Combining cry1Ac with QTL alleles from PI 229358 to improve soybean resistance to lepidopteran pests

A QTL conditioning corn earworm resistance in soybean PI 229358 and a synthetic Bacillus thuringiensis cry1Ac transgene from the recurrent parent 'Jack-Bt' were pyramided into BC2F3 plants by marker-assisted selection. Segregating individuals were genotyped at SSR markers linked to an anitbiosis/antixenosis QTL on linkage group M, and were tested for the presence of cry1Ac. Marker-assisted selection was used during and after the two backcrosses to develop a series of BC2F3 plants with or without the crylAc transgene and the QTL conditioning for resistance BC2F3 plants that were homozygous for parental alleles at markers on LG M, and which either had or lacked cry1Ac, were assigned to one of four possible genotype classes. These plants were used in no-choice, detached leaf feeding bioassays with corn earworm and soybean looper larvae (Lepidoptera: Noctuidae) to evaluate the relative antibiosis in the different genotype classes. Resistance was measured as larval weight gain and degree of foliage consumption. Few larvae of either species survived on leaves expressing the Cry1Ac protein. Though not as great as the effect of Cry1Ac, the PI 229358-derived LG M QTL also had a detrimental effect on larval weights of both pest species, and on defoliation by corn earworm, but did not reduce defoliation by soybean looper. Weights of soybean looper larvae fed foliage from transgenic plants with the PI-derived QTL were lower than those of larvae fed transgenic tissue with the corresponding Jack chromosomal segment. This work demonstrates the usefulness of SSRs for marker-assisted selection in soybean, and shows that combining transgene-and QTL-mediated resistance to lepidopteran pests may be a viable strategy for insect control.