Why did some plant species change pollinators during their evolution? An international team of researchers from the universities of Bonn and Xi’an Jiaotong-Liverpool University Suzhou (China) studied the reproductive systems of three pairs of sister species, where one species is pollinated by insects and the other by hummingbirds. Mechanisms have been discovered that explain the switch from pollination by insects to that of birds. The study has now appeared in the journal Ecology and evolution.
Different strategies have evolved in the pollination of flowering plants. The frequency and effectiveness of the flower visitor play a role. Here, there are major differences between different groups of animals. Worldwide, insects, especially bees, are the most common pollinators. Bees generally have fairly small ranges of activity while other pollinator groups such as hummingbirds fly much longer distances. “It was previously assumed that plants switch their pollinator group from bees to hummingbirds when the activity and therefore the pollination efficiency of bees is too low or too unpredictable, for example in high mountains,” explains Dr Stefan Abrahamczyk from the Nees Institute for Plant Biodiversity at the University of Bonn. For example, in high tropical mountain cloud forests, it is often too wet or too cold for many bees.
But why are there plants in regions of high bee diversity and abundance that have nevertheless shifted to hummingbirds, bats, or even small land mammals such as mice, lemurs, or possums? In the current study, Dr. Abrahamczyk and his colleagues showed that the reasons for the evolutionary switching of pollinator groups are much more complex than expected. When two new species arise from an original species during evolution, for example because their range is divided by mountain folding or an ice age, the two newly formed species are called a sister species pair. .
The researchers analyzed three pairs of sister species from different plant families in terms of reproductive strategies. In each case, one sister species is pollinated by hummingbirds and the other is pollinated by bees. All species descended from bee-pollinated ancestors and are found in regions of North America characterized by high diversity and abundance of bees. Using a series of pollination experiments, it was found that all species pollinated by hummingbirds had significantly higher seed production and seeds had significantly higher germination rates when they resulted pollination with pollen from another individual of the same species.
“From these results, we can conclude that hummingbird pollination has evolved in populations of bee-pollinated species that are particularly dependent on cross-pollination, i.e., they cannot self-pollinate,” says Dr. Abrahmamczyk. Due to their greater range of activity compared to bees and their frequent movement between different plant individuals of the same species, hummingbirds can pollinate especially plants that do not self-pollinate much more effectively than bees.
Bees often visit all the open flowers of a plant before flying to the next. Therefore, bees mainly encourage self-pollination. Compared to hummingbirds, bees have another disadvantage: they groom themselves intensively during flight and deposit combed pollen in their pollen baskets to feed it to their larvae. As a result, only a small part of the pollen reaches the stigma and can fertilize the ovules. Hummingbirds, on the other hand, are not interested in pollen.
“This newly acquired knowledge can also be applied to the evolution of other pollination systems, such as pollination by bats or moths, in terms of frequency and efficiency,” says Dr Abrahamczyk. These results provide a better understanding of the evolution of plant-pollinator interactions. They show that the characteristics of plants and pollinators must be taken into account to understand the evolution of pollination systems.