As public institutions and seed companies incorporate soybean aphid ( Matsumura) resistance genes into soybean [(L.) Merr.] cultivars, it is important to retain resistance to insect defoliators. In order to improve MSU aphid-resistant germplasm for resistance to Japanese beetle ( Newman), a population was derived from a cross between E06906 (MSU) and another aphid-resistant source, LD05-16060 (Uni. of Illinois) that showed lower susceptibility to Japanese beetle. A QTL (Quantitative Trait Loci) mapping approach with a subset of 94 individuals on 15 chromosomes indicated the presence of five QTL (QTL-M, QTL-G, QTL-H, QTL-D1b and QTL-E) previously reported to confer resistance to several other soybean defoliators, and one published QTL to confer Japanese beetle-resistance. More importantly, three new QTL were found on LG-A1, LG-A2, and LG-C2; they were also detected with 234 individuals. Candidate gene analysis for resistant QTL found key enzymes involved in flavonoid biosynthesis pathway, thus a comprehensive flavonoid profiling study was conducted using High Performance Liquid Chromatography/tandem Mass Spectrometry (HPLC/MS/MS) on three susceptible and three resistant lines. 32 distinct peaks corresponding to glycosides or aglycones of Daidzein, Genistein, Glycetein, Kaempferol, Naringenin, and Quercetin were found in damaged and un-damaged leaflets. Higher abundances of flavonoids were found in damaged leaflets of LD05-16060. It appears that differential susceptibility to Japanese beetle observed between LD05 and E06906, can be explained by differences in feeding-deterrent and phago-stimulant flavonoids. Furthermore, this mapping population gave an opportunity to stack and aphid-resistant genes. Pyramiding multiple resistance genes, particularly with different modes of action, has great potential to provide durable resistance. This dissertation also reports MSU soybean breeding program's research on stacking , , , and aphid- resistant genes from a population of 727 F2 individuals derived from two (Plant Introduction) PI s. Four trials in greenhouse and field were conducted for phenotypic evaluations. SSR and SNP markers linked to these genes were used for the genotypic selections. Repeatedly in all trials lines outperformed other lines showing great consistency in their resistance. Additionally, and stacks also provided significantly more resistance than other gene combinations.
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