Resistance to an imidazolinone herbicide is conferred by a gene on chromosome 6DL in the wheat line cv. 9804

An induced mutation of the common wheat (2n == 6x == 42, AABBDD genomes) cultivar ‘‘Fidel’’ has been shown to provide resistance to the imidazolinone class of herbicides. This class of herbicide gives broad-spectrum weed control including the weedy relative of wheat, jointed goatgrass (2n == 4x == 28, CCDD genomes). Because wheat and jointed goatgrass share a common genome, genes present on the D genome may transfer between the two species as a result of natural hybridization and selective pressures. Our objectives were to determine which genome of common wheat contained the herbicide resistance gene in the mutated Fidel and to genetically map its position. We investigated the chromosomal location of this gene using both durum (2n == 4x == 28, AABB genomes) and common wheat (6x) backgrounds. From crosses of durum wheat genotypes as the recurrent parent with mutated Fidel (cv. 9804, resistant), only BC₁ plants containing chromosome 6D (inherited from cv. 9804) were resistant to applications of labeled rates of imazamox, an imidazolinone herbicide. No other D-genome chromosome was absolutely associated with herbicide resistance. To confirm this chromosomal location and genetically map the position of this gene, two populations of F₃ families from the cross of cv. 9804 to the common wheat cultivars ‘‘Cashup’’ and ‘‘Madsen’’ were screened for reaction to imazamox, followed by genetic mapping with microsatellite markers. Two linked microsatellite markers were associated with the resistance trait, and one of them, Xgdm127, was located to chromosome 6D using aneuploid stocks, confirming the location of this gene on 6D. These results indicate that this resistance gene is in the genome that common wheat shares with jointed goatgrass. Therefore, imidazolinone-resistant wheat will need to be carefully managed to minimize the occurrence and spread of resistant jointed goatgrass, whether such plants arise because of hybridization with resistant common wheat or by independent mutation, a frequent occurrence with this herbicide class.Nomenclature: Imazamox; jointed goatgrass, Aegilops cylindrica Host. AEGCY; durum wheat, Triticum durum Desf.; wheat, Triticum aestivum L.