Flowers evolve to have less sex
Every spring, trillions of flowers mate with the help of bees and other animals. They attract pollinators to their flowers with striking colors and nectar. As the animals travel from flower to flower, they take with them pollen, which can fertilize the seeds of other plants.
A new study suggests that people are quickly changing this annual spring ritual. If toxic pesticides and disappearing habitats have driven out populations of bees and other pollinators, some flowers have evolved to fertilize their own seeds more often, rather than those of other plants.
Scientists said they were surprised by the speed of the changes, which occurred in just 20 generations. “That’s rapid evolution,” says Pierre-Olivier Cheptou, an evolutionary ecologist at the University of Montpellier in France who led the study.
Dr. Cheptou was inspired to conduct the research when it became clear that bees and other pollinators were in drastic decline. Would flowers that depend on pollinators for sex, he wondered, find another way to reproduce?
The research focused on a weed plant called the field violet, whose white, yellow and purple flowers are common in fields and along roadsides across Europe.
Field violets typically use bumblebees to reproduce sexually. But they can also use their own pollen to fertilize their own seeds, a process called self-pollination.
Selfing is more convenient than sex, because a flower does not have to wait for a bee to come along. But a self-pollinating flower can only use its own genes to produce new seeds. Sexual reproduction allows flowers to mix their DNA, creating new combinations that make them better prepared for diseases, droughts and other challenges that future generations may face.
To track the evolution of field violets over the past decades, Dr. Cheptou and his colleagues used a stock of seeds collected by the French National Botanical Conservatories in the 1990s and early 2000s.
The researchers compared these old flowers with new flowers from the entire French countryside. After growing the new and old seeds side by side in the laboratory under identical conditions, they found that self-production had increased by 27 percent since the 1990s.
The researchers also compared the anatomy of the plants. Although the overall size of the new field violets had not changed, their flowers had shrunk by 10 percent and they produced 20 percent less nectar.
The researchers suspected that these changes made the new field violets less attractive to bumblebees. To test that idea, they placed bumblebee hives in cages with old and new field violets. And sure enough, the bees paid more visits to the old plants than to the new ones.
As bumblebee populations have declined, the cost of producing nectar and large, attractive flowers may have become a burden on flowers, says Dr Cheptou. Rather than putting energy into attracting pollinators, he speculated, field violets are more successful by focusing them on growth and fighting disease.
The researchers suspect that many other flowers face the same challenge when it comes to their survival, and that they may also be evolving in the same direction. “There is no reason to think that other plants have not evolved,” said Dr. Cheptou.
If true, the plants could be exacerbating a bad situation for pollinating insects. Many pollinators depend on nectar for food; if the plants produce less, the insects will go hungry.
Pollinators and flowers can go into a downward spiral. Less nectar will reduce insect populations even further, making sexual reproduction even less rewarding for the plants.
The coil won’t just be bad for the insects, Dr. warned. Cheptou. If some plants eventually give up sexual reproduction altogether, it is unlikely that they will be able to regain that ability.
In the long term, the genetic limitations of self-determination could push plants toward extinction. “They won’t be able to adapt, so extinction will become more likely,” said Dr. Cheptou.
The results were “impressive, but discouraging,” said Susan Mazer, a botanist at the University of California, Santa Barbara, who was not involved in the study.
Dr. Mazer said the spiral may be even worse than Dr. Cheptou suggested. In addition to a decline in pollinators, flowering plants face other challenges that may prompt them to give up sexual reproduction.
For example, global warming accelerates the growth of flowers. It may be that the time before the flowers fade, during which they can provide nectar to pollinators, becomes shorter.
But Sasha Bishop, an evolutionary biologist at the University of Michigan who was not involved in the study, said some flowers could respond in the opposite way to pollinator declines.
In a study during dawn in the southern United States, she and her colleagues found that the flowers grew larger, not smaller, between 2003 and 2012. The scientists see this shift as a strategy to continue attracting bees as they become less common.
“They could invest in self-fertilization, or they could invest in attracting pollinators,” said Dr. Bishop. “Both outcomes are completely reasonable.”