Inheritance of evolved glyphosate resistance in Conyza canadensis (L.) Cronq.
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- Zelaya, I.A., Owen, M.D.K. & VanGessel, M.J. Theor Appl Genet (2004) 110: 58. doi:10.1007/s00122-004-1804-8
N-(phosphonomethyl)glycine (glyphosate) resistance was previously reported in a horseweed [Conyza (=Erigeron) canadensis (L.) Cronq.] population from Houston, DE (P0R). Recurrent selection was performed on P0R, since the population was composed of susceptible (5%) and resistant (95%) phenotypes. After two cycles of selection at 2.0 kg ae glyphosate ha−1, similar glyphosate rates that reduced plant growth by 50%, glyphosate rates that inflicted 50% mortality in the population, and accumulations of half of the maximum detectable shikimic acid concentration were observed between the parental P0R and the first (RS1) and second (RS2) recurrent generations. In addition, RS1 and RS2 did not segregate for resistance to glyphosate. This suggested that the RS2 population comprised a near-homozygous, glyphosate-resistant line. Whole-plant rate responses estimated a fourfold resistance increase to glyphosate between RS2 and either a pristine Ames, IA (P0P) or a susceptible C. canadensis population from Georgetown, DE (P0S). The genetics of glyphosate resistance in C. canadensis was investigated by performing reciprocal crosses between RS2 and either the P0P or P0S populations. Evaluations of the first (F1) and second (F2) filial generations suggested that glyphosate resistance was governed by an incompletely dominant, single-locus gene (R allele) located in the nuclear genome. The proposed genetic model was confirmed by back-crosses of the F1 to plants that arose from achenes of the original RS2, P0P, or P0S parents. The autogamous nature of C. canadensis, the simple inheritance model of glyphosate resistance, and the fact that heterozygous genotypes (F1) survived glyphosate rates well above those recommended by the manufacturer, predicted a rapid increase in frequency of the R allele under continuous glyphosate selection. The impact of genetics on C. canadensis resistance management is discussed.