HETEROGENEOUS INBRED POPULATIONS ARE USEFUL AS SOURCES OF NEAR-ISOGENIC LINES FOR RAPD MARKER LOCALIZATION

The development and use of RAPD markers for applications in crop improvement has recently generated considerable interest within the plant breeding community. One potential application of RAPDs is their use for ''tagging'' simply-inherited (monogenic) pest-resistance genes and enabling more efficient identification and selection of genotypes carrying specific combinations of resistance genes. In this report, we propose and describe the use of heterogeneous inbred populations as sources of near-isogenic lines (NILs) for targeting RAPD markers linked to major pest resistance genes. The development of these NILs for RAPD marker analyses involved a sequence of line and mass selection during successive generations of inbreeding. DNA bulks derived from the NILs were used to identify a RAPD marker (designated OK14(620), generated by 5'-CCCGCTACAC-3' decamer) that was tightly linked (2.23 +/- 1.33 centiMorgans) to an important rust [Uromyces appendiculatus (Pers.) Unger var. appendiculatus] resistance gene (Ur-3) in common bean (Phaseolus vulgaris L.). The efficiency of this approach was demonstrated by a low rate of false-positives identified, the tightness of the linkage identified, and the ability to detect polymorphism between genomic regions that are representative of the same gene pool of common bean. This method of deriving NILs should find application by researchers interested in utilizing marker-assisted selection for one or more major pest resistance genes. The identification of OK14(620) should help to facilitate continued use of the Ur-3 resistance source and will now enable marker-assisted pyramiding of three different bean rust resistance sources (two previously tagged) to provide effective and stable resistance to this important pathogen.