Identification and characterization of soybean cyst nematode resistance genes using DNA markers

A major partial soybean cyst nematode (SCN; Heterodera glycines) resistance locus, common among various sources of SCN resistance, was identified on linkage group 'G' of the soybean (Glycine max (L.) Merrill) molecular linkage map using restriction fragment length polymorphisms (RFLPs). The 'G' locus explains 51.4% of the total phenotypic variation in SCN disease response in PI 209332, 52.7% in PI 90763, 40% in PI 88788 and 28% in 'Peking' and appears to be additive to partially recessive in gene action. It was also observed that the ability to detect minor SCN resistance loci was complicated by the presence of strong phenotypic effects due to the locus on 'G'. In one experiment, the 'G' locus appeared to be race specific. Marker-assisted selection, based on the 'G' locus, was shown to be 92% accurate in selecting SCN resistant soybean lines compared to 81% obtained with phenotype-based selection. A targeted comparative mapping strategy was employed resulting in a sevenfold increase in marker density within the 14.7 centimorgan (cM) region of interest, which initially had a low density of markers. Qualitative scoring of SCN, using a recombinant inbred soybean population, made it possible to identify recombinants between the two flanking markers (Bng 122 and Bng 173) and the SCN resistance locus. Since cloning a gene by map position is dependent on the ability to uncover tightly linked recombinants around the target gene, the strategy that is described in this thesis facilitated this process. By combining mapping information from four distinct populations, an integrated map of the linkage group 'G' region was created, placing the SCN locus 2.1 cM from the nearest RFLP marker (B53). This thesis demonstrates how various molecular mapping strategies, such as RFLP mapping, comparative genome and integrated mapping, together with qualitative scoring of a quantitative trait, can provide the necessary tools for developing a high resolution map around a quantitative trait locus (QTL) for eventual map-based cloning.