A decade of QTL mapping for cyst nematode resistance in soybean

Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe), the most destructive pest of soybean [Glycine mar (L.) Merrill], is estimated to be responsible for almost nine million megagrams in annual yield loss worldwide. Host plant resistance is the most cost-effective and environmentally friendly method of controlling SCN. Resistance is present among soybean plant introductions (PIs) and related wild species, such as Glycine soja Sieb. and Zucc. Molecular marker technology has ushered in a decade devoted to the identification and characterization of quantitative trait loci (QTL) underlying SCN. These genetic mapping efforts uncovered numerous locations of SCN resistance QTL in many PIs. In more than a decade of mapping SCN resistance QTL, there is some consistency in the results. In almost all studies involving various sources of resistance, the QTL conferring the greatest level of resistance mapped to the region containing rhg1 on linkage group (LG) G. In addition, a major resistance QTL was mapped in many sources to the region containing Rhg4 on LG A2. The mapping of QTL to these regions from many sources suggests that these sources may have resistance genes in common, which has caused concern over the possible dependence on a few resistance genes. Recently, two independent research groups reported cloning candidate genes for rhg1 and Rhg4. Despite these advances, there is some degree of trepidation, especially in the public sector, on the use of rhg1 and Rhg4 genetic mapping and cloning information in SCN resistance breeding because of intellectual property issues.