Introgression from local cultivars is a driver of agricultural adaptation in Argentinian weedy rice

Abstract

Weedy rice, a pervasive and troublesome weed found across the globe, has oftenevolved through fertilization of rice cultivars with little importance of crop-weedgeneflow. In Argentina, weedy rice has been reported as an important constraint sincethe early 1970s, and, in the last few years, strains with herbicide-resistanceare suspectedto evolve. Despite their importance, the origin and genetic composition ofArgentinian weedy rice as well its adaptation to agricultural environments has notbeen explored so far. To study this, we conducted genotyping-by-sequencingon samplesof Argentinian weedy and cultivated rice and compared them with published datafrom weedy, cultivated and wild rice accessions distributed worldwide. In addition, weconducted a phenotypic characterization for weedy-relatedtraits, a herbicide resistancescreening and genotyped accessions for known mutations in the acetolactatesynthase (ALS) gene, which confers herbicide resistance. Our results revealed largephenotypic variability in Argentinian weedy rice. Most strains were resistant to ALS-inhibitingherbicides with a high frequency of the ALS mutation (A122T) present inArgentinian rice cultivars. Argentinian cultivars belonged to the three major geneticgroups of rice: japonica, indica and aus while weeds were mostly aus or aus-indicaadmixed, resembling weedy rice strains from the Southern Cone region. Phylogeneticanalysis supports a single origin for aus-likeSouth American weeds, likely as seed contaminantsfrom the United States, and then admixture with local indica cultivars. Ourfindings demonstrate that crop to weed introgression can facilitate rapid adaptationto agriculture environments.