'Forrest' resistance to the soybean cyst nematode is bigenic:: saturation mapping of the Rhg1 and Rhg4 loci

Field resistance to cyst nematode (SCN) race 3 (Heterodera glycines I.) in soybean [Glycine max (L.) Merr.] cv 'Forrest' is conditioned by two QTLs: the underlying genes are presumed to include Rhg1 on linkage group G and Rhg4 on linkage group A2. A population of recombinant inbred lines (RILs) and two populations of near-isogenic lines (NILs) derived from a cross of Forrest x Essex were used to map the loci affecting resistance to SCN. Bulked segregant analysis, with 512 AFLP primer combinations and microsatellite markers, produced a high-density genetic map for the intervals carrying Rhg1 and Rhg4. The two QTLs involved in resistance to SCN were strongly associated with the AFLP marker E(ATG)M(CGA)87 (P = 0.0001, R-2 = 24.5%) on linkage group G, and the AFLP marker E(CCG)M(AAC)405 (P = 0.0001, R-2 = 26.2%) on linkage group A2. Two-way analysis of variance showed epistasic interaction (P = 0.0001, R-2 = 16%) between the two loci controlling SCN resistance in Essex x Forrest recombinant inbred lines. Considering the two loci as qualitative genes and the resistance as female index FI <5%, jointly the two loci explained over 98% of the resistance. The locations of the two QTLs were confirmed in the NILs populations. Therefore SCN resistance in Forrest x Essex is bigenic. High-efficiency marker-assisted selection can be performed using the markers to develop cultivars with stable resistance to SCN.