Why have some plant species evolved different pollinators? An international team of scientists investigated the reproductive systems of three sister species pairs, one of which is pollinated by insects and the other by hummingbirds. Mechanisms that explain the shift from insect to bird pollination have been discovered. The study has now appeared in the journal Ecology and Evolution.
Findings of Study:
In the pollination of flowering plants, various strategies have evolved. The frequency and efficiency with which the flower visitor visits play a role. There are significant differences between the various animal groups here. Insects, particularly bees, are the most common pollinators on a global scale.
Bees' activity ranges are typically quite limited, whereas other pollinator groups, such as hummingbirds, fly much greater distances. "It was previously assumed that plants switch their pollinator group from bees to hummingbirds when bee activity and thus pollination efficiency is too low or too unpredictable, for example in high mountains," says Dr. Stefan Abrahamczyk of the University of Bonn's Nees Institute for Plant Biodiversity. For example, it is frequently too humid or hot in cloud forests of tropical high mountains.
But why have plants in areas with high bee diversity and abundance switched to hummingbirds, bats, or even small, ground-dwelling mammals like mice, lemurs, or honey possums? Dr. Abrahamczyk and his colleagues demonstrated in the current study that the reasons for pollinator group evolutionary switching are far more complex than previously thought.
A sister species pair is formed when two new species emerge from one original species during evolution, for example, because their distribution range is divided by mountain folding or ice age.
In terms of reproductive strategies, the researchers looked at three sister species pairs from different plant families. In each case, one sister species is pollinated by hummingbirds and the other by bees. All of the species descended from bee-pollinated ancestors and are found in areas of North America with a high diversity and abundance of bees.
Using a series of pollination experiments, it was discovered that all of the hummingbird-pollinated species had significantly higher seed set and germination rates when pollinated with pollen from another plant individual of the same species.
"Based on these findings, we can conclude that hummingbird pollination evolved in populations of bee-pollinated species that are especially reliant on cross-pollination, i.e., cannot self-pollinate," says Dr. Abrahmamczyk. Hummingbirds can pollinate plants that do not self-pollinate much more effectively than bees due to their larger radius of activity and frequent movements between different plant individuals of the same species.
Bees frequently visit all of the open flowers on one plant before moving on to the next. As a result, bees primarily promote self-pollination. Bees have another disadvantage when compared to hummingbirds: they groom extensively while flying and deposit the combed-out pollen in their pollen baskets to feed to their larvae.
As a result, only a small percentage of pollen reaches the stigma and fertilizes the ovules. Hummingbirds, on the other hand, are unconcerned about pollen.
"These newly discovered insights can also be applied to the evolution of other pollination systems, such as a bat or moth pollination, in terms of frequency and efficiency," says Dr. Abrahamczyk. These findings shed light on the evolution of plant-pollinator interactions. They demonstrate that plant and pollinator characteristics must be taken into account when studying the evolution of pollination systems.
(Source: University of Bonn)
