Genetic mapping of soybean cyst nematode race-3 resistance loci in the soybean PI 437.654

Resistance to the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is difficult to evaluate in soybean (Glycine max (L.) Merr.) breeding. PI 437.654 has resistance to more SCN race isolates than any other known soybean. 298 F6:7 recombinant-inbred lines from a cross between PI 437.654 and 'BSR101' were screened for SCN race-3 resistance, 355 RFLP markers and the I locus were genetically mapped, and these markers were tested for association with resistance loci. The Rhg4 resistance locus was within 1 cM of the I locus on linkage group A. Two additional QTLs associated with SCN resistance were located within 3 cM of markers on groups G and M. These two loci were not independent because 91 of 96 lines that had a resistant-parent marker type on group G also had a resistant-parent marker type on group M. Rhg4 and the QTL on G showed a significant interaction by together providing complete resistance to SCN race-3. Individually, the QTL on G had greater effect on resistance than did Rhg4, but neither locus alone provided a degree of resistance much different from the susceptible parent. The nearest markers to the mapped QTLs on groups A and G had allele frequencies from the resistant parent indicating 52 resistant lines in this population, a number not significantly different from the 55 resistant lines found. Therefore, no QTLs from PI 437.654 other than those mapped here are expected to be required for resistance to SCN race-3. All 50 lines that had the PI 437.654 marker type at the nearest marker to each of the QTLs on groups A and G were resistant to SCN race-3. Markers near to these QTLs could be used effectively to select for SCN race-3 resistance, thereby improving the ability to breed SCN-resistant soybean varieties.