THE SEMIDWARF GENE, SD-1, OF RICE (ORYZA-SATIVA L) .2. MOLECULAR MAPPING AND MARKER-ASSISTED SELECTION

To establish the location of the semidwarf gene, sd-1, the anthocyanin activator (A), purple node (Pn), purple auricle (Pau), and the isozym e locus, EstI-2, in relation to DNA markers on the molecular linkage m ap of rice, 20 RFLP markers, previously mapped to the central region o f chromosome 1 (McCouch et al. 1988), were mapped onto an F-2 populati on derived from the cross Taichung 65 (A, Pla, Pau)/Taichung 65 (sd-1) . sd-1 and EstI-2 were determined to be linked most tightly to RFLP ma rkers RG 109 and RG 220, which cosegregated with each other. The dista nce between these RFLP markers and sn-1 was estimated to be 0.8 cM, ba sed on an observed recombination value of 0.8%. The order of genes and markers in this region of chromosome 1 was determined to be sd-1 (Est I-2 - RG220 - RG109) - RG381 - A - Pn - Pau. To test the efficacy of s election for sd-1 based on these linked markers, 50-day-old F-2 seedli ngs derived from another cross, Milyang 23/Gihobyeo, were analyzed for marker genotype. At this age, the semidwarf character could not be cl early detected based on phenotype. In addition, plant height was norma lly distributed in this population, making it difficult to unambiguous ly identify plants carrying sd-1. Thirteen seedlings homozygous for th e sd-1-associated allele at EstI-2, RG220 and RG109, and 13 seedlings homozygous for the Sd-1-associated allele at all three marker loci wer e selected for further genetic analysis. At 20 days after heading, the culm lengths of these 26 plants were measured and the expected phenot ype was confirmed in every case. These 26 plants were then selfed for four generations and Fg lines were again evaluated to determine whethe r any recombination among the three molecular markers, or between thes e markers and the sd-1 gene, could be detected. No recombinants were i dentified, confirming the tight linkage of these loci and the usefulne ss of genotypic selection for this recessive semidwarf character prior to the time when it can be evaluated based on phenotype.