3. Mapping for a new locus causing hybrid sterility in a China landrace (Oryza sativa L.)

D. T. LI¹, S. S. ZHU¹, L. JIANG¹ and J. M. WAN^1,2,3

1) State Key Laboratory of Crop Genetics and Germplasm Enhancement, Jiangsu Provincial Center of Plant Gene Engineering, Nanjing Agricultural University, Nanjing, 210095 China

2) Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing, 100081 China

3) Corresponding Author, E-mail: wanjm@njau.edu.cn, Tel & Fax: +86-25-84396516

Semi-sterile panicles of hybrid between indica and japonica in rice was earlier identified to be due to an allelic interaction at the S5 locus on chromosome 6,where S5ⁱ and S5^j were carried by indica and japonica, respectively. S5ⁿ, referred to as wide compatibility gene, existed in some Aus varieties, such as Dular (Ikehashi and Araki 1986). One of parent carrying neutral allele S5ⁿ does not show hybrid sterility when crossed to both indica and japonica. However, Ketan Nangka, a javanica variety carrying neutral alleles at two loci S5 and S15 (Wan et al. 1996), showed typical sterility when crossed to Tuanguzao, a landrace of Yunnan province in China.

A mapping population of Tuanguzao/Ketan Nangka//Ketan Nangka was used for detecting the gene(s) controlling hybrid sterility in the hybrid between Tuanguzao and Ketan Nangka. Based on 238 F₁ plants, a linkage map consisting of 123 simple sequence repeat (SSR) markers and an expressed sequence tagged (EST) marker was constructed using MAPMAKER/EXP. A genome-wide analysis was carried out to search for genes conferring significant effects on hybrid sterility using QTL Cartographer soft (version 1.15), at a LOD score of 2.89 (the experiment-wise LOD (log of the odds ratio) threshold significance level was determined by computing 1,000 permutations). Two loci were found to cause the hybrid spikelet sterility via female gamete abortion, designated as qSS2 and qSS5 (Table 1). qSS2 located on chromosome 2 linked closely to RM236 and can explain 18.6% of phenotypic variance. qSS5 located on chromosome 5 linked tightly to RM413, and can explain 20.1% of phenotypic variance. By further analysis we found that heterozygote genotype of Tuanguzao/Ketan Nangka was the cause of sterility in backcross population and genetic effects of two loci, qSS2 and qSS5, were additive.

Up to date, two sterility loci, S29(t) and S22(t), have been reported on chromosome 2. S29 (t) was detected to confer hybrid sterility between Bai Mi Fen and Ketan Nangka and located on the long arm of chromosome 2 (Zhu et al. 2005). It appeared to be different from qSS2 detected in this study according to its chromosome location. S22(t) was a pollen sterility locus that produce 50% pollen fertility and normal spikelet fertility in the heterozygous condition (Sobrizal et al. 2000), but qSS2 was a hybrid spikelet semi-sterility locus that caused the hybrid spikelet sterility via female gamete abortion. So qSS2 would be a new locus controlling hybrid sterility on chromosome 2, designated as S32(t) following the hybrid sterility nomenclature. qSS5 was coincided with S31(t) according to the location on chromosome (Zhao et al. 2005).

Three alleles were detected at S32(t) as follows: Ketan Nangka and Tuanguzao carried S32^kn(t) and  S32^ti(t) respectively, an Aus variety, Dular, had a neutral allele S32ⁿ(t). The hybrid genotype of S32^kn(t)/S32^ti(t) showed sterility due to allelic interaction, such an allelic interaction did not occur in S32ⁿ(t)/S32^ti(t) and S32ⁿ(t)/S32^kn(t) genotypes. The tightest linkage markers detected in this paper will be useful for marker-aided transfer of wide compatibility gene (WCG) in hybrid rice breeding.

Table 1. QTLs controlling hybrid sterility in the Tuanguzao/Ketan Nangka//Ketan Nangka population detected at LOD 2.89, with composite interval mapping, using QTL Cartographer, version 1.15

¹ Nearest marker locus of putative QTLs.

References

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, pp. 119-130.

Sobrizal, Y. Matsuzaki, P. L. Sanchez, K. Ikeda and A. Yoshimura, 2000. Identification of a gene for male gamete abortion in backcross progeny of Oryza sativa and Oryza glumaepatula. RGN 17: 59-61.

Zhu, S. S., C. M. Wang, T. Q. Zheng, Z. G. Zhao, H. Ikehashi and J. M. Wan, 2005. A new gene causing hybrid sterility located on chromosome 2 in a remote cross of rice. Plant Breed 124: 1-6.

Wan, J. and H. Ikehashi, 1996. List of hybrid sterility gene loci (HSGLi) in cultivated. rice (Oryza sativa L.). RGN 13: 110-114.

Zhao, Z. G., C. M. Wang, L. Jiang, S. S. Zhu, H. Ikehashi and J. M. Wan, 2005. Identification of a new hybrid sterility gene in rice (Oryza sativa L.). Euphytica (in press).