Gene expression shapes the patterns of parallel evolution of herbicide resistance in the agricultural weedMonochoria vaginalis
AbstractThe evolution of herbicide resistance in weeds is an example of parallel evolution, through which genes encoding herbicide target proteins are repeatedly represented as evolutionary loci. The number of herbicide target-site genes differs among species, and little is known regarding the effects of duplicate gene copies on the evolution of herbicide resistance. We investigated the evolution of herbicide resistance inMonochoria vaginalis, which carries five copies of sulfonylurea target-site acetolactate synthase (ALS) genes. Suspected resistant populations collected across Japan were investigated for herbicide sensitivity andALSgene sequences, followed by functional characterisation andALSgene expression analysis. We identified over 60 resistant populations, all of which carried resistance-conferring amino acid substitutions exclusively inMvALS1orMvALS3. AllMvALS4alleles carried a loss-of-function mutation. Although the enzymatic properties of ALS encoded by these genes were not markedly different, the expression ofMvALS1andMvALS3was prominently higher among allALSgenes. The higher expression ofMvALS1andMvALS3is the driving force of the biased representation of genes during the evolution of herbicide resistance inM. vaginalis. Our findings highlight that gene expression is a key factor in creating evolutionary hotspots.