Common Bean (Phaseolus vulgaris L.) Mutants Defective in Root Nodule Formation: II GENETIC ANALYSIS

Genetic analysis of crosses between two induced, ineffectivelynodulating mutants of common bean, NOD238 and NOD109, revealedthat their mutated nodulation phenotype is under the controlof the same locus in both mutants. The two mutants also resultedallelic for poor pod fertility, the other trait common to themutants. F₁ plants from crosses with their wild types nodulatedeffectively and had wild type pod fertility. Ineffective nodulationand poor pod fertility traits co-segregated in the F₂generationin which plants with the mutant nodulation and pod fertilityphenotypes represented 12.5% of the total population. Analysisin F₃confirmed that these plants were homozygous for both mutatedcharacters. The results indicated both mutant traits studiedare determined by a single recessive allele, named sym-2, whoseinheritance is negatively affected by its pleiotropic effecton pod fertility determining a deficit of ineffectively nodulatingcombinations. In an allelism test with the non-nodulating mutantof common bean NOD125 it was found that ineffective nodulationis controlled at a different locus and that the two loci arenot linked. Key words: Phaseolus valgaris, nitrogen fixation, nodulation mutants, genetics