Ludao is a type of weedy rice in lower Yangtze valley, which grown naturally in field of Lianyungang region (34°33´-34°46´N), Jiangsu Province, China (Jiang et al. 1985). When it crossed to Akihikari, a typical japonica variety, the hybrids showed partial pollen sterility, in which most of abortive pollen was spherical abortive (Fig. 1).

In this study, a genome-wide analysis was performed with a backcross population containing 215 individuals derived from Akihikari //Ludao/Akihikari using a total of 118 simple sequence repeat (SSR) markers and an expressed sequence tag (EST) marker distributed on the entire rice linkage map. As a result, two loci controlling hybrid pollen sterility were detected on chromosomes 3 and 11 respectively, and designated as qPS3 and qPS11, respectively (Table 1, Fig. 2). Between them, qPS3 was linked with the EST marker C0729 with LOD score of 52.6 and PVE (phenotypic variance explained) of 57.9%. Another locus, qPS11 with LOD score of 32.1 was linked with the SSR marker RM552, and accounted for 32.5% of the phenotypic variance of hybrid pollen sterility.

To assess possible effects of interactions between the two loci, a two-way analysis of variance was performed using the genotypes of the most closely linked markers. The results showed that the two loci did not have significant interaction effect and acted independently of each other. The two-locus combination with Akihikari alleles at both loci displayed the highest pollen fertility, and had completely normal fertility of pollens. The plants homozygous only at one of the two loci, especially the heterozygote at qPS3 showed significant reduction of pollen fertility. The lowest pollen fertility was observed in the plants heterozygous at both loci. The results indicated that the genetic effects of two loci were additive, and qPS3 conferred more effect on pollen sterility than qPS11.

So far, two F₁ pollen sterility genes, S19 and Sc (Taguchi et al. 1999, Zhuang et al. 2002) have been located on chromosome 3. By further analysis we determined that both of the known genes appeared to be different from qPS3 detected in this study according to chromosome location and genetic effects. So qPS3 would be a new locus controlling hybrid pollen sterility on chromosome 3, and was tentatively designated as S33(t). Up to now, no locus for hybrid pollen sterility was reported on chromosome 11. Therefore, the locus, qPS11, controlling pollen sterility on chromosome 11 identified in this study, could be another new one, and was tentatively designated as S34(t).

To determine the precise map location of S33(t), we selected 165 highly fertile plants with pollen fertility higher than 80.0% from the backcross population and assayed these individuals using SSR and EST markers surrounding the S33(t) locus. According to the recombination frequencies and with reference to the linkage map constructed in this study, the S33(t) locus was mapped between RM15621 and RM15627, with a map distance of 0.6 cM respectively (Fig. 2). In addition, SSR marker JW78 cosegregated with the S33(t) locus (Fig. 2). These results will be valuable for cloning this gene.

References

Jiang H., J. L. Wu and G. L. Wang, 1985. Studies on Ludao of Lianyungan. Crop Genet. Resources 2: 4-7 (in Chinese).

Taguchi K., K. Doi and A. Yoshimura, 1999. RFLP mapping of S19, a gene for F₁ pollen semi-sterility found in backcross progeny of Oryza sativa and O. glaberrisna. RGN 16: 70-71.

Zhuang C. X., Y. Fu, G. Q. Zhang, M. T. Mei and Y. G. Lu, 2002. Molecular mapping of S-c, an F₁ pollen sterility gene in cultivated rice. Euphytica 127: 133-138.