A new study published in Science shows how modern agriculture has transformed North America's native common water hemp plant into a troublesome agricultural weed. An international team of researchers led by scientists at the University of British Columbia compared 187 water hemp samples from current farms and nearby wetlands to more than 100 historical samples dating back to 1820 that had been housed in museums across North America (UBC).
The plant's genetic makeup has allowed researchers to observe evolution across changing habitats, just as the sequencing of ancient human and neanderthal remains has clarified important questions about humanity's history.
"The genetic variants that help the plant thrive 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 the Department of Botany at UBC.
The researchers discovered hundreds of genes throughout the weed's genome that contribute to its success on farms, with mutations in genes associated with drought tolerance, rapid growth, and herbicide resistance 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 normally consider natural," Dr. Kreiner explained.
The findings could help conservation efforts to protect natural areas in agricultural landscapes reducing gene flow out of agricultural sites. Protecting more isolated natural populations could help limit farms' evolutionary influence. Water hemp is a plant native to North America that was not always a problem. However, due to genetic adaptations such as herbicide resistance, the weed has become nearly impossible to eradicate from farms in recent years.
"While water hemp grows near lakes and streams, the genetic changes we're seeing allow the plant to survive on drier land and grow quickly enough to outcompete crops," said co-author Dr. Sarah Otto, Killam University Professor at the University of British Columbia. "Because of how strongly it has been selected to thrive alongside human agricultural activities, waterhemp has evolved to become more of a weed."
Notably, five of the seven herbicide-resistant mutations discovered in current samples were not found in historical samples. "Modern farms impose a strong filter that determines which plant species and mutations can persist over time," Dr. Kreiner explained. "When the plant's genes were sequenced, herbicides emerged as one of the most powerful agricultural filters determining which plants survive and which die."
Waterhemp, with any of the seven herbicide-resistant mutations, has produced 1.2 times as many surviving offspring per year since 1960 as plants without the mutations. Herbicide-resistant mutations have also been discovered in natural habitats, albeit at a lower frequency, raising concerns about the costs of these adaptations for plant life in non-agricultural settings.
"Being resistant can be costly to a plant in the absence of herbicide applications, so the changes happening on farms are affecting the fitness of the plant in the wild," Dr. Kreiner explained. Agricultural practices have also altered the distribution of specific genetic variants across the landscape. A weedy southwestern variety has spread its genes eastward across North America over the last 60 years, spreading its genes into local populations due to its competitive advantage in agricultural contexts.
"These findings highlight the enormous potential of studying historical genomes to understand plant adaptation on short timescales," says co-author and University of Toronto Professor of Ecology and Evolutionary Biology Dr. Stephen Wright.
"Expanding this research across scales and species will allow us to understand better 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," says co-author and the University of Toronto Professor John Stinchcombe.
(Source: University of British Columbia)
