Date: Tuesday, January 17, 2012
Time: 11:20 AM
Time: 11:20 AM
Room: Pacific Salon 4-5 (2nd Floor)
Strong selection by herbicides has resulted in the widespread evolution and spread of herbicide resistance in agricultural weed populations. During resistance evolution and spread, local mutation and selection, interpopulation gene flow, and founder events lead to the spatial structuring of genetic variation across the agricultural landscape. To elucidate the origins and invasion routes of glyphosate-resistant plants within orchards and vineyards across the Central Valley of California, we sampled and genotyped 1,262 Conyza canadensis individuals from 43 populations at 12 microsatellite loci and 1,077 C. bonariensis individuals from 40 populations at 15 loci. Seedling offspring from each population were phenotyped for resistance or susceptibility to glyphosate at one and/or two times the recommended field rate. The model-based Bayesian clustering analysis STRUCTURE identified two distinct genetic clusters at the highest hierarchical level within both species. However, the geographical distribution of proportional assignments of populations to the genetic clusters differed markedly between species. For C. canadensis, populations exhibited a strong north-south distribution of genetic clusters, which corresponded to the geographical distribution of resistant and susceptible phenotypes. For C. bonariensis, most populations assigned partially to both genetic clusters, with one cluster predominant at the center of the sampled range and the second cluster predominant at the extremes. The majority of populations contained resistant phenotypes, but populations in the center of the range revealed more consistent and less variable glyphosate responses. Our results strongly suggest that the processes underlying the evolution and spread of glyphosate resistance differ for the two species.