Animal – Flower Interactions: Pollination

The flower – insect symbiosis is one of the most successful partnerships in biology. I’m talking, of course, about pollination (bee goes in flower to get nectar, picks up pollen on the way, goes to another flower). Flowers often have specialised designs and secrete specific molecules to attract insects. In fact, it is this diversity, and the resulting coevolution of flowers and their insect pollinators, that are key to the angiosperms’ explosive radiation.

Before starting off on different ways flowers attract insects, it’s important to note that insects sense the world much differently than we do. For example, honey bees have much worse visual resolution than us. They may have thousands of eyes, but the overall resolution is ~ 100 times worse than the human eye. This means they need to get very close to a flower in order to actually recognise it. That said, their colour perception is substantially more complex than in mammals.

They have three types of ommatidia. Each type contains six green receptor cells (which allow them to see motion), but they differ in the other cell types they contain: one has two UV receptors, another has two blue receptors and another has one of each. Each ommatidium type sees colour differently, and none of them sees it as a human does (we have red, green and blue in our eyes, with individuals having different amounts of each). This tiny summary doesn’t do the bee’s colour perception system justice without going into how the all the inputs are processed, but that would be too complicated for here. Suffice to say that a bee’s brain has to learn how to distinguish the information from each ommatidium in order to get proper colour vision; a bee’s vision improves as it gets older.

But even this seemingly chaotic visual system does not hold a candle to the complexity of the odor perception of a bee. In each antenna, there are at least 130 different types of odor receptors, making up around 60000 receptors! Again, going into details would be overkill here, but it’s enough to say that despite so many odor receptors, we can group them into four broad categories, according to which chemicals they respond to.

Bee Odor Perception. Chittka & Raine, 2006.

What I’m trying to say is that bees see and smell the world much differently than us. A flower that is plain white in our eyes may have UV patterns scrawled all over it; a seemingly odorless flower may smell like ass to a bee. This kind of thinking is absolutely essential to any zoologist: humans are just one species, and not even a model species at that. Your human physiology, behaviour or perception is nothing special, but do not try to project it on any other taxon outside of the primate tree – and leave it far away from my adorably organised, segmented, exoskeleton-clad arthropods.

But we’re getting side-tracked here, so let’s get back to pollination. I mentioned coevolution at the beginning. Looking at wind-pollinated and insect-pollinated flowers, we can see where natural selection has acted: the actual flower. Insect-pollinated flowers vary wildly in shape, colour and scent – all in order to seduce their pollinators.

Often, they are adapted to varying degrees to which animal pollinates them. Beetle-pollinated flowers tend to smell like food (in beetle’s cases, feces is a popular choice). Bee-pollinated flowers are markedly different, being specially adapted to appeal to bees: they have bright colours (yellow, blue, violet) that the bee’s visual system picks up and differentiates easily. They often have markings on them (which we cannot see except with special filters – see this link).

Huernia zebrina

The shape of the flower also changes: beetles have a mouth straight on their head, so those flowers pollinated by them are open for easy access (see picture above). Compare this to those pollinated by butterflies: butterflies have a very long proboscis, so the flower can be tube-like with a small ‘landing platform’ for the insect to stand on (see below for a flower pollinated by humminbirds, who are in a similar situation due to their long, thin tongue, but do not need a landing platform).

Tristerix colymbosus

Flowering time can also vary, with moth- (or bat-) pollinated flowers opening only at night.

The way the flower’s sex organs are arranged is also important. As a reminder (to be honest, I forget them all the time): stigma is female and where the pollen has to go, stamen is male and where pollen gets picked up. The usual set-up is the stigmas being towards the outside, so pollen from other flowers gets deposited during the insect’s entry, with the stamens found further down, on the way to the nectar.

All this variation in the flower’s look and smell is very tightly regulated. The smell is caused by various specific volatile compounds produced by metabolism and released not accidentally, but by specialised, controlled structures in the petals. Flower colour is even more specific. It is caused by various compounds called carotenoids (yellow, orange, red), anthocyanins (red, blue, purple) and other molecule families. The structures of these compounds, in typical organic chemistry fashion, are variable, producing slightly different hues. In addition, the final colour is determined by other factors such as pH and the contents of the cell’s vacuole (which in turn depends on the soil). These are then all arranged in patterns dictated by gene expression. These may be simple lines, but some are highly specific and can even be examples of biomimicry: some orchids have petal colours and shapes that, to a male bee, look exactly like a female. The male bee then actually copulates with the flower (in the extreme cases, the flower also releases compounds that copy the female pheromones!)

This was just an introduction to the fascinating world of insect – flower interactions. Before concluding, one word on coevolution. As I said earlier, coevolving with insects is one of the key innovations of the flowering plants that allowed them to dominate the planet so quickly. But some may call this parasitism instead of symbiosis. After all, the bee doesn’t care about pollination. It just wants to drink the nectar because it tastes good and it’s very energetic. Looking at it this way, flowers are parasites on insects. But that doesn’t mean it’s not coevolution; parasitism nothing more than a very specialised case of coevolution. But even though the bee doesn’t consciously pollinate, it is given a reward by the plant – thus making it a symbiotic interaction.

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