Research finds how intensive agriculture turned wild plant into pervasive weed

British Columbia [Canada], December 9 (ANI): New research published in Science demonstrates how the development of modern agriculture has transformed the native common water hemp plant of North America into a troublesome agricultural weed.
187 water hemp samples from current farms and nearby wetlands were compared with more than 100 historical samples dating back as far as 1820 that had been housed in museums across North America by an international team of researchers led by scientists at the University of British Columbia (UBC). The genetic makeup of the plant over the past 200 years allowed the researchers to observe evolution in action across changing habitats, just as the sequencing of ancient human and neanderthal remains has clarified important questions about the history of humanity.
“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 neighbouring habitats that we’d usually think were natural,” said Dr Kreiner.
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 water hemp 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 water hemp 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.”
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 has 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 its genes into local populations as a result of its 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. (ANI)