Hybrid sterility commonly found in Indica-Japonica crosses of rice (Oryza sativa L.) has been proved to be caused by an allelic interaction at the S-5 locus on chromosome 6, where Indica and Japonica rice have S-5ⁱ and S-5^j, respectively, while some Javanica varieties, such as Ketan Nangka, possess a neutral allele S-5ⁿ. The S-5ⁱ/S-5^j genotype is semi-sterile due to partial abortion of female gametes carrying S-5^j, but both the S-5ⁿ/S-5ⁱ and S-5ⁿ/S-5^j genotypes are fertile. The S-5ⁿ allele is thus called a "wide compatibility gene (WCG)" (Ikehashi and Araki 1986), and used to overcome hybrid sterility in rice (Ikehashi and Araki 1991). However, Ketan Nangka, the donor of WCG was found to be typical hybrid sterility when crossed to a landrace, Bai Mi Fen, of Yunnan province in China. (Ikehashi and Araki 1987). In this study, we identified a novel gene responsible for F₁ hybrid sterility and mapped it on SSR linkage map.

A backcross, Ketan Nangka / Bai Mi Fen// Ketan Nangka, was made in the summer of 2000 in Nanjing. Then, eighty F₁ plants from this backcross as well as the parents and the F₁ crosses were grown in a single plant in 2001 at the experimental fields of Nanjing Agricultural

University in Jiangsu province of China. The spikelet fertility was scored as seed setting rate on the upper halves of 3 panicles for each plant. The distribution of spikelet fertility in the backcross population was clearly bimodal with an apparent valley at approximately 55% fertility (Fig. 1). The segregation of highly fertile and partly sterile individuals deviated from 1:1 ratio, with more individuals in the high-fertility group than in the low-fertility group. This deviation implied that a substantial portion of female gamete with Bai Mi Fen alleles was aborted in the F₁ hybrids.

A genome-wide analysis was performed using a total of 143 simple sequence repeat (SSR) markers and an EST marker covering the entire rice linkage map. Composite interval mapping was performed to identify QTLs by using the software package QTL CARTOGRAPHER (Basten et al. 1998). As a result, two loci were found to cause independently the hybrid sterility via female gamete abortion. Of the two loci, the one on chromosome 4 was identified to be S-9 (Wan et al. 1996) by its chromosomal location, linked with the SSR marker RM185, and the other one on chromosome 2 was different from all the previously reported hybrid sterility loci (Kubo et al. 2001; Wan et al. 1995, 1996, 1998), and tentatively designated as S-29(t) following the hybrid sterility nomenclature (Fig.2), linked with the marker RM425. The spikelet fertility in the backcross population was significantly differentiated by genotypes at RM425 and RM185, and the homozygotes exhibited significantly higher fertility than the heterozygote (Table 1).

On the basis of allelic interaction which causes female gamete abortion, two alleles were found: S-29^kn(t) in Ketan Nangka and S-29^bi(t) in Bai Mi Fen. In the heterozygote, S-29^kn(t)/ S-29^bi(t), which was semi-sterile, female gametes carrying S-29^bi(t) were aborted. Because the F₁ hybrids of Ketan Nangka/Dular and Bai Mi Fen/Dular showed normal fertility, Dular has been demonstrated to have neutral allele, S-9ⁿ, at S-9 locus (Wan et al. 1996). This result suggests that Dular possess the neutral allele, S-29ⁿ(t), at S-29(t) locus.

References

Basten, C.J., B.S. Weir and Z-B. Zeng, 1998. QTL CARTOGRAPHER: a reference manual and tutorial for QTL mapping. Department of Statistics, North Carolina State University, Raleigh, North Carolina, USA.

Ikehashi, H. and H. Araki, 1986. Genetics of F₁ sterility in remote crosses of rice. In "Rice Genetics", IRRI, P.O. Box 933, Manila, Philippines, 119-130.

Ikehashi, H. and H. Araki, 1987. Screening and genetic analysis of wide-compatibility in F₁ hybrid of distant crosses in rice (Oryza sativa L.). Ibaraki: Technical Bulletin of Tropical Agriculture Center.

Ikehashi, H., 1991. Genetics of hybrid sterility in wide hybridization in rice. In "Biotechnology in Agriculture and Forestry, Vol.14. Rice" Y.P.S. Bajajed, Springer-Verlag, Berlin Heidelberg 113-127.

Kubo, T. and A. Yoshimura, 2001. Linkage analysis of an F₁ sterility gene in Japonica/Indica cross of rice. RGN 18: 52-54.

Wan, J. and H. Ikehashi, 1995. Identification of a new locus S-16 causing hybrid sterility in native rice varieties (Oryza sativa L.) from Tai-hu Lake region and Yunnan province, China. Breeding Science 45: 161-170.

Wan, J., Y. Yamaguchi, H. Kato and H. Ikehashi, 1996. Two new loci for hybrid sterility in cultivated rice (Oryza sativa L.). Thero. Appl. Genet. 92: 183-190.

Wan, J., H. Ikehashi, M. Sakai1, H. Horisue1 and T. Imbe, 1998. Mapping of hybrid sterility gene S17 of rice (Oryza sativa L.) by isozyme and RFLP markers. RGN 15: 151-154.