What is a Pollination Syndrome?
When you hear the word ‘syndrome’ your first thought isn’t likely to be about flowers. In the context of pollination you have to look at the definition of the word syndrome: “A group of symptoms which consistently occur together, or a condition characterized by a set of associated symptoms.” What a pollination syndrome refers to is a group of flower characteristics (the ‘symptoms’) like color, scent, and petal shape that are found in particular combinations. These sets of flower characteristics can be indicative of the animals that visit them (the ‘condition’) for pollen or nectar. When you look at the dominant animal visitors to flowers of a particular pollination syndrome, it almost seems like the flower evolved specifically for that animal to visit.
Take for instance a hummingbird feeding on an Ipomopsis flower (pictured below). The petals of the Ipomopsis flower are fused to form a long tube and at the base of the tube are glands secreting sugary nectar, which is what the hummingbird likes to feed on. The flowers are also red, just like hummingbird feeders and feeder mixes that people put up in their yards. Not only does the hummingbird’s long bill reach comfortably to the end of the flower, but hummingbirds can also see red incredibly well. Meanwhile, bumblebees can’t reach the nectar through the opening of the flower, so they will make their own by biting a hole in the petals. We can characterize this set of flower traits (long floral tube, red, nectar-filled) as catering almost specifically to a hummingbird, thus labelling Ipomopsis as a hummingbird-pollinated flower.
While this group of floral traits is perfect for a hummingbird, what benefit does this provide the plant, or is the plant just being nice? And why would a flower want to exclude bumblebees, which are known to be excellent pollinators? It turns out that this match with hummingbirds is driven by the ‘self-interest’ of the Ipomopsis. Hummingbirds are far more likely visit plants with red, tubular, nectar-filled flowers, so by entrusting only hummingbirds with its pollen Ipomopsis plants are less likely to have pollen deposited on the wrong species of plant (there are only so many red, tubular flowers to choose from in a floral community), and more likely to receive pollen from another Ipomopsis plant. While this seems like a unique and special relationship, the evolution of these two organisms to match each other, has occurred many times through evolutionary history. This pollination syndrome has evolved several times in distantly related groups of plants including columbine, honeysuckle, snow plant and many more. This phenomenon of unrelated groups of organisms evolving the same adaptations is known as convergent evolution. Similar pressures (the reproductive benefits of floral visitation by hummingbirds) seem to consistently yield the same traits regardless of that plant’s genetic background.
This phenomenon of plants matching their pollinators goes far beyond catering to hummingbirds. Other plants have evolved sets of traits to attract different groups of pollinators. There are moth-pollinated, bumblebee-pollinated, bat-pollinated, fly, beetle, rodent and even gecko-pollinated plants. All of these plants have evolved a particular set of traits well-suited to attract a specific group of animals, often to the exclusion of other animals. A classic example that likely contributed to the idea of pollination syndromes was the story of Darwin’s hawkmoth. Darwin was sent an orchid specimen that a
colleague had collected in Madagascar. The orchid was white, with an extremely long spur extending down from the floral tube, filled with nectar. Darwin, having never seen this orchid before, predicted that it was pollinated by a moth with an equally long tongue. Years later, other entomologists confirmed that Darwin was correct, and that the orchid was pollinated by a hawk moth known as Xanthopan morganii praedicta, which has a tongue almost 10 inches long, a perfect fit for Darwin’s Orchid.
Buzz Pollination, the Loneliest Pollination Syndrome
Until recently, the interaction known as buzz pollination was not usually considered in scientific conversations about pollination syndromes. Buzz pollination is a pollination syndrome defined by anthers (the floral structures that hold the pollen) that have a narrow opening at the tip (poricidal anthers) and must be vibrated at a particular frequency to release pollen. Bees are the only animals that can reliably access this securely stored pollen. To do this, bees “uncouple” their wing muscles from their wings and vibrate their muscles; a process known as sonication. If you are observing bumblebees on a flowering bush and you listen carefully you may be able to hear them sonicating. Just like the Ipomopsis, buzz pollinated plants (tomatoes for instance) are able to restrict access from other animals, while maintaining access to a particular group of high-quality pollinators. This confers a benefit to the plant by ensuring its pollen is collected by a pollinator likely to visit another flower of the same species, and by protecting its pollen from low-quality pollinators. Buzz pollinated plants are not just restricted to bees, but specifically to the subset of bees that can sonicate the pollen from the plants, which constitutes about 58% of all bee species (Cardinal et al. 2018). As with other pollination syndromes, buzz pollination has evolved several times across distantly related lineages. One peculiarity about buzz pollination is that in many lineages where it has evolved, the flowers have forgone providing nectar as a floral reward for pollinators, and instead only offer their well-guarded pollen. This quality (amongst others) has led to some fairly interesting new types of pollination syndromes.
What’s the Buzz about Buzz Pollination?
A recent scientific paper published in the academic journal New Phytologist explored the idea that adaptations for buzz pollination can prevent a lineage of plants from evolving the “traditional” pollination syndromes, instead taking alternative evolutionary pathways to attract the same group of pollinators or even forming new pollination syndromes (Dellinger et al. 2018). As mentioned above, many plants that have evolved buzz pollination have forgone producing nectar in favor of providing pollen as a nutritious reward for pollinating animals. This was the case in the ancestor of the researched group of plants, the Merianeae, which has since evolved multiple different pollination syndromes. None of those descendants of the Merianeae that changed their pollination syndrome away from buzz pollination have converged on any of the “traditional” pollination syndromes. However, some of the Merianine species still responded to the benefits of pollination by hummingbirds. While the architecture for the typical location of a nectary was lost long ago in the Merianeae, 7 of the 19 investigated species developed nectar glands on their filaments (the “stem” that holds up the anther) and anthers that open downward (depositing pollen toward a pollinator accessing the nectar). The researchers found this group of species to be pollinated almost exclusively by hummingbirds. This is a strange case where similar selective pressures lead to a different floral shape, yet still attracted hummingbirds.
‘p’ indicates the pore, or the opening in the anther; ‘a’ indicates an appendage for manipulating pollinator behavior. Photos and diagrams from Dellinger et al. 2018
But buzz pollination didn’t just cause reinvention of a proven floral syndrome, it also provided the opportunity for evolutionary innovation. The Merianeae also evolved a new pollination syndrome that is one of my favorites: Passerine pollination. Passerines are also known as the perching birds (sparrows, robins, chickadees, etc.), which are not known for their pollen-feeding capabilities. In the case of passerine pollination, the anthers of the flower are enlarged, nutritious, and brightly colored, which is thought to be highly attractive to Passerines, who often forage for similar-looking berries. But how does the pollination happen if the birds are just plucking the anthers off the plant? Inside of these enlarged anthers is a pocket of air, and when grabbed by a bird the anther acts like a bellows, ejecting pollen out of the anther and onto the bird’s bill and face feathers. This pollen can then be transferred to another flower on a successive floral visit.
This study explored a few interesting concepts. The one that interested me the most is that selection has to work with what an organism has in the present. If a plant lost it’s nectaries, it has to evolve another way to attract pollinators, whether by evolving nectaries again, or by an entirely novel method. From a broader perspective, this means that the context (what type of animal community was present, what type of climate, etc) in which selective pressures are applied to an organism is crucial in determining the response to that pressure.
Works Cited:
Cardinal, S., Buchmann, S. L., & Russell, A. L. (2018). The evolution of floral sonication, a pollen foraging behavior used by bees (Anthophila). Evolution, 72(3), 590–600. https://doi.org/10.1111/evo.13446
Dellinger, A. S., Chartier, M., Fern, D., Penneys, D. S., Alvear, M., Michelangeli, A., … Armbruster, W. S. (2018). Beyond buzz-pollination – departures from an adaptive plateau lead to new pollination syndromes. New Phytologist, (October). https://doi.org/10.1111/nph.15468
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