Plants fight for pollen space from pollinators

Scientists from South Africa and Brazil have provided empirical evidence that pollen grains from rival plants can compete with each other for space on pollinators, thereby influencing which pollen will or will not reach the next flower.

In an article published in The American naturalistThey argue that because plants can manipulate where and how much pollen is placed on the bodies of pollinators, plants may have evolved strategies similar to sperm manipulation in animals.

Some animals and insects have evolved complex structures on their penises, thought to remove rival males’ sperm from the females’ reproductive tracts before depositing their own. However, unlike animals, plant mating does not involve direct contact between flowers. Therefore, pollen manipulation (pollen carries plant sperm) must take place before the pollen reaches another flower, on the body of the arriving pollinators.

In the case of Hypenea macrantha, a deeply red flower endemic to Brazil, the researchers set up an experimental simulation, captured by slow-motion video footage, showing how the flower uses a catapult-like mechanism to efficiently remove pollen rival flowers of the beak. of a hummingbird skull, safely placing its own pollen in one place.

This mechanism, called explosive pollen placement, is not unknown in the plant kingdom. However, this is the first time the researchers can provide empirical evidence of the mechanism’s effectiveness by physically counting the number of quantum dot-marked pollens removed by the explosion.

Professor Bruce Anderson, an evolutionary ecologist in the Department of Botany and Zoology at Stellenbosch University and first author of the paper, says their findings provide evidence for the idea of ​​competitive pollen elimination in plants.

He explains: “Flowers visited by hummingbirds deposit their pollen on the hummingbird’s beak, but there is very little room for the pollen to be deposited. The flowers have evolved a catapult mechanism where pollen is thrown onto the hummingbird’s beak. Ballistic grains dislodge previously deposited grains from rival plants, allowing the flower to place its own grains on a cleaner beak, thereby increasing its chances of reproductive success. »

In other words, a demonstration of efficient pollen removal by floral burst would provide the first evidence that male-male competition may have contributed to the evolution of this trait, they write in the paper.

For Anderson, this discovery shows that plants can compete in ways previously unimaginable.

“Until recently, no one had ever thought to look for these types of structures in plants. On the one hand, the flowers of one individual never directly interact with the flowers of another individual when “This makes it difficult for flowers to manipulate the male gametes of other flowers like animals can,” he explains.

But gamete manipulation can take place, not on other flowers, but on pollinators, Anderson comments: “Imagine a pollinator arriving at a flower, covered in rival pollen from previously visited plants. The flower may have difficulty placing pollen on the pollinator because all the space is taken up by grains from rival plants and, furthermore, even if the flower manages to place pollen on the pollinator, its chances of reproductive success may be weak because it must compete with all other rival pollen grains for access to the ovules of the next flower visited.

“So what can a flower do? Like animals with penis ornaments, it can strategize to clear rival grains from pollinators before placing their own grains.”

Professor Vinícius Brito, a botanist at the Federal University of Uberlândia in Brazil and co-author, previously said the floral burst was thought to mainly help place pollen grains on or even encourage pollinators to fly towards new plants, thus dispersing pollen over greater distances.

“Our data, however, suggest that this mechanism may actually displace pollen from previous flowers, thereby improving male reproductive success by increasing competition for space on pollinator bodies,” he concludes.