IDENTIFICATION OF RAPD MARKERS LINKED TO A MAJOR RUST RESISTANCE GENEBLOCK IN COMMON BEAN

Rust in bean (Phaseolus vulgaris L.), caused by Uromyces appendiculatu s (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 s eries of major genes which necessitate the development of efficient se lection strategies. Our objective was focused on the identification of RAPD (random amplified polymorphic DNA) markers linked to a major bea n rust resistance gene block enabling marker-based selection and facil itating resistance gene pyramiding into susceptible bean germplasm. Us ing pooled DNA samples of genotyped individuals from two segregating p opulations, we identified two RAPD markers linked to the gene block of interest. One such RAPD, OF10(970) (generated by a 5'-GGAAGCTTGG-3' d ecamer), was found to be closely linked (2.15 +/- 1.50 centi Morgans) in coupling with the resistance gene block. The other identified RAPD, OI19(460) (generated by a 5'-AATGCGGGAG-3' decamer), was shown to be more tightly linked (also in coupling) than OF10(970) as no recombinan ts were detected among 97 BC6F2 segregating individuals in the mapping population. Analysis of a collection of resistant and susceptible cul tivars and experimental lines, of both Mesoamerican and Andean origin, revealed that: (1) recombination between OF10(970) and the gene block has occurred as evidenced by the presence of the DNA fragment in seve ral susceptible genotypes, (2) recombination between OI19(460) and the gene block has also occurred indicating that the marker is not locate d within the gene block itself, and (3) marker-facilitated selection u sing these RAPD markers, and another previously identified, will enabl e gene pyramiding in Andean germplasm and certain Mesoamerican bean ra ces in which the resistance gene block does not traditionally exist. O bservations of variable recombination among Mesoamerican bean races su ggested suppression of recombination between introgressed segments and divergent recurrent back grounds.