High Resolution Mapping of the Indica-Derived Rice Blast Resistance Genes. I. Pi-b. Masaru Miyamoto. (1)National Institute of Agrobiological Resources (NIAR), Kan'non-dai, Tsukuba, Ibaraki 305, Japan; (2)Ibaraki University, Faculty of Biological Sciences, Ami 3998, Ibaraki 300-03, Japan; (3) Ibaraki Plant Biotechnology Institute, Ago, Iwama, Ibaraki 309-02, Japan. Ikuo Ando(4), Krystyna Rybka (1,5), Osamu Kodama (2), Shinji Kawasaki (1,6); (1)National Institute of Agrobiological Resources (NIAR), Kan'non-dai, Tsukuba, Ibaraki 305, Japan; (2) Ibaraki University, Faculty of Biological Sciences, Ami 3998, Ibaraki 300-03, Japan; (4) National Agricultural Research Center (NARC), Kan'non-dai, Tsukuba, Ibaraki 305, Japan; (5) Plant Breeding and Acclimatization Institute, Radzikow, 05-870 Blonie, Poland; (6) PRESTO; Research Development Corporation of Japan, Honcho, Kawaguchi, Saitama 332, Japan. MPMI 9:06-13. Accepted 25 September 1995, Copyright 1996 The American Phytopathological Society.

Rice is a good material for studying positional cloning of disease resistance genes because its ratio of physical to genetic distance is small (100 to 300 kb/cM), and abundant genetic information is available. As a first step in cloning, the rice blast resistance gene Pi-b was finely mapped near the terminal region of chromosome 2 using restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers. Various near-isogenic lines (NILs) developed by the introgression of the indica-derived Pi-b gene into japonica recurrent parents facilitated its rapid and accurate localization and also the F2 analyses. Although Pi-b was introgressed independently from four cultivars, all the donors showed the same RFLP profile for all the nearby probes used, whereas there was some polymorphism among japonica recurrent parents, suggesting a single origin of the resistance gene. Pi-b cosegregated with RAPD (b-1) and RFLP (RZ123) markers, and it was bracketed between flanking RFLP markers at 0.5 cM on the centromeric side, and 1.9 cM on the telomeric side. The centromeric and telomeric limits of the indica regions of NILs BL-1 and BL-7 were closer to Pi-b than the flanking nucleic markers on each side, respectively, providing better (<0.5 and <1.9 cM) delineating markers. A long-range Southern blot with rare-cutter restriction enzymes revealed an 85-kb common band between the two 0-cM markers. These markers provide an excellent environment for the positional cloning of Pi-b.