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 impro vement has recently generated considerable interest within the plant b reeding community. One potential application of RAPDs is their use for ''tagging'' simply-inherited (monogenic) pest-resistance genes and en abling more efficient identification and selection of genotypes carryi ng specific combinations of resistance genes. In this report, we propo se and describe the use of heterogeneous inbred populations as sources of near-isogenic lines (NILs) for targeting RAPD markers linked to ma jor pest resistance genes. The development of these NILs for RAPD mark er analyses involved a sequence of line and mass selection during succ essive 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 centiMo rgans) to an important rust [Uromyces appendiculatus (Pers.) Unger var . appendiculatus] resistance gene (Ur-3) in common bean (Phaseolus vul garis L.). The efficiency of this approach was demonstrated by a low r ate of false-positives identified, the tightness of the linkage identi fied, and the ability to detect polymorphism between genomic regions t hat are representative of the same gene pool of common bean. This meth od of deriving NILs should find application by researchers interested in utilizing marker-assisted selection for one or more major pest resi stance genes. The identification of OK14(620) should help to facilitat e continued use of the Ur-3 resistance source and will now enable mark er-assisted pyramiding of three different bean rust resistance sources (two previously tagged) to provide effective and stable resistance to this important pathogen.