| The utility of combining two approaches for improving insect resistance in soybean was evaluated through field studies with a transgenic line, mapping of resistance quantitative trait loci (QTLs), and marker-assisted selection to combine transgenic resistance and host plant resistance. Jack-Bt, a transgenic line expressing a cry1Ac transgene from Bacillus thuringiensis ssp. kurstaki was evaluated for resistance to corn earworm (Helicoverpa zea Boddie), soybean looper (Pseudoplusia includens Walker), and velvetbean caterpillar (Anticarsia gemmatalis Hübner) in a 1996 field study. Plants in mesh-covered cages were artificially infested with larvae and resistance was measured as percent defoliation. Additional cage studies with corn earworm and velvetbean caterpillar were conducted in 1997 and 1998, and natural infestation studies were also conducted in 1998. These studies indicated that, in comparison with untransformed ‘Jack’ and the resistant breeding line GatIR81-296, Jack-Bt has a high level of resistance to velvetbean caterpillar and the lesser cornstalk borer, and good resistance to the corn earworm. Jack-Bt resistance to soybean looper was significantly better than that of Jack, but was lower than resistance to the other pest species.; Simple sequence repeats (SSRs) were used to remap three antixenosis (nonpreference) QTLs which had been previously mapped to linkage groups (LGs) D1b, H, and M of soybean PI 229358 using restriction fragment length polymorphisms (RFLPs). SSRs known to be linked to RFLP loci close to the three QTLs were screened for length polymorphisms in a ‘Cobb’ (susceptible) × PI 229358 cross. An F2 population of 100 plants from this cross was genotyped at six SSR loci on LG D1b, seven on LG H, and nine on LG M. These data were then used to construct integrated RFLP-SSR linkage maps of the regions containing the QTLs, and were combined with data for corn earworm defoliation of F2:3 plants to remap the positions of the three QTLs. Interval mapping, single factor analysis of variance, and multiple regression were used to determine the most likely positions of the QTLs relative to the available markers. In addition to improving the resolution of the QTL maps, the identification of SSRs closely linked to the QTLs provided markers useful in a concurrent marker-assisted selection project.; A backcross scheme was implemented to introgress the LG H and LG M antixenosis QTLs from PI 229358 into a predominantly Jack-Bt genetic background, so that the value of combining these two types of resistance could be tested. SSR markers were used to select plants in the BC2F1, BC 2F2 and BC2F3 generations that had desirable genotypes, and selected BC2F3 plants were classified into one of eight classes representing all possible combinations for presence or absence of the cry1Ac transgene and the two QTLs. Resistance of these plants was evaluated based on percent defoliation in detached leaf bioassays with corn earworm and velvetbean caterpillar. First and second instar larvae of both pest species were highly sensitive to the Cry1Ac protein, and damage to leaves from plants carrying the transgene was rare. |
