Scientists from South Africa and Brazil have provided empirical evidence that pollen grains from rival plants may compete for space on pollinators, thus influencing whose pollen is likely to reach the next flower.
In an article published in The American Naturalist this week, the researchers argue that because plants can manipulate where and how much pollen is placed on the bodies of pollinators, they may have developed strategies akin to sperm manipulation in animals.
In the animal kingdom, some species have evolved complex structures on their reproductive organs that are thought to remove the sperm of rival males from the reproductive tracts of females before depositing their own. Unlike animals, however, plant mating does not involve direct contact between flowers. Consequently, the manipulation of pollen—pollen carries plant sperm—needs to occur before the pollen reaches another flower, specifically on the bodies of arriving pollinators.
The study focuses on Hypenea macrantha, a deeply red flower endemic to Brazil. The researchers set up an experimental simulation, captured by slow-motion video footage, demonstrating how the flower employs a catapult-like mechanism to effectively remove pollen from rival flowers from the bill of a hummingbird. This mechanism, known as explosive pollen placement, is not entirely unknown in the plant kingdom.
However, this study marks the first time that researchers have provided empirical evidence of the efficacy of this mechanism by physically counting the number of quantum dot-labelled pollen grains removed by the explosive action.
Professor Bruce Anderson, an evolutionary ecologist in Stellenbosch University’s Department of Botany and Zoology and the first author of the article, states that their findings support the idea of competitive pollen removal in plants.
“Flowers visited by hummingbirds deposit their pollen on the hummingbird bills, but there is very little space for the pollen to be deposited. Flowers have evolved a catapult mechanism where pollen is shot at the bill of the hummingbird. The force of the ballistic grains dislodges previously deposited grains from rival plants, allowing the flower to place its own grains onto a cleaner bill, thus increasing its chances of reproductive success.”
In essence, demonstrating effective pollen removal by floral explosion provides the first evidence that male-male competition may have contributed to the evolution of this trait, as the researchers note in their article. Professor Anderson said this finding reveals that plants may be competing with one another in previously unimagined ways.
“Until recently, no one has ever thought of looking for these kinds of structures in plants. For one thing, the flowers of one individual never interact directly with the flowers of another individual when mating occurs. This makes it hard for flowers to manipulate the male gametes of other flowers as animals can do,” he said.
However, gamete manipulation may take place not on other flowers but on pollinators. “Imagine a pollinator arriving at a flower, covered in rival pollen from previously visited plants. The flower may find it difficult to place pollen on the pollinator because all of the space is taken up by the grains of rival plants. Furthermore, even if the flower does manage to place pollen on the pollinator, its chances of reproductive success may be low as it has to compete with all the other rival pollen grains for access to the ovules of the next flower visited,” he adds.
“So, what can a flower do? Like animals with penis adornments, it can evolve strategies to clean the rival grains from pollinators before placing its own grains,” Professor Anderson comments.
Co-author Professor Vinícius Brito, a botanist at the Federal University of Uberlândia in Brazil, notes that previously, floral explosion was thought to primarily aid in placing pollen grains on pollinators or even startling them into flying to new plants, thus dispersing pollen over greater distances.
“Our data suggest, however, that this mechanism may actually displace pollen from previous flowers, enhancing male reproductive success by increasing competition for space on the pollinators’ bodies,” he said.
This groundbreaking research not only sheds light on the complex interactions between plants and their pollinators but also opens up new avenues for understanding plant reproductive strategies. As scientists continue to explore the intricacies of plant competition, the findings from this study may lead to further discoveries about the evolutionary adaptations that enable plants to thrive in their environments.