MBE Advance Access originally published online on March 10, 2004
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Mol. Biol. Evol. 21(5):884-892. 2004
DOI: 10.1093/molbev/msh095
© 2004 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038
Nucleotide Variability at the Acetyl Coenzyme A Carboxylase Gene and the Signature of Herbicide Selection in the Grass Weed Alopecurus myosuroides (Huds.)
Institut National de la Recherche Agronomique, UMR Biologie et Gestion des Adventices, Dijon Cédex, France
E-mail: delye{at}dijon.inra.fr.
Acetyl coenzyme A carboxylase (ACCase) is the target of highly effective herbicides. We investigated the nucleotide variability of the ACCase gene in a sample of 18 black-grass (Alopecurus myosuroides [Huds.]) populations to search for the signature of herbicide selection. Sequencing 3,396 bp encompassing ACCase herbicide-binding domain in 86 individuals revealed 92 polymorphisms, which formed 72 haplotypes. The ratio of nonsynonymous versus synonymous substitutions was very low, in agreement with ACCase being a vital metabolic enzyme. Within black grass, most nonsynonymous substitutions were related to resistance to ACCase-inhibiting herbicides. Differentiation between populations was strong, in contrast to expectations for an allogamous, annual plant. Significant H tests revealed recent hitchhiking events within populations. These results were consistent with recent and local positive selection. We propose that, although they have only been used since at most 15 black-grass generations, ACCase-inhibiting herbicides have exerted a positive selection targeting resistant haplotypes that has been strong enough to have a marked effect upon ACCase nucleotide diversity. A minimum-spanning network of nonrecombinant haplotypes revealed multiple, independent apparitions of resistance-associated mutations. This study provides the first evidence for the signature of ongoing, recent, pesticide selection upon variation at the gene encoding the targeted enzyme in natural plant populations.
Key Words: acetyl coenzyme A carboxylase • Alopecurus myosuroides • herbicide • local selection • population structure
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