By Phil Williams
University of Georgia

For the first time, University of Georgia researchers have identified morning glory families that are tolerant to glyphosate. These noxious vines could cause problems for the country's farmers.

"Our study suggests that serious and immediate consideration should be given to developing regional strategies for managing the evolution of tolerance in morning glories," said Regina Baucom, a UGA doctoral student who directed the research.

Baucom and UGA assistant professor of genetics Rodney Mauricio co-authored the study. It's being published this week in the Proceedings of the National Academy of Sciences. The research was funded by the National Science Foundation and a research grant from Sigma Xi.

The tolerance of some morning glories to glyphosate is a naturally occurring trait, not something caused by applying Roundup and other herbicides that contain the chemical. Glyphosate is used on farm crops and millions of home lawns and gardens.

The problem is that the chemical does kill most morning glories effectively, so that the tolerant ones could be the "last weed standing," leaving farmers without an effective means of control.

The current study doesn't address the practical concerns of agriculture. Rather, it examines genetically how morning glories -- both those that aren't killed by glyphosate and those that are -- lose or maintain the ability to produce offspring for future generations.

The issues are complex. The use of herbicides and pesticides has allowed dramatic increases in food production in the past century. But, as the paper in PNAS points out, the repeated use of herbicides exerting strong selection pressure on crop weeds has led to more than 250 documented cases of herbicide resistance, and "this process is likely to accelerate with increased reliance on herbicides."

Glyphosate has been available since 1974. But to date, only six cases of glyphosate resistance in plants have been reported among the 250 documented cases of herbicide resistance.

The makers of the best-known glyphosate herbicide developed Roundup-Ready canola, corn, cotton, soybeans and sugar beets. Glyphosate doesn't harm these crop varieties, so farmers can use it to kill weeds and increase yields.

"Our interviews with farmers in the Southeast suggest that morning glories can tolerate applications of glyphosate," Baucom said. "And, in some cases, increasing concentrations of the herbicide have been required to control it."

Such an increase in tolerance to the chemical gives researchers a unique opportunity to study the evolutionary genetics of a novel trait. It may help them and others slow the rate of evolution of tolerance in morning glories.

What Baucom and Mauricio found was that, in at least one natural population of morning glories they studied, there is a substantial genetic variation for tolerance, meaning that the "evolutionary door" is wide open.

For evolution by natural selection to succeed, there must be genetic variation with a population and a significant selective force. This study is a case-in-point of evolution by selection -- human-mediated evolution, similar to the evolution of bacteria resistant to antibiotics.

"Given the continued presence of glyphosate, the number of tolerant individuals should increase within the population over time," the scientists reported, "as might the overall level of tolerance of the population."

Glyphosate is a relatively recent tool in the fight against weeds. This fact has led the scientists to conclude that the tolerance trait in this wild population was naturally occurring, not caused by use of the herbicide.

The presence of genetic variation, however, doesn't guarantee in itself that tolerance to glyphosate will evolve. The "net selection" requirement for tolerance is acted on by two components: fitness costs and benefits. The benefit of being tolerant must outweigh any sort of cost.

If the benefits of being able to tolerate the chemical outweigh the costs, the tolerant individuals will produce offspring for future generations and susceptible ones won't.

Costs are thought to include diverting important nutrients and resources away from reproduction into the trait conferring the ability to be tolerant.

This research has shown positive directional selection for tolerance to glyphosate. So, by applying glyphosate, plants that are tolerant to it produce more seeds than those that are susceptible.

Perhaps more key for the farmer, however, is the finding that in an environment devoid of glyphosate, tolerant families produce many fewer seeds or offspring than susceptible families.

This is evidence of a fitness cost of tolerance, and this information can be used in managing or controlling the further evolution of tolerance in morning glories by arguing for not spraying Roundup in certain years.

Since the issues are so complex, new strategies will have to be considered to control increasing numbers of glyphosate-tolerant varieties.

"Hers [Baucom's] is the first demonstration of a fitness cost of tolerance to glyphosate," Mauricio said. "This finding, along with an analysis suggesting a critical evolutionary threshold has been crossed, will be of broad interest to scientists and policymakers."

Morning glories aren't at the level of such nuisance weeds as musk thistles in crops, but they're still a widespread problem for farmers. The new evidence for genetic variation of tolerance in morning glories, however, points toward a potential problem with no easy solutions.

"For glyphosate, such strategies could involve something as simple as periodically spraying with alternate herbicides, as long as there is little cross-tolerance with glyphosate," the authors said.

"If, however, there is cross-tolerance with other causes of plant damage, such as hail, herbivores or pathogens," they said, "alternative spraying regimes may not be a viable mechanism for controlling the evolution of glyphosate tolerance."

(Phil Williams is the director of public relations with the University of Georgia Franklin College of Arts and Sciences.)