In crosses between distantly related species, reproductive barriers are frequently observed. Quantitative Trait Loci (QTL) analysis was conducted to comprehensively understand the F₁ pollen sterility found in hybrids between cultivated rice (Oryza sativa L.) and its wild progenitor, O. rufipogon Griff. A Japonica variety, Taichung 65 (T65) was crossed with two O. rufipogon accessions (W1962 originated from China and IRGC105715 originated from Cambodia), and BC₁F₁ mapping populations were obtained by backcrossing with T65. The mapping populations consisted of 68 (W1962 population) and 62 (IRGC105715 population) plants. The pollen fertility was examined by I₂-KI staining. Pollen of the F₁ plants and the BC₁F₁ populations was observed by a microscope (more than 200 pollen grains per plant). Genotyping of the BC₁F₁ populations derived from W1962 and IRGC105715 was done using 98 and 102 simple sequence repeat (SSR) markers, respectively. The linkage maps were constructed using the software MAPMAKER/EXP3.0. QTL analysis was conducted by Windows QTL Cartographer V2.0 using both simple interval mapping (SIM) and composite interval mapping (CIM). Critical threshold values of LOD scores for QTL detection were calculated using QTL Cartographer V2.0 at experiment-wise significance level of 0.05 by conducting 1,000 permutation tests.

Pollen fertility of F₁ plants was 46.2% in the T65/W1962 cross and 48.2% in the T65/IRGC105715 cross. Frequency distribution of pollen fertility was continuous in both BC₁F₁ populations (Fig. 1). The linkage maps based on BC₁F₁ populations were constructed. Critical threshold values of LOD scores were equivalent to LOD=2.8 in both populations. In W1962 population, QTLs were detected on chromosomes 4 and 10, and explained 54.0% of the total phenotypic variance (Fig. 2A, Table 1). In IRGC105715 population, QTLs were detected on chromosomes 4 and 7, and explained 58.8% of the total phenotypic variance (Fig. 2B, Table 1). All the four QTLs decreased pollen fertility at heterozygous condition.

The QTL on chromosome 10 in W1962 population was detected near S18 (Doi et al. 1998). S21 and S23 (Doi et al. 1999, Sobrizal et al. 2000) were located near the QTL on chromosome 7 in IRGC105715 population. The two QTLs detected on chromosome 4 in both populations were near S28 (Sobrizal et al. 2002). There is a possibility that the QTLs detected in the present study might be alleles of these known loci.

This work was supported in part by a grant from the Ministry of Agriculture, Forestry and Fisheries of Japan (Green Technology Project GD-2003). The wild accession, W1962, used in this study was kindly provided from National Institute of Genetics, Mishima, Japan, supported by the National BioResource Project, MEXT, Japan. The authors are grateful to Genetic Resources Center, IRRI, Philippines, for providing IRGC105715.

References

Doi K., K. Taguchi and A. Yoshimura, 1998. A new locus affecting high F₁ pollen sterility found in backcross progenies of Japonica rice and African rice. RGN 15: 146-148.

Doi K., K. Taguchi and A. Yoshimura, 1999. RFLP mapping of S20 and S21 for F₁ pollen semi-sterility found in backcross progeny of Oryza sativa and O. glaberrima RGN 16: 65-68.

Sobrizal, Y. Matsuzaki and A. Yoshimura, 2002. Mapping of pollen semi-sterility gene, S28(t), on rice chromosome 4. RGN 19: 80-82.