Inheritance of evolved glyphosate resistance in Conyza canadensis (L.) Cronq.

N-(phosphonomethyl) glycine ( glyphosate) resistance was previously reported in a horseweed [Conyza (= Erigeron) canadensis ( L.) Cronq.] population from Houston, DE (P-0(R)). Recurrent selection was performed on P-0(R), 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 P-0(R) 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 (P-0(P)) or a susceptible C. canadensis population from Georgetown, DE (P-0(S)). The genetics of glyphosate resistance in C. canadensis was investigated by performing reciprocal crosses between RS2 and either the P-0(P) or P-0(S) populations. Evaluations of the first (F-1) and second (F-2) 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 F-1 to plants that arose from achenes of the original RS2, P-0(P), or P-0(S) parents. The autogamous nature of C. canadensis, the simple inheritance model of glyphosate resistance, and the fact that heterozygous genotypes (F-1) 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.