Molecular Tagging of the bc‐3 Gene for Introgression into Andean Common Bean

Abstract

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 necrasis‐inducing strains of this virus into the USA. The recessive bc‐3 gene confers resistance against all known strains of this pathogen. We describe here experiments to develop a relatively easy‐to‐use procedure to introgress the bc‐3 gene into elite bean cultivars. First, we employedb ulked segregant analysis to identify RAPD markers linked to the bc‐3 locus. The ROCII/350/420 marker was codominant 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 Mesoameriean gene pool, probably of race Mesoamerica. Polymorphism of the RAPD markers in a Davis common bean mapping population (BAT93 ✕ Jalo EEP558) allowed us to map the markers and, by inference, the bc‐3 gene to linkage group D6. Second, we used sequence information from the cloned RAPD fragments to design longer, more reliable PCR primers that differentiate individuals homozygous for the resistance allele from susceptible genotypes in segregating populations of Andean origin. Third, we developed a marker tagging system that used a simplified DNAe xtraction technique and a PCR‐based assay to identify the genotype of common bean plants at the bc‐3 disease resistance locus. This simplified marker assisted selection system is expected to eliminate the need for costly quarantines and progeny tests in breeding programs for common be an of Andean origin.