Identification of RAPD markers linked to a major rust resistance gene block in common bean.

Rust in bean (Phaseolus vulgaris L.), caused byUromyces appendiculatus (Pers.) Unger var.appendiculatus [ =U. phaseoli (Reben) Wint.], is a major disease problem and production constraint in many parts of the world. The predominant form of genetic control of the pathogen is a series of major genes which necessitate the development of efficient selection strategies. Our objective was focused on the identification of RAPD (random amplified polymorphic DNA) markers linked to a major bean rust resistance gene block enabling marker-based selection and facilitating resistance gene pyramiding into susceptible bean germplasm. Using pooled DNA samples of genotyped individuals from two segregating populations, we identified two RAPD markers linked to the gene block of interest. One such RAPD, OF10970 (generated by a 5'-GGAAGCTTGG-3' decamer), was found to be closely linked (2.15±1.50 centi Morgans) in coupling with the resistance gene block. The other identified RAPD, OI19460 (generated by a 5'-AATGCGGGAG-3' decamer), was shown to be more tightly linked (also in coupling) than OF10970 as no recombinants were detected among 97 BC6F2 segregating individuals in the mapping population. Analysis of a collection of resistant and susceptible cultivars and experimental lines, of both Mesoamerican and Andean origin, revealed that: (1) recombination between OF10970 and the gene block has occurred as evidenced by the presence of the DNA fragment in several susceptible genotypes, (2) recombination between OI19460 and the gene block has also occurred indicating that the marker is not located within the gene block itself, and (3) marker-facilitated selection using these RAPD markers, and another previously identified, will enable gene pyramiding in Andean germplasm and certain Mesoamerican bean races in which the resistance gene block does not traditionally exist. Observations of variable recombination among Mesoamerican bean races suggested suppression of recombination between introgressed segments and divergent recurrent backgrounds.