Supergenes drove the rapid spread of common ragweed

The ever-increasing expansion of the common ragweed plant, known scientifically as Ambrosia artemisiifolia, is leaving allergy sufferers worldwide in prolonged discomfort as the pollen season intensifies. 

Notoriously invasive, common ragweed has now managed to venture as far north as Norway, demonstrating its extraordinary adaptability. However, it’s currently meeting resistance due to Norway’s challenging climate.

“Common ragweed can be found in Norway, but for now it has no stable populations,” said Vanessa Carina Bieker, a postdoctoral researcher at the Norwegian University of Science and Technology’s (NTNU) University Museum Department of Natural History. 

Bieker stumbled upon this alien species in Oslo during the fall of last year. As of now, Norway’s environmental conditions seem to be stalling the plant’s progress, though an increase in temperature might facilitate its survival, given its track record of rapid global expansion.

Scientists attribute the unrelenting proliferation of the ragweed plant to the role of “supergenes,” large blocks of genes that enable the plant to readily adapt to new conditions. 

Supergenes are inherited as a single unit by the subsequent generation, significantly impacting the plant’s characteristics, such as size and flowering time. These changes, in turn, increase the plant’s chances of survival and reproduction under diverse climatic conditions.

“These supergenes affect the plants’ characteristics, such as their size or when they flower, and are in turn important for the plants to be able to survive and reproduce under new climate conditions,” explained Professor Michael D. Martin.

The researchers collaborated with scientists from various other  institutions, including Dr. Kathryn Hodgins of Monash University, to study these supergenes and their role in the ragweed’s invasiveness. The groundbreaking research is published in the journal Nature Communications.

Native to North America, common ragweed made its way to Europe in the 19th century, most likely through imported seeds and horse feed. However, the plant’s story of expansion isn’t unique; it is part of a larger global issue of invasive species disrupting local ecosystems.

Invasive species pose a serious environmental threat. They disrupt existing biodiversity, often leading to destructive consequences for local wildlife. 

Professor Martin emphasizes the need for further understanding of invasive species: “We have to learn how invasive species evolve and spread before we can learn to control them. Then we can counteract the damage they do and limit their effect on species diversity, the economy, and our health.”

In the global battle against invasive species, studying the genetic material of plants is a crucial strategy to help understand their rapid spread and adaptability. 

“When we identify the changes in DNA that enable such invasive species to spread so quickly, we gain a new weapon in our arsenal when we have to fight them,” explained Bieker. 

The research team sequenced the complete genetic code of over 600 plant specimens, including those of common ragweed. 

The genetic detective work carried out by the team led them to diverse museum collections and herbariums across North America and Europe, where they found specimens dating back as far as 190 years. 

By comparing the genetic material of these historical samples with living plants, the researchers could identify alterations that have transpired over the years.

“When you compare these old specimens with the living plants you find outside today, you can also identify the changes in the genetic material that have taken place over the years. And this helps pinpoint which genes have contributed to common ragweed spreading so quickly,” said Bieker. 

The analysis revealed that there are gene variants which thrive under certain climate conditions, and genetic segments that undergo modifications over time.

One of the team’s significant findings is the genetic divergence between ragweed populations in North America, its native continent, and Europe. This difference underpins fundamental principles of genetics and evolutionary theory. 

As the environmental conditions around a plant shift, for instance, when they land on a new continent, certain genes might give individual plants advantages over others. These genetic variants boost reproduction rates, enabling the advantageous genes to be more frequently inherited by the next generation.

“The supergenes in the European plants enabled them to evolve rapidly during the invasion,” explained Professor Martin. These supergenes have facilitated ragweed’s colonization of vast regions of Europe, modifying vital plant characteristics like size and lifecycle timing.

“These are properties that have had a major impact on ragweed’s success and therefore the plants’ production of highly allergenic pollen,” said Martin, emphasizing the repercussions of ragweed’s spread on global health, particularly for allergy sufferers.

What is remarkable about this study is not only its findings, but also the techniques that were used for the analysis. The rapid advancements in genetics over recent years have enabled research methods previously thought impossible. This investigation, in particular, demonstrates how innovative genetic methodologies can breathe new life into old scientific collections, extracting fresh and enlightening insights.

“This research shows the power in studying old specimens in herbariums and other museum collections. They can inform us about the recent spread, and sometimes very rapid evolution, of invasive species that have been strongly influenced by human activity,” said Martin.

In the future, these findings might serve as the basis for effective strategies to curb the spread of common ragweed. While the prospect is a long-term one, it certainly offers a glimmer of hope to those who suffer from seasonal allergies.

More about invasive species

Invasive species are plants, animals, or pathogens that are non-native (or alien) to an ecosystem and whose introduction causes or is likely to cause harm. They can affect the environment, the economy, or even human health.

Environmental Impacts

Invasive species can disrupt biodiversity by outcompeting native species for resources such as food, water, and habitat. Some invasive species are predators or pathogens that directly kill native species. Others alter habitats in ways that native species cannot tolerate or to which they cannot adapt. In the worst cases, invasive species can cause local extinctions of native species and drastically alter ecosystem functions.

Economic Impacts

Invasive species can cause extensive economic damage. They can harm agricultural productivity through crop damage and loss of livestock. In forests, invasive insects and diseases can kill large numbers of trees, affecting timber production. Invasive aquatic plants and animals can disrupt commercial and recreational fishing, boating, and other water-related industries. Furthermore, managing invasive species is costly.

Human Health Impacts

Some invasive species can harm human health. For instance, some invasive plants produce allergens. Alien insects like mosquitoes can transmit diseases. And some invasive animals, such as rodents, are carriers of various diseases.

Examples of invasive species

The zebra mussel in the Great Lakes of North America, the brown tree snake in Guam, the cane toad in Australia, and the lionfish in the Caribbean and the Southeast U.S. waters are all examples of invasive species.

Managing and controlling alien species is a significant conservation challenge. It involves preventing their introduction, detecting and eradicating newly established invaders, and controlling and mitigating the impacts of established invasive species. 

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