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 Ich inohe) is difficult to evaluate in soybean [Glycine max (L.) Merr.] br eeding. PI 437.654 has resistance to more SCN race isolates than any o ther known soybean. We screened 298 F-6:7 recombinant-inbred lines fro m a cross between PI 437.654 and 'BSR101' for SCN race-3 resistance, g enetically mapped 355 RFLP markers and the I locus, and tested these m arkers for association with resistance loci. The Rhg(4) resistance loc us was within 1 cM of the I Locus on linkage group A. Two additional Q TLs 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. Rhg(4) and the QTL on G show ed a significant interaction by together providing complete resistance to SCN race-3. Individually, the QTL on G had greater effect on resis tance than did Rhg(4), but neither Locus alone provided a degree of re sistance much different from the susceptible parent. The nearest marke rs to the mapped QTLs on groups A and C had allele frequencies from th e 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 e xpected 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 Q TLs on groups A and G were resistant to SCN race-3. We believe markers near to these QTLs can be used effectively to select for SCN race-3 r esistance, thereby improving the ability to breed SCN-resistant soybea n varieties.