IDENTIFICATION AND POTENTIAL USE OF A MOLECULAR MARKER FOR RUST RESISTANCE IN COMMON BEAN

The Up2 gene of common bean (Phaseolus vulgaris L.) is an important source of dominant genetic resistance to the bean rust pathogen [Uromyces appendiculatus (Pers. ex Pers.) Unger var 'appendiculatus' [syn U. phaseoli (Reben) Wint.]. Up2 in combination with other rust resistance genes may be used to obtain potentially stable genetic resistance. It is difficult, however, to combine rust resistance genes effective against a single race due to epistatic interactions that frequently occur between them. A strategy that employed bulked DNA samples formed separately from the DNA of three BC6F2 individuals with Up2 and three without Up2 as contrasting near-isogenic lines (NILs) was used to identify random amplified polymorphic DNA fragments (RAPDs) tightly linked to the Up2 locus. Only 1 of 931 fragments amplified by 167 10-mer primers of arbitrary sequence in the polymerase chain reaction (PCR) was polymorphic. The RAPD marker (OA14(1100)) amplified by the 5'-TCTGTGCTGG-3' primer was repeatable and its presence and absence easy to score. No recombination was observed between OA14(1100) and the dominant Up2 allele within a segregating BC6F2 population of 84 individuals. This result suggests that OA14(1100) and Up2 are tightly linked. Andean and Mesoamerican bean germ plasm, with and without the Up2 allele, were assayed for the presence of OA14(1100). Apparently, the marker is of Andean origin because all Andean lines, with or without the Up2 allele, contained the marker, and the marker was absent in all Mesoamerican germ plasm except the lines to which Up-2 had been purposely transferred. These results suggest that OA14(1100) will be most useful for pyramiding Up2 with other rust resistance genes into germ plasm of Mesoamerican origin where the marker does not traditionally exist. The use of bulked DNA samples may have concentrated resources toward the identification of RAPDs that were tightly linked to the target locus. Marker-based selection may provide an alternative to the time-consuming testcrosses required to pyramid bean rust resistance genes that exhibit epistasis.