Fine mapping of quantitative trait loci Hd-1, Hd-2 and Hd-3, controlling heading date of rice, as single Mendelian factors

Fine mapping was carried out on three putative QTLs (tentatively designated as Hd-l to Hd-3) of five such QTLs controlling heading date in rice that had been earlier identified using an F-2 population derived from a cross between a japonica variety, 'Nipponbare', and an indica variety,'Kasalath', using progeny backcrossed with 'Nipponbare' as the recurrent parent. One BC3F2 and two BC3F1 plants, in which the target QTL regions were heterozygous and most other chromosomal regions were homozygous for the 'Nipponbare' allele, were selected as the experimental material. Self-pollinated progeny (BC3F2 and BC3F3) Of the BC3F1 or BC3F2 showed continuous variation in days to heading. By means of progeny testing based on BC3F3 or BC3F4 lines, we determined the genotypes of each BC3F2 or BC3F3 individual at target QTLs. Their segregation patterns fitted Mendelian inheritance ratios. When the results obtained by RFLP analysis and progeny tests were combined, Hd-1, Hd-2 and Hd-3 were mapped precisely on chromosomes 6, 7 and 6, respectively, of a rice RFLP linkage map. The results demonstrated that QTLs can be treated as Mendelian factors. Moreover, these precise locations were in good agreement with the regions estimated by QTL analysis of the initial Fz population, demonstrating the high reliability of QTL mapping using a high-density linkage map.