The soybean aphid has become one of the most devastating pests to soybean in North America since its invasion in 2000. Soybean breeders have been seeking resistance sources and incorporating the resistance into elite cultivars since then. My research focuses on mapping and fine mapping soybean aphid resistance genes in three aphid resistance sources PI 567597C, PI 567537 and PI 567585A. The first objective of this study was to characterize the inheritance pattern of soybean aphid resistance gene/genes in PI 567597C and pinpoint the resistance gene/genes with genetic markers. Four populations segregating for aphid resistance from PI 567597C were studied. Phenotypic analysis by Chi-square test showed that aphid resistance in PI 567597C was controlled by a single partially dominant gene. QTL analysis using population 050107 located the aphid resistance gene on chromosome 16 which explained 90% of the variation, named Rag3e. Data from population 050018 excluded possible QTLs on chromosomes with known aphid resistance QTLs. Data from two populations confirmed the aphid resistance gene in different genetic backgrounds.The second objective of this study was to narrow the Rag3e QTL region in PI 567597C to facilitate marker assisted selection. residual heterozygous lines (RHL) were selected from two segregating populations. SNP markers discovered from the next generation sequencing data enabled the fine mapping process. Rag3e was pinpointed to a 60kb interval on chromosome 16 with seven candidate genes.The third objective of this study was to use RHLs to fine map aphid resistance QTL, Rag3d, from PI 567585A, to develop single nucleotide polymorphism (SNP) markers for marker assisted selection and to find candidate genes for functional study. RHLs were selected from two segregating populations. Important RHLs were genotyped with the SoySNP50K chip to help identifying recombination breaking points. Rag3d was fine mapped to a 46kb interval on chromosome 16 with five candidate genes.The fourth objective of this study was to use SNP markers to fine map aphid resistance QTL Rag3b from PI 567537, to identify candidate genes for Rag3b and to develop genetic markers for marker assisted selection. Three F2 populations derived from the cross between aphid resistant and the susceptible parents were used to validate Rag3b and screen for recombination breaking points. The Rag3b QTL was narrowed to a 199kb region on chromosome 16 with twelve candidate genes.
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