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A centuries-old, interdependent relationship that contributes to food systems and the stability of ecosystems around the world could be changing.
Many flowering plants can self-pollinate or transfer pollen between their own blossoms to generate and propagate seeds, but most of these plants depend on pollinators such as butterflies and bees to reproduce.
Now — amid declines in many pollinator populations — a new study into the evolution of one flower species’ mating system has revealed a remarkable shift that could worsen the challenges facing the plants’ insect partners.
According to Samson Acoca-Pidolle, who led the study published December 19 in the journal New Phytologist, the reproductive evolution of flowers may be linked to environmental changes, such as habitat destruction and the rapid continued decline of the biodiversity of pollinators.
By comparing seeds of wild field violets collected decades ago in France with the plant’s modern descendants, Acoca-Pidolle and his colleagues found that today’s flowers are smaller and produce less nectar due to increased self-pollination, which has direct consequences for the behavior of pollinators. . According to the study, pansies of the past were less self-fertile and attracted many more pollinators than those of today.
“It seems that only traits involved in the interaction between plants and pollinators evolve,” says Acoca-Pidolle, a doctoral researcher at the University of Montpellier. The changes could limit the plants’ ability to adapt to future environmental changes and impact “the entire floral biodiversity” – potentially reducing the genetic, species and ecosystem variation of flowering plants.
“This could increase pollinator declines and create a vicious feedback cycle,” co-author Pierre-Olivier Cheptou told CNN. If plants produce less nectar, there will be less food available for pollinators, which in turn will accelerate the rate at which animal numbers decline, he explained.
“The main message is that we are currently seeing the evolutionary breakdown of plant pollinators in the wild,” says Cheptou, an evolutionary ecologist at the French National Center for Scientific Research and professor at the University of Montpellier.
Bringing plants back to life
Using a method called ‘resurrection ecology’ to conduct the research, the research team germinated the seeds of four populations of wild field violets, scientifically known as Viola arvensis, collected in the Paris region in the 1990s and early 2000s.
Some propagules, or parts of a plant that can be used to grow a new plant, can remain in the seed stage for “a very long time,” Acoca-Pidolle explains. They are alive, but with a very low metabolism. “It’s like a long nap,” he said.
In 2021, the team collected field violets from the exact spot where the ancestral seeds were collected 20 to 30 years earlier. The scientists then conducted a population genetic analysis that looked at rates of self-pollination and changes in heterozygosity, or genetic variability, as well as changes in floral traits related to pollinator attraction.
In a sample of 4,000 flowers, the rate of self-fertilization rose from about 50% for the flowers collected two to three decades earlier to about 80% for their naturally occurring offspring, the authors found. Meanwhile, the surfaces of the “resurrected” flowers were 10% larger, produced 20% more nectar and were visited by more bumblebees than their modern counterparts.
‘An insurance policy’
An increase in the number of self-pollinating, or “self-pollinating,” flowering plant species isn’t always a bad thing, says Gretchen LeBuhn, a professor of biology at San Francisco State University who has studied interactions between pollinators and plants.
“The way you can think about (self)thinking is a kind of persistence strategy,” says LeBuhn, who was not involved in the study. While increasing self-reliance often leads to a decrease in genetic variation in a population, among several other negative consequences, it can also preserve the population, she added. “Just like an insurance policy.”
Decreasing genetic variation within a plant population is important because plants with a reservoir of genetic variation can respond better to major environmental changes, effectively reducing the risk of extinction.
But reading the new paper, part of her “actually thought that an increase in self-reliance means that the population will be preserved,” LeBuhn said. “If plants can persist over time and pollinator populations increase again, this could suggest that this is a mechanism for species conservation.”
However, it is unclear whether that evolutionary shift can be reversed, although the new research suggests that a depletion of the genetic diversity of a plant population is expected over time, according to Acoca-Pidolle.
“Some scientists believe there may be a tipping point beyond which a plant cannot go back,” he noted, adding that the evolutionary transition is classically considered “irreversible.” Investigating whether these wild violets have the ability to recover from the effects of self-gratification is the next big question, Acoca-Pidolle said.
In the meantime, it’s important to recognize that the authors don’t really have the data on what happened to pollinators 20 to 30 years ago, LeBuhn said. “The one thing they can’t document is the magnitude of the difference in pollinators at these sites then and now,” she said — which stems from gaps in historically widespread pollinator monitoring.
“(The study) is a very important demonstration of the close links between plant and pollinator communities,” LeBuhn said. “I think the next step in research is to understand what the implications are for pollinators.”
The lasting footprint of humanity
Other recent studies have found that declining pollinator populations, an offshoot of harmful human activities, threaten the future of food crops and the survival of the many species that depend on them.
The growing body of research strengthens the case for urgent conservation measures – such as developing and protecting flower-rich habitats that act as flowering and nesting habitats – to help reverse the global decline of pollinators, Acoca-Pidolle said.
“Our impact is not just killing some individual plants… we are putting them on an evolutionary path that could be bad for them,” Acoca-Pidolle told CNN. “And even after we’re gone, we’ll have a footprint on this evolutionary trajectory of many species, of the planet’s biodiversity, for a long time.”
Ayurella Hoorn-Muller has reported for Axios and Climate Central. Her book “Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South” will be published this spring.
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