Wild pansy: These plants have shrunk their flowers and are giving up their pollinators | Science

The planet is running out of pollinators. The collapse may be worse among bumblebees than in honeybees. Without them, who will pollinate the plants that need them for fertilization? Well, they alone to themselves. The self-fertilization rate of a wild plant has increased by almost 30%. And if you no longer need to attract them, what good are flowers and nectar? Both attributes of wild thought (Viola arvensis) have decreased when compared to specimens from 30 years ago now resurrected. It is just one species of the thousands that exist and has only been observed in France, but it could be opening the way to a world without flowers.

Angiosperms, flowering plants, appeared on Earth about 130 million years ago and it only took a few more years to fill the entire planet with color. Charles Darwin, the father of the theory of evolution, seems to have been bothered by them. In a letter to his best friend, the botanist and explorer Joseph Hooker, he told her: “The apparent rapid development of all higher plants in recent geological times is an abominable mystery,” he wrote at the time. In the rapid success of this abomination, plants found allies in insects, birds and, why not, dinosaurs, who helped them fertilize each other through pollen grains, the male gametophytes. Today, 80% of wild plant species and 70% of cultivated ones depend to a greater or lesser degree on pollinators. Therefore, the decline of populations and entire species of insects, reduced by half in the areas most altered by humans, could have an enormous impact on the world’s flora.

The Paris region, in northern France, is not the entire planet, but botanists there have observed what could happen in the rest of the globe. In recent years, they were noticing that wild pansy flowers were less showy. At the same time, the reduction in pollinating insect populations seemed evident to them. Could both phenomena be related? To prove this, they turned to what they call the ecology of resurrection.

Samson Acoca-Pidolle, researcher at the University of Montpellier, explains what it is: “It consists of using the latency property of some stage of life to store individuals for a long period. In our case, some seeds collected between the 90s and 2000 and stored in refrigerators at the National Botanical Conservatories.” In 2021, they recovered the wild pansy seeds from their hibernation and returned to the same fields where they had obtained them and collected new ones, from the present, to compare them. “This methodology is powerful because we can compare ancestry and offspring in exactly the same conditions, in a common garden,” adds Acoca-Pidolle.

The results of the sowing, published in the scientific journal New Phytologist, they are disturbing. They took both groups of seeds, the resurrected ones and the current ones, to greenhouses in four different places. At each location they designed the same experiment. In isolated areas with mosquito netting, they planted about thirty seedlings from each lineage. When April arrived, they introduced bee hives to pollinate them and produced a second generation. In total, 792 plants. They investigated them from all possible sides. They analyzed its genome, the frequency of insect visits, vegetative growth rates and, in particular, all the flowering parameters: length of the corolla, width of the lip, length of the spur, also that of the sepals or duration. of anthesis, the formation of the flower, of the first five flowers that appeared on each plant.

Of the seven parameters, only the length of the sepals, that kind of protectors under the petals, was not the same. In the rest, everything had changed between the wild thought of now and that of 30 years ago. Specifically, the current plants have shrunk their floral area by 10%. They also had fewer nectar guides. They are one of the most sophisticated adaptations that flowering plants have developed and are what they say, visual patterns that guide the insect to where the nectar and pollen are. In some flowers they are visible to the human eye, but in others, such as the sunflower, which appears to be a monotonous yellow, in reality it has other yellows, but the stripes are in the range of ultraviolet light, only visible to pollinators. . One last and definitive fact: in the four sites where they carried out the experiments, the resurrected lineage of the past produced on average 20% more nectar.

In parallel, the authors of the experiments observed two other trends. On the one hand, and confirmed at a genetic level, they verified that the self-fertilization rate of present-day plants is 27% higher than in those of the past. Since the origin of vascular plants, many species have developed the ability to fertilize themselves. It is an unbeatable strategy in principle, they do not need anyone to reproduce. But it has a problem, the reduction of genetic diversity through inbreeding makes the organism more vulnerable and less elastic to face environmental changes and, of course, multiplies the risk of inheriting a harmful mutation. In current flowers, botanists detected less herkogamy, the distance between stamens and pistils, between the male and female sexual organs, to facilitate self-fertilization.

“Self-fertilization is the extreme form of inbreeding and in plants (and all organisms) it affects their size, their survival”

Pierre-Olivier Cheptou, researcher at the French National Center for Scientific Research

“Self-fertilization is the extreme form of inbreeding and in plants (and all organisms) it affects their size, their survival…” says Pierre-Olivier Cheptou, researcher at the National Center for Scientific Research (CNRS). French) and supervisor of the work of Acoca-Pidolle. In fact, they saw that the number of seeds produced by current plants compared to those of the past was somewhat lower, but it would be necessary to see how this lower yield evolves in the future.

The enormous change would be due to the increasing difficulty that wild pansy have in recruiting pollinators. The decline of these mutualistic insects would be making the flowers and nectar that attracted them unnecessary, elements in which plants invest a good part of their resources. In fact, in the experiments, they confirmed that the bumblebees came in smaller numbers and frequency to the present specimens. “We were surprised to discover that these plants evolve so quickly. “Our results show that ancient interactions linking pansies with their pollinators are rapidly disappearing,” Acoca-Pidolle said. The same idea highlights her tutor, the speed of change. “What our study shows is that they are evolving to do without their pollinators,” highlights Cheptou. “They are evolving toward self-pollination, which works in the short term, but may limit their ability to adapt to future environmental changes.”

Professor Michael Lenhard heads a plant organ genetics laboratory at the University of Potsdam, Germany. Unrelated to experiments with wild pansy, he has investigated self-pollination syndrome. The basic characteristics of the syndrome are changes in the location of the sexual organs (shortening herkogamy) and in the morphology of the flowers. Lenhard agrees that one of the results is the loss of attractiveness of these ornaments: “Especially when the self-fertilization syndrome with its different components, such as smaller size and lower odor production, is already strongly established. In this case, two important signals for attracting pollinators are reduced (visual, flower size, and olfactory, aroma), which makes the flowers less striking and less attractive to pollinators.”

The German scientist also agrees with the French: “In the near future, I think we may see more plant populations evolving with a higher rate of self-pollination and self-fertilization, if the number of pollinators continues to decline.” In the long term, he also believes that it is not a good strategy: “If this effect became really strong, it would probably be detrimental to the populations in question, since self-fertilized populations/species tend to have a lower capacity for evolutionary adaptation and a higher extinction rate.”

“This transition has existed naturally, it is common among plants, but global change is accelerating it. “The speed is what is dramatic”

Sergio Ramos, botany researcher at the University of Zurich, Switzerland

From the University of Zurich (Switzerland), researcher Sergio Ramos remembers that plant self-fertilization has always been there. “It is not an isolated phenomenon, all plants, all groups of plants have experienced it, and in fact it is one of the most important, most frequent and most consistent evolutionary transitions,” he says. A few years ago Ramos carried out a series of experiments with cabbages, which boast intense yellow flowers. Like the rest of the plants, they have to attract pollinating insects, but they cannot be too attractive or they will also attract herbivorous insects. In their experiments, they used plants with the same origin that they divided into four groups and played with the presence/absence of bumblebees and/or caterpillars of the cabbage butterfly, a voracious herbivore. By the eighth generation, the flowers of each were very different. Those exposed to pollinators had larger flowers and released more fragrance. Meanwhile, those that suffered the scourge of the caterpillars had reduced their floral attractiveness, but increased the amount of toxic metabolites, to scare away the herbivores.

“It was one of the first experimental examples that this transition occurs very quickly,” says Ramos. But there are other works that have also manipulated the presence or absence of insects and “what has been seen is that when plants do not have pollen movement, there is no interbreeding, mediated by insects, just after a few generations, it begins to seen to evolve into self-reproduction, he adds. For Ramos, what is different now is, once again, the speed of change: “This transition has existed naturally, it is common among plants, but global change is accelerating it. The speed is what is dramatic. Evolutionary biologists could not imagine being able to see these changes in real time. To me that is what seems beautiful and at the same time alarming”

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