Reproductive isolation is one of the main obstacles to the practical use of super-heterosis between indica and japonica hybrid rice. Discovery of wide-compatibility genes(WCG) shed a light on intersubspecific hybrid rice (Ikehashi et al. 1987; Wan et al. 1996). However, those intersubspecific hybrids, completely fertile under normal condition with the aid of WCGs, are still sensitive to relatively low temperature. This kind of sterility , named low temperaturesensitive sterility (LTSS), was found mainly due to the low-temperature-sensitive pollen sterility (LTSPS, Li et al.1996, 1997), and remained one of the major problems in intersubspecific hybrid rice breeding (Ikeda 1994, Li et al. 1996). In this study, we tried to illuminate the

genetic basis underlying LTSPS by quantitative trait loci (QTLs) analysis.

157 F₂ plants derived from 3037 (indica) and 02428 (japonica, a wide-compatibility varietiy with S₅ⁿ) (Zou et al. 1989) were planted together with the F₁ hybrid and the parents in the experimental farm of the Agricultural Academy Jiangsu Province, Nanjing, China, 2002. Field management was basically the same as usual except the delay of seeding time. The June 20 planting provided a relatively low temperature stress (21~23C) for spikelet development dur-

ing booting stage, especially for pollen development. Anthers were collected from spikelets just before flowering, and pollen grains from crushed anthers were suspended in potassium iodide solution (KI-I₂). All round and strongly stained pollen were scored as normal fertile and irregular-shaped yellowish or unstained pollen grains were scored as sterile (Chaudary et al. 1981). Frequency distribution of pollen fertility for F₂ population, along with F₁ and parents are shown in Fig. 1.

The linkage map was constructed with 108 SSR markers (developed by McCouch et al. 2002) spanning 1857.8 cM along the rice genome with average interval of 17.20 cM and used for QTL mapping with MapMaker/QTL 1.1b(Lincoln et al. 1993). LOD score of 2.0 was used as criteria to indicate the putative QTLs.

Two putative loci, namely qLTSPS2 and qLTSPS5, were detected on chromosomes 2 and 5 by interval mapping, which explained 15.6% and 11.9% of phenotypic variation respectively, with additive effects of 0.021 and 0.045, dominant effects of -0.246 and -0.215, and degrees of dominance of 11.7 and 4.8, respectively (Table 1, Fig. 2). The additive effects of the two QTLs have been contributed by the japonica cv 02428. The gene action is overdominance, which coincided with the phenotypic appearance shown in Table 2, where the average pollen fertility of heterozygotes from F₂ population on either locus was lower than that of homozygotes. The entire genome was searched at a 0.001 probability level for digenic interaction for the trait with two-way ANOVA using all possible two-locus combination of marker genotypes. No epistatic loci were detected, which is similar to the low temperature-sensitive sterility (Li et al. 1997)

The two QTLs for LTSPS described here were detected with relative low LOD scores as threshold level and only account for a total of 27.5% of the phenotypic variation, suggesting that there should be many other minor QTLs for LTSPS, some of which would be detected by a LOD score lower than the threshold level used in this study. Further analysis of the data present here is now underway.

References

Chaudary, R.C., S.S. Virmani and G.W. Khush, 1981. Patterns of pollen abortion in some cytoplasmic-genetic male sterile lines of rice, Oryza, 18: 140-142.

Li, H.B., J. Wang, A.M. Liu, K.D. Liu, Q.F. Zhang and J.S. Zou, 1997. Genetics of low-temperature-sensitive sterility in indica-japonica hybrids of rice as determined by RFLP analysis, Theor. Appl. Genet., 95: 1092-1097.

Li, H.B., Q. Zhang, A.M. Liu, J.S. Zou and Z.M. Chen, 1996. A genetic analysis of low-temperature-sensitive sterility in indica-japolica rice hybrids, Plant Breeding, 115: 305-309.

Ikehashi, H. and H. Araki, 1987. Screening and genetic analysis of wide-com-patibility in F1 hybrid of distant crosses in rice (Oryza sativa L.). In: Technical Bulletin of Tropical Agriculture Research Center, No. 23. Tsukuba, Ibaraki, Japan. P: 1-79.

Ikeda, R., 1994. Research on hybrid rice in Japan-progress and future direction (in Japanese). J. Agric. Sci., 49: 478-492.

Lincoln, S.E., M.J. Daly and E.S. Lander, 1993. Mapping genes controlling quantitative traits ueing MAPMAKER/QTL version 1.1. Whitehead Institute technical Reports.

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

McCouch, S.R., L. Teytelman, Y. Xu, K.B. Lobos, K. Clare, M. Walton, B. Fu, R. Maghirang, Z. Li, Y. Xing, Q. Zhang, I. Kono, M. Yano, R. Fjellstrom, G. DeClerck, D. Schneider, S. Cartinhour, D. Ware and L. Stein, 2002.
Development and mapping of 2,240 new SSR markers for rice (Oryza sativa L.). DNA Res., 9: 199-207.

Zou, J.S., Y.Q. Nie and Q.M. Pan, 1989. The utilization of wide-compatibility line 02428 in the indica-japonica hybrids, Chinese J. Agri. Sci., 1: 6-14.