Wild abortive cytoplasmic male sterility (CMS-WA) has been utilized in the commercial hybrid rice production for more than two decades. Two fertility restoring genes were located on chromosomes 7 and 10 based on primary trisomic analysis (Bharaj et al. 1995), and the later one was confirmed by DNA marker based mapping (Yao et al. 1997; Tan et al. 1998). Another major gene was mapped on chromosome 1 (Zhang et al. 1997; Yao et al. 1997). In this study, an F₆ population from the cross Zhenshan 97B/Milyang 46 was obtained by single seed descent method. A single plant of each F₆ line was crossed to the CMS-WA line Zhenshan 97A. In 1999, 12 plants of each of the 227 test crosses were grown in the paddy field in CNRRI. Spikelet fertility (SF) was scored, which showed a bimodal distribution (Fig. 1).

A linkage map consisting of 115 RFLP markers was constructed and used for QTL mapping with MAPMAKER/QTL 1.1b (Lincoln et al. 1992). Three QTL were detected (Table 1, Fig. 2), of which qRf-10 was shown to have major effect for spikelet fertility

restoration, while qRf-1 and qRf-7 displayed minor effects. Another QTL, qRf-11, was detected when the data set was restricted to 93 crosses having maternal homozygotes (11 type ) at RZ811. These 4 QTL jointly accounted for 57.5% of the total phenotypic variation.

When the data set was restricted to 104 crosses which had heterozygotes (12 type) at RZ811 and the spikelet fertility was higher than 0.30, two other QTL (qSF-1 and qSF-7) for SF were detected. The two QTL showed effects for increasing SF when qRf-10 was present, but they did not have such effects when qRf-10 was absent (Table 2). This indicated that the effects of qSF-1 and qSF-7 for increasing the spikelet fertility depended on the presence of qRf-10.

Genetic effects were also estimated using markers closest to the Rf locus (Table 3). The genetic effect estimated when heterozyogotes were observed at all four marker loci was almost exactly equal to the sum of the value when heterozygote was observed at only one locus. However, it was found that the effects of minor QTL varied greatly depending on whether qRf-10 was present. When qRf-10 was absent, the effects of the three

minor QTL were enhanced by each other. When qRf-10 was present, the three minor QTL acted in a way similar to classical duplicate genes.

References

Bharaj T.S., S.S. Virmani and G.S. Khush, 1995. Chromosomal location of fertility restoring genes for 'wild abortive' cytoplasmic male sterility using primary trisomics in rice. Euphytica 83: 169-173.

Lincoln S., M. Daley and E. Lander, 1992. Mapping genes controlling quantitative traits with MAPMAKER/ QTL1.1.Whitehead Institute Technical Report, 3rd edition, Whitehead Institute, Cambrige, Mass.

Tan X.L., A. Vanavichit, S. Amornsilpa and S. Trangoonrung, 1998. Genetic analysis of rice CMS-WA fertility restoration based on QTL mapping. Theor. Appl. Genet. 96: 994-999.

Yao F.Y., C.G. Xu, S.B. Yu, J.X. Li, Y.J. Gao, X.H. Li and Q.F. Zhang, 1997. Mapping and genetic analysis of two fertility restorer loci in the wild-abortive cytoplasmic male sterility system of rice (Oryza sativa L.). Euphytica 98: 183-187.

Zhang G., T.S. Bharaj, Y. Lu, S.S. Virmani and N. Nuang, 1997. Mapping of the Rf-3 nuclear fertility restoring gene for WA cytoplasmic male sterility in rice using RAPD and RFLP markers. Theor. Appl. Genet. 94: 27-33.