MOLECULAR TAGGING OF THE BC-3 GENE FOR INTROGRESSION INTO ANDEAN COMMON BEAN

Alternative genes for resistance to Bean Common Mosaic Virus in common bean (Phaseolus vulgaris L.) are necessary as a result of the recent introduction of necrosis-inducing strains of this virus into the USA. The recessive bc-3 gene confers resistance against all known strains o f this pathogen. We describe here experiments to develop a relatively easy-to-use procedure to introgress the bc-3 gene into elite bean cult ivars. First, we employed bulked segregant analysis to identify RAPD m arkers linked to the bc-3 locus. The ROC11/350/420 marker was codomina nt with the bc-3 gene and the ROC20/460 marker was dominant and linked in trans. A survey of cultivated materials allowed us to identify the likely evolutionary origin of the bc-3 resistance allele as a member of the Mesoamerican gene pool, probably of race Mesoamerica. Polymorph ism of the RAPD markers in a Davis common bean mapping population (BAT 93 x Jalo EEP558) allowed us to map the markers and, by inference, the bc-3 gene to linkage group D6. Second, we used sequence information f rom the cloned RAPD fragments to design longer, more reliable PCR prim ers that differentiate individuals homozygous for the resistance allel e from susceptible genotypes in segregating populations of Andean orig in. Third, we developed a marker tagging system that used a simplified DNA extraction technique and a PCR-based assay to identify the genoty pe of common bean plants at the bc-3 disease resistance locus. This si mplified marker assisted selection system is expected to eliminate the need for costly quarantines and progeny tests in breeding programs fo r common bean of Andean origin.