Segregation of pollen semi-sterility was observed in a BC₄F₂ population derived from backcrossing O. sativa cv. Taichung 65 / Oryza glumaepatula (IRGC105668) with Taichung 65 as a recurrent parent. A segregant showing pollen semi-sterility was self-pollinated. Then, the genetic analysis of pollen semi-sterility was performed in BC₄F₃ population. BC₄F₃ population have retained O. glumaepatula chromosomal segments in heterozygous condition on chromosomes 3 and 4, and homozygous condition on chromosome 8 (Sobrizal et al. 1999).

The BC₄F₃ population (N=94) showed a clear-cut bimodal distribution for pollen fertility when classified into normal and semi-sterile plants (Fig. 1). There were 58 normal and 36 semi-sterile segregants. Semi-sterile plants showed 50% pollen and normal spikelet fertility. Segregation analysis between pollen semi-sterility and RFLP markers on chromosomes 3 and 4 revealed that pollen semi-sterility was associated with RFLP marker XNpb237 on chromosome 4 (Table 1). At XNpb237 locus, 58 plants showing normal pollen fertility were homozygous for O. glumaepatula allele; 35 plants showing pollen semi-sterility were heterozygous and one plant was homozygous for Taichung 65 allele (Table 1). Only heterozygous plants produced semi-sterile pollens. These results indicate that the gene controlling pollen semi-sterility was located near XNpb237 and acted in heterozygous condition. One pollen semi-sterility plant

carrying the Taichung 65 homozygous allele at XNpb237 locus was considered as a recombinant between the loci of pollen semi-sterility and XNpb237. There was no homozygous plant for Taichung 65 allele at XNpb237 in BC₄F₂ population, suggesting that the pollen grains carrying Taichung 65 allele were aborted.

So far, four genes controlling pollen sterility, S12, S22(t), S23(t) and S27(t), have been reported in the hybrids of O. sativa and O. glumaepatula. The chromosomal location of S12 is unknown (Sano 1994). S22(t) and S23(t) are located on the short arm of chromosome 2 (Sobrizal et al. 2000a) and on the long arm of chromosome 7 (Sobrizal et al. 2000b), respectively. S27(t) is located on the long arm of chromosome 8 (Sobrizal et al. 2001). Since no gene for F₁ pollen sterility on chromosome 4 has been reported, the gene identified in this study was designated as S28(t). Linkage map of chromosome 4 showed that S28(t) was located near RFLP marker Xnpb237 with a map distance of 0.5 cM (Fig. 2).

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

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