Hybridization between imidazolinone-resistant wheat (Triticum aestivum L.) and jointed goatgrass (Aegilops cylindrical Host.) and selection pressure impacts on proportion of resistance alleles Public Deposited



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  • Imazamox-resistant wheat (Clearfield®) cultivars carry the Imi1 gene, which confers resistance to the imidazolinone (IMI) herbicide imazamox. Imazamox provides selective control of jointed goatgrass and other weeds in IMI-resistant wheat. Imi1 gene flow between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of the Imi1 gene in wheat by jointed goatgrass hybrids in Eastern Oregon. Tissue and spikes from hybrids were collected and Imi1 presence was detected by PCR assays. We assessed hybrid yield components and explored how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and type of system (agricultural or non-agricultural) or management practice in the commercial fields was determined. A total of 128 sites were surveyed over the two years. Of 1,410 plants sampled, 1,100 were positive for the Imi1 gene of which 1,087 were heterozygousand 13 samples were homozygous for the gene. The 13 homozygous plants provide evidence that they are of backcross generations because they no longer carry the wild type allele. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Non-agricultural sites or fields with IMI-resistant wheat production back-to-back, were two factors associated with a greater proportion of IMI-resistant hybrids. These results indicate that it is important to choose field management practices that reduce the production of IMI-resistant hybrids, and to manage non-agricultural areas with jointed goatgrass infestations to prevent introgression of the Imi1 gene in these hybridization zones. The most economic and environmental friendly method to selectively control the pathogen Oculimacula yallundae in winter wheat is the use of resistant wheat cultivars. These cultivars carry the Pch1 gene, which provides resistance to foot rot. Once the Imi1 and Pch1 genes are introgressed into a jointed goatgrass population, their intraspecific movement and fate in the progeny remains largely unstudied. Therefore, field experiments were conducted using Imi1 and Pch1 resistance genes introgressed into a single jointed goatgrass population and selection pressure treatments were applied. The progeny were genotyped to detect the presence of the resistance alleles in order to determine proportion and the level of gene flow. In addition, selection pressure effects on yield components were analyzed. The herbicide-resistance allele proportion in the progeny was greater when parent plants were treated with imazamox. The disease-resistance allele proportion did not differ among the selection pressure treatments in the first year, but was greater with disease in the second year. The herbicide-resistance gene flow was greater with herbicide selection pressure in the first year but did not differ in the second year. Disease resistance gene flow did not differ among the selection pressure treatments. Because the resistance allele proportion increased in the two experiments for herbicide and in experiment one for disease resistance, it is likely that once introgression takes place, the increase of the resistance alleles in subsequent generations will reach fixation, with selection pressure. In addition, selection pressure treatments reduced yield components in the parental plants compared with the control treatment. This study revealed that there was no fitness cost associated with IMIresistant or foot rot resistance in jointed goatgrass in the absence of selection pressure. The knowledge of how selection pressure at the field level influences the resistance gene flow and the proportion with which the resistance genes occur in the progeny is important to prevent resistance spread. In addition, it lays the ground work for researchers to continue investigating the impacts of selection pressure on resistance genes in subsequent generations or other genes of ecological significance such as drought, cold or salt tolerance.
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