SignificanceGlyphosate is a nonselective herbicide used around the globe for weed control in glyphosate-resistant (GR) and noncrop situations. The extensive and exclusive use of glyphosate has led to the evolution of herbicide resistance in many crop weeds. The molecular target of glyphosate, the 5-enolpyruvlyshikimate-3-phosphate synthase (EPSPS) gene, confers resistance upon amplification and was first documented in GR Amaranthus palmeri. We now report that amplified EPSPS copies in GR A. palmeri are present in the form of extrachromosomal circular DNA molecules (eccDNAs) with various conformations. We discovered that eccDNAs are transmitted to the next generation by tethering to mitotic and meiotic chromosomes. These results represent a report of extrachromosomal structures that drive rapid adaptive evolution in higher organisms.

This study shows that multiple herbicide resistance in Italian ryegrass populations in California, USA, is due to both target-site and non-target-site resistance mechanisms. © 2017 Society of Chemical Industry.

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A population of Palmer amaranth (Amaranthus palmeri S. Watson) in Kansas (KSR) was found to have evolved resistance to three commonly used herbicides, i.e., chlorsulfuron, atrazine, and mesotrione. Our previous research confirmed the predominance of metabolic resistance to these herbicides in KSR, although a small percentage of plants also showed target-site alteration conferring resistance. In this research, we investigated the inheritance of resistance to these three herbicides in KSR Palmer amaranth. F1 and F2 progeny were generated by a conventional breeding approach. On the basis of the response of F1 and F2 progeny to three herbicide applications and χ2 analyses, we found that the resistance of KSR Palmer amaranth to chlorsulfuron and atrazine is controlled by single nuclear genes. However, resistance to mesotrione was found to be imparted by multiple nuclear alleles in this population. Single-gene resistance can spread rapidly, while multiple-gene traits are slower to evolve and spread. Regardless, adoption of integrated weed management strategies can minimize the spread of resistance.

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