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A QTL (Quantitative Trait Locus) is chromosomal location of a gene controlling a specific phenotypic characteristic (trait). This trait might be governed by two or more genes and may be affected by environmental interaction. The USA soybean seed composition, when averaged over years and states, is 18.7% oil and 35.3% protein. Soybean seed provides cooking oil for humans and protein for livestock. Concurrent genetic improvement of seed protein (pro) and oil content has been difficult to achieve due to the negative genetic correlation of the two traits. This negative correlation could be due to a pair of tightly linked protein and oil QTLs, whose individual alleles are linkage-paired to give rise to high pro - low oil or low pro – high oil, phenotypes, OR it could be due to just one pleiotropic QTL, whose two alleles have inverse effects on both oil and protein. This thesis objective is to find oil QTLs with minimal effect on protein. Three F2 populations were developed by mating of two high oil lines with each other and with Williams 82, a current high-yield cultivar. About 500 individual F2 plants in each population produced F2.3 seed progenies and then F2.4 seed progenies that were phenotyped for seed protein and oil content. Selective genotyping was used to genotype F2 plant progenitors of only the highest and the lowest seed oil deciles of F2.4 seed progenies. A 1536 SNP locus assay chip was used for genotyping. In the three mapping populations, eight seed oil QTLs with LOD scores greater than 3.0 were detected and mapped on seven linkage groups using R/qtl software. Six statistically significant seed oil QTLs on LG-C2 (Chr6), LG-M (Chr7), LG-B1 (Chr11), LG-F (Chr13), LG-E (Chr15), and LG-L (Chr19) were detected using genome-wide permutation tests (α = 0.05). Of the seed oil QTLs detected in this study, only the seed oil QTLs on LG-F (Chr13) have no significant impact on seed protein content. For improving the seed oil content in high yielding soybean cultivars, S17276 allele (Chr13) from the parental line RMLPC1-311-128-128 may be useful to soybean breeders to improve soybean seed oil content without effecting on seed protein content.
Advisor: James E. Specht