Summary: New evidence suggests that herbicide resistance in weeds is more likely to occur from pre-existing genetic variation than from new mutations.
After exposing more than 70 million grain amaranth seeds to a soil-based herbicide, researchers were not able to find a single herbicide-resistant mutant. Though preliminary, the findings suggest that the mutation rate in amaranth is very low, and that low-level herbicide application contributes little — if anything — to the onset of new mutations conferring resistance, researchers say…
Any major stress that does not kill a plant can contribute to genetic mutations in its seeds and pollen, said University of Illinois crop sciences professor Patrick Tranel, who led the new research. Even the ultraviolet light in sunlight can stress a plant and increase the likelihood of mutations in its offspring, he said. Such mutations increase genetic diversity, which can be useful to a species’ survival.
“Resistance to herbicides comes from genetic variation in a population,” Tranel said. “If an individual weed has the right mutation that allows it to survive a particular herbicide, that individual will survive and pass the trait to its progeny.”
The relative contribution of new mutations to the problem of herbicide resistance is poorly understood, Tranel said. He and his colleagues hoped to determine the baseline mutation rate for a plant of the genus Amaranthus, a group that includes waterhemp, Palmer amaranth and other problematic agricultural weeds. They also wanted to test whether herbicide applications that failed to kill the plant increased that baseline rate…
Very few of the test plants overcame the herbicide treatment. Rigorous testing revealed that those rare plants that did survive were the offspring of seeds of weedy amaranth species that already carried the resistance genes…
“Herbicide resistance is an evolutionary process, and evolutionary processes are mathematical,” Tranel said. “If you know more precisely how plants will behave under different environmental conditions, you can develop equations that will predict how fast resistance will evolve.” Paper. (paywall) – Federico A. Casale, Darci A. Giacomini, Patrick J. Tranel. Empirical investigation of mutation rate for herbicide resistance. Weed Science, 2019; 1 DOI: 10.1017/wsc.2019.19 More.
Note: “New evidence suggests that herbicide resistance in weeds is more likely to occur from pre-existing genetic variation than from new mutations” vs. “Herbicide resistance is an evolutionary process, and evolutionary processes are mathematical”
It sounds as though the necessary evolution occurred a long time ago and that a Darwinian process just isn’t happening. But they are not likely allowed to discuss it that way.
See also: How Maize Corn’s Wild Ancestor Teosinte Prevents Maize From Breeding With It