Soybean cyst nematode (SCN; Heterodera glycines) is one of the most challenging disease problems of soybeans. Genetic resistance to SCN is complex and involves multiple unlinked genes. To identify genes for SCN resistance, four distinct F_2 populations and one recombinant inbred population segregating for resistance were analysed using restriction fragment length polymorphism (RFLP) markers throughout the soybean genome. Among all the populations, four independent partial resistant loci were significantly associated with SCN resistance. One of these loci, located on the top of linkage group G, behaved as a major partial resistance gene and was common in all the populations studied. This locus explained at least 49% of the total variation in resistance (based on r-square estimates) in one population (PI 209332 × Evans) and between 23 and 45 % of the variation in resistance in the remaining populations. The locus on G mapped to a specific interval defined by two flanking RFLP markers (Bng 173 and Bng 122) approximately 12 cM apart. This information was used to characterize 147 F_4 and F_5 lines in the conventional SCN breeding programme that derived from a total of 16 crosses. These lines were assayed in parallel with both greenhouse SCN tests and RFLPs in the region of interest on linkage group G. Selection for the resistant parent allele on the basis of DNA markers identified lines with, on average, a 43% decrease in the SCN index of parasitism. Moreover, marker selection uncovered lines with interesting crossovers very tightly linked to the SCN resistance gene - crossovers that may lead to lines with increased yield, together with high levels of resistance.
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