Genetics and Inheritance of Nontarget-Site Resistances to Atrazine and Mesotrione in a Waterhemp (Amaranthus tuberculatus) Population from Illinois

A waterhemp population (McLean County resistant, MCR) from McLean County, Illinois is resistant to both mesotrione and atrazine by elevated rates of herbicide metabolism. Research was conducted to investigate the inheritance of these resistance traits. Resistant and sensitive plants were crossed to obtain reciprocal F 1 populations, which were then used to create pseudo-F 2 and backcross (to sensitive parent; BC S ) populations. The various populations were evaluated with whole-plant herbicide efficacy studies in a greenhouse. The responses of the F 1 populations to both mesotrione and atrazine were intermediate when compared with parental populations. In the case of atrazine, BC S and F 2 populations segregated 1 : 1 and 1 : 3, respectively, for susceptibility (S) : resistance (R), at a dose that controlled the sensitive parent but not the F 1 or resistant parent. For mesotrione, variability was observed within the F 1 populations, suggesting that mesotrione resistance is multigenic and the resistant parents used in the cross were not homozygous at the resistance loci. Furthermore, at low mesotrione doses, more F 2 plants survived than expected on the basis of a single-gene trait, whereas at high doses, fewer F 2 plants survived than expected. Dry weight data confirmed the conclusions obtained from survival data. Specifically, atrazine responses segregated into two discrete classes (R and S) in both the F 2 and BC S populations, whereas mesotrione responses showed continuous distributions of phenotypes in F 2 and BC S populations. We conclude that metabolism-based atrazine resistance in MCR is conferred by a single major gene, whereas inheritance of mesotrione resistance in this population is complex.