Utilization of QTL in Soybean Breeding Programs.

Marker-assisted selection (MAS) is widely used in soybean breeding in both the public and private sectors. The use of MAS in soybean breeding was spurred by investment in the development of marker technologies and genetic maps in the species. The sequencing of the soybean genome has provided further resources for marker development and genetic mapping. The use of MAS in soybean breeding programs has been especially successful in selecting genes controlling resistance to diseases and pests including soybean cyst nematodes (SCN). Resistance to SCN is a logical choice for MAS because this pest is estimated to cause the greatest yield loss of any soybean disease or pests in the USA. In addition, phenotypically testing plants for this resistance is expensive and difficult. To phenotypically test for SCN resistance, plants must be inoculated with nematodes, grown for 30 days, and then the number of nematode females on the roots estimated or counted. SCN resistance was one of the first resistance traits mapped in soybean and quantitative trait loci (QTL) controlling resistance have been mapped to almost every linkage group. The most important SCN resistance QTL mapped is rhg1 as it has a large impact on resistance and it is the main resistance gene in elite soybean germplasm in the USA. rhg1 was originally mapped using RFLP markers, later with SSR markers, and now with SNP markers. The gene was fine mapped and recently cloned, which will provide perfect marker for the selection of rhg1. Overall, the use of MAS for rhg1 and other SCN resistance genes has resulted in the development and release of more SCN resistant cultivars than would have been released through conventional breeding methods.