Inheritance of resistance to S-metolachlor in a waterhemp (Amaranthus tuberculatus) population from central Illinois

Abstract Waterhemp [ Amaranthus tuberculatus (Moq.) Sauer] is a dioecious weed that has evolved resistance to very-long-chain fatty acid (VLCFA) elongase-inhibiting herbicides via rapid metabolism. Although detoxification enzyme activities are associated with S -metolachlor resistance in two multiple herbicide-resistant (MHR) A. tuberculatus populations from Illinois, the genetic basis of resistance is unknown. Therefore, our goal was to investigate inheritance of S -metolachlor resistance in the Stanford, Illinois Resistant (SIR) population. Specifically, our research objectives were to: i) generate a uniformly resistant, full-sib near inbred line (DK 3-2 ) via three generations of recurrent selection for resistance using preemergence S -metolachlor, ii) develop A. tuberculatus populations segregating for S -metolachlor resistance via reciprocal single plant (one male × one female) full-sib mating of DK 3-2 and a VLCFA inhibiting herbicide-sensitive population, SEN, iii) quantify S -metolachlor resistance levels in parental lines and their F 1 progenies via greenhouse dose-response analysis, and iv) evaluate inheritance of S -metolachlor resistance in F 2 progenies. Dose-response analysis using 6 to 8 S -metolachlor concentrations (0.015–15.0 μM; varying per population) generated lethal dose (LD) estimates of 50% (LD 50 ) and 90% (LD 90 ) for SIR, SEN, DK 3-2 and F 1 progenies. Lethal dose estimates indicated DK 3-2 has a higher magnitude of S -metolachlor resistance than the SIR population, demonstrating single crosses significantly increased S -metolachlor resistance in DK 3-2 . Levels of S -metolachlor resistance in F 1 populations were intermediate compared to DK 3-2 and SEN. Segregation of S -metolachlor resistance in F 2 families from the paternal-derived lines fit a single-gene model (R:S = 3:1), indicating a single, dominant gene confers S -metolachlor resistance in SIR. However, F 2 segregation results from the maternal-derived lines fit a duplicate recessive epistasis model (R:S = 9:7), indicating a second recessive gene may also modify S -metolachlor resistance in SIR. Results and germplasm derived from this research can assist in identifying the gene(s) conferring resistance to S -metolachlor in A. tuberculatus .