The origin of ALS-inhibitor resistance in European populations of Amaranthus palmeri

Abstract

Resistant populations of Amaranthus palmeri to acetolactate synthase (ALS) inhibitors are present in Spain and Italy, but the evolutionary origin of resistance remains unknown. This study assesses the response of eleven populations of A. palmeri to ALS inhibitors and employs a haplotype-based genetic method to investigate the origin and distribution of resistance. All populations were resistant to thifensulfuron-methyl (THIF) and imazamox (IMA). The mutation in the Trp-574-Leu allele of the ALS gene was the primary cause of cross-resistance in both countries. Mutant alleles at 376-Glu and 197-Thr were also found in two Spanish populations. Genetic analyses revealed the presence of seven distinct haplotypes in the Spanish populations and three in the Italian ones. No haplotype was common to both countries. ALS-resistant populations were introduced to Italy and Spain from outside Europe. Their evolutionary histories differ, and the introduction events are independent. Resistance to ALS inhibitors spread over short and long distances through seed dispersal. The higher number and genetic diversity among Spanish haplotypes indicate recurrent invasions.