10. Brittle node, a new marker for chromosome 2

International Rice Research Institute, P.O. Box 933, Manila, Philippines

Four brittle culm rice mutants, designated as bc-1 to bc-4, have been reported. Culms, leaves and panicles of these mutants break easily. Dr. R.N. Misra of CRRI, Cuttack supplied us seeds of a novel type of brittle mutant whose tillers break at the node when gently pulled (Fig. 1). The leaf blades break from leaf sheaths at the juncture upon gentle pulling. In contrast to brittle culm mutants, other plant parts are not brittle. The F\1\ of the mutant with IR36 was

Fig. 1. Normal plant on left and brittle node (bc-5) plant on right. The tillers of the bn plants break at the nodes (break points shown by arrow) when gently pulled

normal and the F\2\ segregated in a ratio of 3 normal to one mutant thereby showing the monogenic recessive nature of the mutant. Gene symbol bc-5 is assigned to this brittle node mutant. The mutant was crossed with 10 of the 12 primary trisomics of rice and F\2\ or BC populations were examined. It showed disomic inheritance in the progenies of 9 trisomics but showed trisomic inheritance in the backcross population of triplo 2. Since the F\1\ of triplo 2 with bc-5 was highly sterile, it was backcrossed to bc-5. Of the 91 diploid plants of the backcross population, only 24 were bc-5 and none of the 20 trisomic plants was mutant. These results clearly show that bc-5 is located on chromosome 2. Since it is easily scored at seedling stage, bc-5 is a useful marker for linkage group 2.