Plant Molecular Genetics, The University of Tennessee and Center for Legume Research, Knoxville, TN 37901-1071, U.S.A.
Recombinant inbred lines (RILs) as well as an F2 segregating population of soybean Glycine max facilitated the mapping of two expressed sequence tags involved in early nodulation and subsequent nitrogen fixation in soybean. For the early nodulin gene enod2, the parents of RILs, Minsoy and Noir1, showed a polymorphism (5.5 vs 5.9 kb) after EcoRV digestion. Restriction fragment length polymorphism (RFLP) patterns of 42 RILs were analyzed with the MAPMAKER program, linking enod2 to the seed coat color gene, I, with a distance of 11.1 cM on linkage group U3 of RIL map. enod2 and I are located close to Rhg4, a soybean cyst nematode resistance gene, and a locus for seed coat hardness. The molecular marker pA-110 and seed coat color were used to integrate enod2 on an F2 segregating population (72 plants) generated from a cross between cv. Bragg and G. soja PI468.397. enod2 was mapped in the same order as on the RIL map but 18.5 cM from the I locus on the TN map. A microsatellite from the 5′ region of enod2b was mapped in the same position, demonstrating that enod2b and not enod2a was mapped. An RFLP for lbc3 (leghemoglobin) segregated independently from enod2 and the nts-1 supernodulating locus suggesting that in soybean symbiotically significant loci (including rj1, Rj2, and rj6) are not clustered.