To produce the substitution lines, recombinant inbred lines derived from the cross between Japonica variety Asominori and Indica variety 1R24 (Tsunematsu et a!. 1996) were used for original crosses and backcrosses with Asominori. Some of the BC3F2 plants derived by selfing BC3F1 plants showed very low spikelet fertility (approximately 10% fertility). The difference between fertile and sterile plants was clearly recognized. The segregation of fertile and sterile plants agreed with a 3:1 ratio in some lines, suggesting that the sterility was governed by a recessive gene. This recessive gene is designated as s(t).

To determine the chromosomal location of s(t), introgressed chromosomal region of BC3F1 plants segregating the sterile plants in their selfed progenies was compared. Because the common region of chromosome 12 was introgressed in the BC3F1 plants, the gene controlling the spikelet sterility was inferred to be on chromosome 12. Subsequently, several RFLP markers in the putative region of chromosome 12 were used for RFLP mapping. A BC3F2 population (n=57) which showed clear 3:1 segregation was used as a mapping population. All the RFLP markers used agreed with 1:2:1 segregation ratio in this BC3F2 population. Most of the sterile plants were homozygous for 1R24 allele at the respective RFLP loci of chromosome 12 and close linkage between s(t) and the RFLP loci was detected. The RFLP marker closest to s(t) was G148 (Fig. 1).