How Modern Agriculture Turned a Wild Plant Into a Problematic Weed ~~ University of British Columbia

 How Science turned a Wild Plant into a Problematic Weed

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Waterhemp can drastically reduce corn and soy yields, as seen on the right in a corn field in Essex County. Credit: Julia Kreiner, University of British Columbia

New research has discovered that agriculture is causing rapid evolutionary changes not only on farms but also in wild species in adjacent areas.

An international team of researchers at the University of British Columbia has uncovered how the expansion of modern agriculture has transformed a North American native plant, the common waterhemp, into a detrimental agricultural weed.

The study, published in Science, compared 187 samples of waterhemp from contemporary farms and adjacent wetlands to over 100 historical samples dating back to 1820 that were stored in museums across North America. By analyzing the genetic makeup of the plant over the last two centuries, the researchers were able to observe evolution in action in different environments, much like how studying ancient human and neanderthal remains can reveal key insights into human history.

A 155-year-old waterhemp herbarium specimen from the Missouri Botanical Garden Herbarium. Credit: Julia Kreiner, University of British Columbia

“The genetic variants that help the plant do well in modern agricultural settings have risen to high frequencies remarkably quickly since agricultural intensification in the 1960s,” said first author Dr. Julia Kreiner, a postdoctoral researcher in UBC’s Department of Botany.

The researchers discovered hundreds of genes across the weed’s genome that aid its success on farms, with mutations in genes related to drought tolerance, rapid growth, and resistance to herbicides appearing frequently. “The types of changes we’re imposing in agricultural environments are so strong that they have consequences in neighboring habitats that we’d usually think were natural,” said Dr. Kreiner.

Lead author Dr. Julia Kreiner performing DNA extractions of historical herbarium samples in the ancient DNA lab in Tuebingen, Germany. Credit: Julia Kreiner, University of British Columbia

The findings could inform conservation efforts to preserve natural areas in landscapes dominated by agriculture. Reducing gene flow out of agricultural sites and choosing more isolated natural populations for protection could help limit the evolutionary influence of farms.

Common waterhemp is native to North America and was not always a problematic plant. Yet in recent years, the weed has become nearly impossible to eradicate from farms thanks to genetic adaptations including herbicide resistance.

“While waterhemp typically grows near lakes and streams, the genetic shifts that we’re seeing allow the plant to survive on drier land and to grow quickly to outcompete crops,” said co-author Dr. Sarah Otto, Killam University Professor at the University of British Columbia. “Waterhemp has basically evolved to become more of a weed given how strongly it’s been selected to thrive alongside human agricultural activities.”

Waterhemp occurring in natural habitats, the sandy merging of a lake in southern Illinois. Credit: Julia Kreiner, University of British Columbia

Notably, five out of seven herbicide-resistant mutations found in current samples were absent from the historical samples. “Modern farms impose a strong filter determining which plant species and mutations can persist through time,” said Dr. Kreiner. “Sequencing the plant’s genes, herbicides stood out as one of the strongest agricultural filters determining which plants survive and which die.”

Waterhemp carrying any of the seven herbicide-resistant mutations have produced an average of 1.2 times as many surviving offspring per year since 1960 compared to plants that don’t have the mutations.

Herbicide-resistant mutations were also discovered in natural habitats, albeit at a lower frequency, which raises questions about the costs of these adaptations for plant life in non-agricultural settings. “In the absence of herbicide applications, being resistant can actually be costly to a plant, so the changes happening on the farms are impacting the fitness of the plant in the wild,” said Dr. Kreiner.

Agricultural practices have also reshaped where particular genetic variants are found across the landscape. Over the last 60 years, a weedy southwestern variety has made an increasing progression eastward across North America, spreading their genes into local populations as a result of their competitive edge in agricultural contexts.

“These results highlight the enormous potential of studying historical genomes to understand plant adaptation on short timescales,” says Dr. Stephen Wright, co-author and Professor in Ecology and Evolutionary Biology at the University of Toronto. “Expanding this research across scales and species will broaden our understanding of how farming and climate change are driving rapid plant evolution.”

“Understanding the fate of these variants and how they affect plants in non-farm, ‘wild’ populations is an important next step for our work,” according to Professor John Stinchcombe of the University of Toronto, a coauthor on the study.

Reference: “Rapid weed adaptation and range expansion in response to agriculture over the past two centuries” by Julia M. Kreiner, Sergio M. Latorre, Hernán A. Burbano, John R. Stinchcombe, Sarah P. Otto, Detlef Weigel and Stephen I. Wright, 8 December 2022, Science.
DOI: 10.1126/science.abo7293