The plants heterozygous for the putative region Of the gene for heading date on chromosome 10 (hereafter referred as EJ(t)) were selected using RFLP markers and a BC3F5 population consisting of 147 plants was used as the mapping population. The seeds of the mapping population were sown on December 11, 1997 and seedlings were transplanted on January 1, 1998 at the International Rice Research Institute, Los Bafios, Philippines.

In the mapping population, early-heading (60-67 days to heading (DTH)) plants and late heading (69-84 DTH) plants segregated in a ratio of 105 : 42, fitting the expected 3: 1 ratio (x2= 1.000, P=0.32). The dominant allele from 0. glaberrima caused early heading. RFLP analysis revealed that Ef(t) was mapped on the same position as C1369 on chromosome 10 in this population size. The two loci Ef(t) and C1369 were located between the flanking markers C234 and G37 which are linked with distance of 1.1 cM from each other (Fig. 1).

Dominant allele of Efi shortens the days to heading especially under the short-day condition (Tsai 1997). Ef(t) showed similar characteristics to Efi (unpublished data). Moreover, Efi was located on the identical region on chromosome 10 (Sato et a!. 1988, Yoshimura et a!. 1997). These results indicate that there is a possibility that Ef(t) is an allele of Efi locus.

This study was supported in part by the Program for Promotion of Basic Research Activities for Innovative Biosciences. The authors are grateful to Drs. G. S. Khush and E. A. Angeles, IRRI, Philippines, who kindly planted the mapping population.