The evolution of herbicide resistance in weeds is an example of parallel evolution, throughwhich genes encoding herbicide target proteins are repeatedly represented as evolutionarytargets. The number of herbicide target-site genes differs among species, and little is knownregarding the effects of duplicate gene copies on the evolution of herbicide resistance.
We investigated the evolution of herbicide resistance in Monochoria vaginalis, which car-ries ﬁve copies of sulfonylurea target-site acetolactate synthase (ALS) genes. Suspected resis-tant populations collected across Japan were investigated for herbicide sensitivity and ALSgene sequences, followed by functional characterization and ALS gene expression analysis.
We identiﬁed over 60 resistant populations, all of which carried resistance-conferring aminoacid substitutions exclusively in MvALS1 or MvALS3. All MvALS4 alleles carried a loss-of-function mutation. Although the enzymatic properties of ALS encoded by these genes werenot markedly different, the expression of MvALS1 and MvALS3 was prominently higheramong all ALS genes.
The higher expression of MvALS1 and MvALS3 is the driving force of the biased represen-tation of genes during the evolution of herbicide resistance in M. vaginalis. Our ﬁndings high-light that gene expression is a key factor in creating evolutionary hotspots.