High Resolution Mapping of the/ndica-Derived Rice Blast Resistance Genes. I.Pi-b

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.