Heading date of rice is controlled by complex mechanisms that respond to several environmental conditions, such as daylength and temperature. Factors controlling heading date include basic vegetative growth and photoperiod sensitivity. Enhancer-Se1 (En-Se1), one of the photoperiod sensitivity genes, was discovered in a perennial wild rice O. rufipogon (W593) which dramatically delays heading in the presence of another photoperiod sensitivity gene Se1 (Sano 1992). En-Se1 was reported to be located on chromosome 6, between wx (Waxy endosperm) and Lcr (Low crossability) (Sano 1992), but it has not been mapped on any molecular linkage map. In this study, we carried out genetic mapping of En-Se1 using restriction fragment length polymorphism (RFLP) markers.

Near isogenic lines T65WxS6 (W593A) and T65wxSe1S6 (W593C) contain segments of chromosome 6 of O. rufipogon (W593) from a S6 gene (Hybrid sterility-6), to the Wx, and from the S6 to the Se1, respectively, in the genetic background of T65wx (Sano 1992). An F₂ population of 198 plants was produced by crossing W593A and W593C. These F₂ plants were grown from April to August in an experimental paddy field in Tsukuba, Japan. More than half of the F₂ plants showed very late heading. Short-day treatment was given to these plants to obtain self-pollinated seeds.

Total DNA of F₂ plants was extracted from fresh leaf tissue by the CTAB method (Murray and Thompson 1980). Southern hybridization analysis was carried out following the procedure of Yamamoto et al. (1998). DNA clones were selected from the distal side of the short arm of chromosome 6 of a high-density linkage map (Harushima et al

1998) as probes for the RFLP analysis.

A total of 12 RFLP markers showed polymorphism between W593A and W593C, and were used for linkage mapping (Fig. 1). Numbers of recombinations detected between adjacent markers are summarized in Fig. 1. Plants in which recombinations occurred in this region were used for a progeny test to determine the genotype at the En- Se1 locus. Days to heading of 50 self-pollinated progeny of each plant were measured. Genotypes of F₂ plants whose progeny headed from the middle to the end of September were determined to be W593C type (en-Se1/en-Se1). Plants whose progeny headed from the middle of September to the end of November were determined to be heterozygous (En- Se1/en-Se1). Plants whose progeny headed from the middle to the end of November were determined to be W593A type (En-Se1/En-Se1). From these results along with the results of RFLP mapping, En-Se1 was mapped between RFLP markers C764 and R845, with 3 and 7 recombinants, respectively. No recombination occurred with RFLP markers B174 and C1032 (Fig. 1).

References

Murray, M.G. and W.F. Thompson, 1980. Rapid isolation of high molecular weight plant DNA. Nucl. Acid Res. 8: 4321-4325.

Harushima, Y., M. Yano, A. Shomura, M. Sato, T. Shimano, Y. Kuboki, T. Yamamoto, S.Y. Lin, B.A. Antonio, A. Parco, H. Kajiya, N. Huang, K. Yamamoto, Y. Nagamura, N. Kurata, G.S. Khush and T. Sasaki, 1998. A high density rice genetic linkage map with 2275 markers using a single F2 population. Genetics 148: 479-494.

Sano, Y., 1992. Genetic comparisons of chromosome 6 between wild and cultivated rice. Jpn. J. Breed. 42: 561-572.

Yamamoto, T., Y. Kuboki, S.Y. Lin, T. Sasaki and M. Yano, 1998. Fine scale mapping of quantitative trait loci, Hd-1, Hd-2 and Hd-3, controlling heading date of rice, as single Mendelian factors. Theor. Appl. Genet. 97: 37-44.