By Phil Williams
University of Georgia

Morning glories are beloved mailbox flowers all over rural
America. But to farmers, they’re something else: a noxious weed
that can lower yields and choke harvesters. For 30 years, the
herbicide glyphosate has kept them out of farm fields, but
something is changing.

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.)