The genetic locus controlling supernodulation in soybean (Glycine max L.) co-segregates tightly with a cloned molecular marker.

The genetic locus (nts) controlling nitrate-tolerant nodulation, supernodulation, and diminished autoregulation of nodulation of soybean (Glycine max (L.) Merill) was mapped tightly to the pA-132 molecular marker using a restriction fragment length polymorphism (RFLP) detected by subclone pUTG-132a. The nts (nitrate-tolerant symbiotic) locus of soybean was previously detected after its inactivation by chemical mutagenesis. Mutant plant lines were characterized by abundant nodulation (supernodulation) and tolerance to the inhibitory effects of nitrate on nodule cell proliferation and nitrogen fixation. The large number of RFLPs between G. max line nts382 (homozygous for the recessive nts allele) and the more primitive soybean G. soja (PI468.397) allowed the detection of co-segregation of several diagnostic markers with the supernodulation locus in F2 families. We located the nts locus on the tentative RFLP linkage group E about 10 cM distal to pA-36 and directly next to marker pA-132. This very close linkage of the molecular marker and the nts locus may allow the application of this clone as a diagnostic probe in breeding programs as well as an entry point for the isolation of the nts gene.