Evidence of multiple origins of glyphosate resistance evolution in Lolium multiflorum

Abstract

The multitude of herbicide resistance patterns in the weed Lolium multiflorum L. is a remarkable example of the rapid adaptation to anthropogenic-driven disturbance. Recently, resistance to glyphosate was identified in multiple populations of L. multiflorum in Oregon. We used phenotypic approaches, as well as population genomic and gene expression analyses, to determine if known mechanisms were responsible for glyphosate resistance, if resistance phenotypes evolved independently in different populations, and to identify potential loci contributing to resistance. We found no evidence of genetic alterations or expression changes at known target and non-target sites of glyphosate resistance. Population genomic analyses indicated that resistant populations tended to have largely distinct ancestry from one another, with little evidence of admixture, suggesting that glyphosate resistance did not spread among populations via gene flow. Rather, resistance appears to have evolved independently on different genetic backgrounds. We also detected potential loci associated with the resistance phenotype, some of which encode proteins with potential effects on herbicide metabolism. Our results suggest that Oregon populations of L. multiflorum evolved resistance to glyphosate due to a novel mechanism. Future studies that characterize the gene or genes involved in resistance will be necessary to confirm this conclusion.