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first_imgBy Phil WilliamsUniversity of GeorgiaMorning glories are beloved mailbox flowers all over ruralAmerica. But to farmers, they’re something else: a noxious weedthat can lower yields and choke harvesters. For 30 years, theherbicide glyphosate has kept them out of farm fields, butsomething is changing.For the first time, University of Georgia researchers haveidentified morning glory families that are tolerant toglyphosate. These noxious vines could cause problems for thecountry’s farmers.”Our study suggests that serious and immediate considerationshould be given to developing regional strategies for managingthe evolution of tolerance in morning glories,” said ReginaBaucom, a UGA doctoral student who directed the research.Baucom and UGA assistant professor of genetics Rodney Mauricioco-authored the study. It’s being published this week in theProceedings of the National Academy of Sciences. The research wasfunded by the National Science Foundation and a research grantfrom Sigma Xi.The tolerance of some morning glories to glyphosate is anaturally occurring trait, not something caused by applyingRoundup and other herbicides that contain the chemical.Glyphosate is used on farm crops and millions of home lawns andgardens.The problem is that the chemical does kill most morning glorieseffectively, so that the tolerant ones could be the “last weedstanding,” leaving farmers without an effective means of control.The current study doesn’t address the practical concerns ofagriculture. 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 futuregenerations.The issues are complex. The use of herbicides and pesticides hasallowed dramatic increases in food production in the pastcentury. But, as the paper in PNAS points out, the repeated useof herbicides exerting strong selection pressure on crop weedshas led to more than 250 documented cases of herbicideresistance, and “this process is likely to accelerate withincreased reliance on herbicides.”Glyphosate has been available since 1974. But to date, only sixcases of glyphosate resistance in plants have been reported amongthe 250 documented cases of herbicide resistance.The makers of the best-known glyphosate herbicide developedRoundup-Ready canola, corn, cotton, soybeans and sugar beets.Glyphosate doesn’t harm these crop varieties, so farmers can useit to kill weeds and increase yields.”Our interviews with farmers in the Southeast suggest thatmorning glories can tolerate applications of glyphosate,” Baucomsaid. “And, in some cases, increasing concentrations of theherbicide have been required to control it.”Such an increase in tolerance to the chemical gives researchers aunique opportunity to study the evolutionary genetics of a noveltrait. It may help them and others slow the rate of evolution oftolerance in morning glories.What Baucom and Mauricio found was that, in at least one naturalpopulation of morning glories they studied, there is asubstantial genetic variation for tolerance, meaning that the”evolutionary door” is wide open.For evolution by natural selection to succeed, there must begenetic variation with a population and a significant selectiveforce. This study is a case-in-point of evolution by selection –human-mediated evolution, similar to the evolution of bacteriaresistant to antibiotics.”Given the continued presence of glyphosate, the number oftolerant individuals should increase within the population overtime,” the scientists reported, “as might the overall level oftolerance of the population.”Glyphosate is a relatively recent tool in the fight againstweeds. This fact has led the scientists to conclude that thetolerance trait in this wild population was naturally occurring,not caused by use of the herbicide.The presence of genetic variation, however, doesn’t guarantee initself that tolerance to glyphosate will evolve. The “netselection” requirement for tolerance is acted on by twocomponents: fitness costs and benefits. The benefit of beingtolerant must outweigh any sort of cost.If the benefits of being able to tolerate the chemical outweighthe costs, the tolerant individuals will produce offspring forfuture generations and susceptible ones won’t.Costs are thought to include diverting important nutrients andresources away from reproduction into the trait conferring theability to be tolerant.This research has shown positive directional selection fortolerance to glyphosate. So, by applying glyphosate, plants thatare tolerant to it produce more seeds than those that aresusceptible.Perhaps more key for the farmer, however, is the finding that inan environment devoid of glyphosate, tolerant families producemany fewer seeds or offspring than susceptible families.This is evidence of a fitness cost of tolerance, and thisinformation can be used in managing or controlling the furtherevolution of tolerance in morning glories by arguing for notspraying Roundup in certain years.Since the issues are so complex, new strategies will have to beconsidered to control increasing numbers of glyphosate-tolerantvarieties.”Hers [Baucom’s] is the first demonstration of a fitness cost oftolerance to glyphosate,” Mauricio said. “This finding, alongwith an analysis suggesting a critical evolutionary threshold hasbeen crossed, will be of broad interest to scientists andpolicymakers.”Morning glories aren’t at the level of such nuisance weeds asmusk thistles in crops, but they’re still a widespread problemfor farmers. The new evidence for genetic variation of tolerancein morning glories, however, points toward a potential problemwith no easy solutions.”For glyphosate, such strategies could involve something assimple as periodically spraying with alternate herbicides, aslong as there is little cross-tolerance with glyphosate,” theauthors said.”If, however, there is cross-tolerance with other causes of plantdamage, such as hail, herbivores or pathogens,” they said,”alternative spraying regimes may not be a viable mechanism forcontrolling the evolution of glyphosate tolerance.”(Phil Williams is the director of public relations with theUniversity of Georgia Franklin College of Arts and Sciences.)last_img

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