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| Genetics professor Rodney Mauricio (left) and doctoral student Regina Baucom have identified morning glory families that are tolerant of the most common herbicides. (Photo by Peter Frey) | |
Glory be |
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| Scientists ID morning glory families that could cause problems for farmers | |
| By Phil Williams |
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Morning glories are
beloved mailbox flowers all over rural America, but to farmers, they
are something else: a noxious weed that can lower yields and choke
harvesting combines. For some
Now however, for the first time, UGA scientists have identified -morning glory families that are tolerant to glyphosate, and that could therefore 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,” says Regina Baucom, a doctoral student at UGA, who directed the research. Baucom and assistant professor of genetics Rodney Mauricio co-authored the study, which was published 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. Applying RoundUp and other herbicides containing glyphosate does not cause tolerance, but it does kill all but the glyphosate-tolerant plants. The tolerant ones are left as the “last weed standing,” and farmers are left without an effective means of control. Baucom and Mauricio’s study does not address the practical concerns of agriculture however. Rather, it examines genetically how morning glories—both those that are not 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. The makers of the best-known glyphosate herbicide have developed RoundUp-Ready canola, corn, cotton, soybeans and sugar beets—crop varieties that aren’t harmed by glyphosate, which means it can be used to kill weeds and increase yields. “Our interviews with farmers in the Southeast suggest that morning glories can tolerate applications of glyphosate,” says Baucom, “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 and 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 within 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. The presence of genetic variation, however, does not in itself guarantee that tolerance to glyphosate will evolve. There must also be a “net selection” for tolerance, determined by fitness costs and benefits: the “benefit” of being tolerant must outweigh any “cost” of being tolerant. The argument is similar to economic cost/benefit analyses. In the ecological realm, the production of offspring can be thought of as the economic goal. Costs are thought to be caused by diverting important nutrients and resources away from reproduction into the trait(s) conferring the ability to be tolerant. Costs are evident only in an environment in which the benefit of tolerance is not needed—that is, in an environment without glyphosate. Thus, if the benefits of tolerance outweigh the costs, then glyphosate-tolerant plants can increase in the population by the action of selection. In fact, the research shows that there is positive directional selection for tolerance to glyphosate, meaning that when glyphosate is used, plants that are tolerant to the herbicide produce more seeds than those that are susceptible (given that susceptible individuals either die or produce almost no seed). More significantly, however, in an environment devoid of glyphosate the tolerant families produce many fewer seeds or offspring than susceptible families. This is evidence of a fitness cost of tolerance, and suggests the possibility of managing or controlling the further evolution of tolerance in morning glories by 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,” says Mauricio. “This finding, along with an analysis suggesting a critical evolutionary threshold has been crossed, will be of broad interest to scientists and policymakers.” |
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