25. RFLP mapping of S20 and S21 for F1 pollen semi-sterility found in backcross
  the backcross populations between Oryza sativa and 0. glaberrima. They were first detected as QTLs, using the BC2F1 mapping population obtained from the cross between Japonica rice (Oryza sativa L. cv. Taichung 65) as a recurrent parent and African rice (0. glaberrima Steud., Acc. IRGC 104038) as a donor parent (Doi et al. 1998a).

     Backcrossing with Taichung 65 was continued after the QTL analysis. For the mapping of the QTL detected near the RFLP markers C1057 and R2401 on the short arm of chromosome 7, a BC4FI plant heterozygous for the RFLP markers was selected and back- crossed with Taichung 65 to produce the BCSFI mapping population. The plant was homozygous for Taichung 65 allele in regions around the other QTLs and Si. Similarly, the mapping population for QTLs found in the long arm of chromosome 7 was constructed (Table 1). Pollen fertility of the plants in the mapping population was observed as described by Taguchi et a!. (1999).

     In the two mapping populations (A and B), semi-sterile plants and normal plants segregated and they were clearly distinguished (Table 1). The semi-sterile plants showed pollen fertility of approximately 50% (Fig. 1) and their spikelet fertility was almost normal. Segregation ratio of the semi-sterile and normal plants fitted well with the expected monogenic 1: 1 segregation in both populations (Table 1). RFLP mapping using the population A revealed that the semi-sterility locus was located in the region expected by the QTL analysis. Since no sterility locus was reported in this region, this locus on the short arm of chromosome 7 was designated as S20 (Fig. 2A). In the same manner, the other locus was located on the long arm of chromosome 7 using the population B and it was designated as S21 (Fig. 2B).

Table 1. Mapping populations (BC5 F1) used in the present sutdy