RFLP MAPPING OF THE GENES-CONTROLLING HYBRID BREAKDOWN IN RICE (ORYZA-SATIVA L.)

Complementary recessive genes hwd1 and hwd2 controlling hybrid breakdo wn (weakness of F-2 and later generations) were mapped in rice using R FLP markers. These genes produce a plant that is shorter and has fewer tillers than normal plants when the two loci have only one or no domi nant allele at both loci. A cultivar with two dominant alleles at the hwd1 locus and a cultivar with two dominant alleles at the hwd2 locus were crossed with a double recessive tester line. Linkage analysis was carried out for each gene independently in two F2 populations derived from these crosses. hwd1 was mapped on the distal region of rice gene tic linkage map for chromosome 10, flanked by RFLP markers C701 and R2 309 at a distance of 0.9 centiMorgans (cM) and 0.6 cM, respectively. h wd2 was mapped in the central region of rice genetic linkage map for c hromosome 7, tightly linked with 4 RFLP markers without detectable rec ombination. The usefulness of RFLP mapping and map information for the genes controlling reproductive barriers are discussed in the context of breeding using diverse rice germplasm, especially gene introduction by marker-aided selection.