F₁ hybrids between the Japonica and the Indica rice varieties show various degrees of pollen and spikelet sterility. Since the mode of inheritance of hybrid sterility is complex, many sterility genes have not been investigated as yet. Our objectives are to precisely localize the sterility genes on molecular linkage map and elucidate the genetic basis of hybrid sterility. In this study, we identified a new gene for F₁ pollen sterility in Japonica-Indica hybrid.

In our previous study (Kubo et al. 1999), we developed IR24 (Indica var.) chromosome segment substitution lines (CSSLs) in Asominori (Japonica var.) genetic background through successive backcrosses and marker-assisted selections. In the BC₃F₂ and BC₃F₃ populations, segregation of pollen sterility was observed. The sterile segregants showed less than 50% pollen fertility. The genotype of BC₃F₂ line was almost identical to Asominori genome, excluding a part of chromosomes 1, 5, 6 and 10.

A BC₄F₁ population obtained from backcrossing a pollen sterile segregant with Asominori was evaluated for pollen fertility and used for RFLP analysis. The BC₄F₁ population consisting of 71 individuals was classified into two groups, normal and pollen sterile with a total of 34 and 37 plants, respectively. The segregation agreed with the theoretical ratio of 1:1, suggesting that the pollen sterility was controlled by a single nuclear gene. To determine the chromosomal position of the causal gene, we examined the RFLP genotypes of the individuals. At RFLP marker R830 on chromosome 5, the pollen sterile

plants were heterozygotes, and normal plants were homozygotes for Asominori alleles. This result indicated that the hybrid sterility gene was closely linked to R830 on chromosome 5. Since no gene for hybrid sterility has been identified on chromosome 5, we have tentatively designated this gene as S24(t). S24(t) is located between R830 and R3166 with a map distance of 1.4 and 2.9 cM, respectively (Fig. 1).

This study was supported in part by Bio-oriented Technology Research Advancement Institution (BRAIN), Japan.

Reference

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.

Kubo, T., K. Nakamura and A. Yoshimura, 1999. Development of a series of Indica chromosome segment substitution lines in Japonica background of rice. RGN 16: 104-106.