically charged. The result is that pollen grains are drawn to animals when they touch or even just get close to flowers – whether they are there for the pollen or not. 

As animals forage at multiple flowers, they inadvertently transport some grains of pollen between flowers of different plants. Fortunately for animals and plants, relatively few pollen grains are required for plant reproduction, so there is plenty to go around for everyone. 

FROM POLLINATION TO SEED 

A seed is an amazing structure. It is long-term storage capsule comprised of a tough outer covering that encases a dormant embryonic plant and its food supply. Astonishingly, some seeds can remain viable, just waiting for the right conditions for thousands of years. The ability of seeds to delay germination until conditions are favourable was a major leap toward plants being able to colonize nearly all dry land. Prior to seeds, plants relied on spores for sexual reproduction – and that meant they could thrive only in damp environments. 

Seeds only develop after pollination. Once a pollen grain makes it to the stigma of a flower, the tube cell inside the grain begins to grow down through the style, creating a pollen tube. As it grows, the generative cell divides, resulting in two sperm cells. The sperm are transported down the pollen tube, into the ovary, where they can then enter an ovule. An egg, along with other types of cells, are present inside the ovule. 

For a flowering plant, fertilization is not quite as straightforward as sperm and egg uniting. That “regular” type of fertilization also needs to occur, but in order to produce a seed, one of the two sperm that travels down the pollen tube must fertilize one of the other cells inside the ovule, a polar cell. Whereas the fertilized egg goes on to become an embryonic plant, the fertilized polar cell grows into the endosperm – the food source for the baby plant. After fertilization, the ovule becomes the tough seed coat — and the ovary develops into a fruit. 

FRUIT: MORE THAN JUST FRUIT! 

This brings us to the importance of pollinators in agriculture: pollination is a prerequisite for the development of fruit! 

Fruits come in many shapes and sizes – including a whole lot of things, like the fluffy pappus from a dandelion, that you probably never thought of as being fruit. In general terms, a fruit’s job is to get a seed from its parent plant to a favourable location to germinate and grow. In an ideal world, this would be far away from the parent so the two plants don’t have to compete for resources.  

Edible fruits come from plants that make use of animals for transporting their seeds. Seeds are eaten with the tasty treat and then – ahem – “deposited” later, often far from the site of ingestion. That tough seed coat plays a major role in the seed’s ability to make it through the digestion process unscathed! 

NOT ALL HEROES WEAR CAPES 

Over millennia, humans have selectively bred plants to create fruits, like bananas, that scarcely resemble their ancestors. We have amplified the characteristics we like, such as sugar content and size [2]. Despite all those modifications – not to mention our innumerable technological innovations — we still mainly [3] rely on a plethora of insect, bird, and mammal pollinators to carry out the necessary pollination. 

Even though the Western Honey Bee is the most economically important agricultural pollinator globally, it is important not to forget the contributions of a host of other species. Non-honey bee pollinators also contribute to crop pollination. Outside of agriculture, pollinators are crucial for maintaining sufficient plant diversity that translates into healthy ecosystems – and a healthier planet overall.   

To help everyone appreciate some lesser-known pollinators, here is a collection of photos I took last week on the grounds at NOD’s new Trenton facility: future home of our Honey Bee Health and Education Centre! Click on the images in the album to learn about the animal and plant species.

 

FOOTNOTES 

[1] The “banana” flavouring of banana candies is the result of a compound called isoamyl acetate (aka, isopentyl acetate) – a molecule produced by actual bananas. The Gros Michel banana contains a naturally higher concentration of isoamyl acetate than does the Cavendish, and the difference may explain why most people find banana-flavoured foods don’t taste like real bananas. However, the full history of banana flavouring and its relationship to the Gros Michel remains somewhat murky. 

Biology often makes use of the same chemical in multiple unrelated and surprising contexts. Intriguingly, isopentyl acetate is also one of the main components of honey bee alarm pheromone. So, when people say that alarm pheromone smells like bananas, that is not a coincidence: it is the same chemical.

[2] I would be remiss to not mention here the important fact that not all food crops are fruits! For example, the bulk of the staple crops that feed the world — cereal grains and rice — are seeds. Whereas humans have bred fruit crops for larger-than-usual fruits, with grains, we’ve selected for larger-than-usual endosperm within seeds. Crops like potatoes are tubers, which are not roots, but enlarged underground stems. Lettuce and spinach are leaves. Broccoli and cauliflower are flower buds that are harvested before they mature. None of these crops require pollination by animals to harvest.

It is an often-repeated myth that humans would starve if pollinators were to go extinct. This is not true in an immediate sense, although our diets would certainly become less diverse. But because pollinators are intimately connected to all levels of ecosystems, the long-term impact of a pollinator collapse would almost certainly be catastrophic – although specifically how it would all play out is a matter of considerable uncertainty.

[3] In another interesting banana-related aside, bananas are one of a few fruits that do not require pollination at all – insect, wind, or otherwise. The ovary tissue of the banana grows into a fruit without the twin fertilization of egg and polar cells, a phenomenon known as parthenocarpy. Hence, the reason modern bananas lack seeds.

 

About Heather Broccard-Bell, Ph.D.

Dr. Heather Broccard-Bell is the Honey Bee Health Researcher at NOD Apiary Products. She is a scientist and educator with over 15 years research and teaching experience. Heather has been focused on investigating issues surrounding honey bee health and communication since 2014. When Heather’s not in the lab, you can usually find her in the bee yard or on a trail hiking with her many pawed pals. If you’d like to ask Dr. Heather Broccard-Bell more about her work or this article, you can email her at heatherb@nodglobal.com 

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