23. A new gene for F1 pollen sterility located on chromosome 12 in Japonica / Indica cross of rice

Plant Breeding Laboratory, Faculty of Agriculture, Graduate School, Kyushu University, Fukuoka, 812-8581 Japan

F1 hybrids between Japonica and Indica rice varieties show various degrees of pollen and spikelet sterility. Although a large number of studies have been made on hybrid sterility, many sterility genes have not been fully investigated. During the process of developing Indica chromosome segment substitution lines with Japonica genetic background (Kubo et al. 1999), we observed segregation of hybrid sterility. In this study, we identified a gene responsible for F1 pollen sterility and mapped it on RFLP linkage map.

We developed IR24 (Indica variety) chromosome segment substitution lines (CSSLs) with Asominori (Japonica variety) genetic background by successive backcrossing and marker-assisted selections (Kubo et al. 1999). Segregations of pollen sterility were observed in some of the BC3F2 and BC3F3 populations. To identify and map the causal gene, a segregating BC5F1 population (N=73) derived from backcrossing a substitution line of chromosome 12 with Asominori was used to evaluate pollen fertility and perform RFLP analysis. The BC5F1 population clearly segregated into two groups: pollen fertile (more than 95%) and semi-sterile (20- 65%), with a total of 41 and 32 plants, respectively (Fig. 1).

This segregation fitted to the theoretical 1:1 ratio (chi2 =1.11) based on a single nuclear gene. The gene for pollen sterility was designated as S25(t). RFLP analysis in BC5F1 population showed that the fertile and semi-sterile plants were Asominori homozygous and heterozygous alleles, respectively, at G193 on chromosome 12. Thus, S25(t) caused pollen semi-sterility in heterozygous condition and co-segregated with G193. Linkage analysis using other RFLP markers at the region showed that S25(t) was located between G24 and G189 with a map distance of 1.4cM and 5.5cM, respectively (Fig. 2).

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


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