Since mid 1980s, the sterility in indica-japonica crosses has been explained by allelic interaction at locus S-5. Indica and Japonica varieties carry S-5i and S-5j, alleles, respectively. Some varieties have a neutral allele, S-5n. The genotype of S-5i/S-5j produces semi-sterile panicles due to partial abortion of female gametes carrying the allele S-5j. Such abortion does not occur in S-5n/S-5iand S-5n/S-5j genotypes. The

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donor of S-5n is referred to as wide compatible variety (WCV) (Ikehashi and Araki 1986). This allele has been incorporated into indica or japonica varieties to overcome hybrid sterility in hybrid rice breeding (Ikehashi 1991; Yuan 1992; Zou et al. 1992). So far, the S-5n has been effective in a large number of indica-japonica crosses. Out of more than one thousand varieties from China a few varieties showed hybrid sterility in their crosses to WCVs as shown in Table 1. Recently, an RFLP marker RG213 on Chromosome 6 has been found to be closely linked to the S-5 allele and is being used to select wide compatible lines (Zheng et al. 1992; Yanagihara et al. 1995). However, in wider ranges of varietal testing, some WCVs exhibited hybrid sterility when crossed to some varieties from the Indian subcontinent or to some native rices in China. Further genetic analyses of the hybrid sterility gene loci (HSGLi) were carried out as summarized below.

Table 1. Applicability of WCG (S-5n) to Chinese varieties

fer.: panicles of a hybrid gave a fertility heigher than 70%.

ster.: panicles of a hybrid gave a fertility lower than 70%.

1 ) Methods for analyzing hybrid sterility gene loci (HSGLi)

A large number of three-way crosses (A/B//C) were made after confirming that a hybrid A/C produced semi-sterile panicles and another hybrid B/C was fertile. The progeny of A/B//C segregated semi-sterile plants expected from A/C and fertile ones expected from B/C in a ratio of 1:1. When a backcross A/C//C was made, the progeny resulted in semi-sterile plants expected from A/C and fertile ones from C/C in a certain ratio. In the backcrosses, F1 plants were used as a female parent to find distortion of marker genotypes which was caused by abortion of female gamete carrying one of alleles. Such genetic markers as cosegregating with the semi-sterility were surveyed to

112 Rice Genetics Newsletter Vol. 13

identify a locus for the semi-sterility.

Allelic differences at each new locus were estimated following the model of allelic interaction at S-5. For three given varieties, A, B and N, if a hybrid A/B shows gamete abortion at a HSGL S-X, but N/A and N/B do not show any distorted segregation of markers for S-X, the variety N is determined to possess a neutral allele S-Xn at the new locus. 2) Identified HSGLi and markers

A number of new HSGLi have been idenfifeid (Table 2). A locus S-7 was detected in hybrids between Aus varieties (summer rice in Indian Sub-continent) and some Javanica varieties (Yanagihara et al. 1992). A locus S-8 was detected in a hybrid between a Korean Indica variety and some Javanica varieties (Wan et al. 1993). A locus S-9 was detected in hybrids between Aus varieties and some Javanica varieties (Wan et al. 1996a). A locus S-15 was found in hybrids between an Aus variety Dular (WCV) and some IRRI lines (Wan et al. 1996a). A locus S-16 was identified located near Est-5 on Chromosome I in hybrids between Ketan Nangka and local varieties from the Tai-hu Lake region and Yunnan Province of China (Wan and Ikehashi 1995a). One more locus S-17(t) was identified in crosses between Penuh Baru a U and the Japonica varieties (Wan and Ikehashi 1995b). Isozyme analyses were carried out according to the methods of Ishikawa et al., (1989) and Glaszmann et al. (1988).

Table 2. Loci for hybrid sterility

From these genetic analyses a set of tester varieties have been identified and are listed in Table 3. Since their alleles at HSGLi have been analyzed, these varieties may be used to identify respective alleles in other varieties. 3) The nature of hybrid sterility genes

An irradiated mutant Miyukimochi (Toda 1982) was analyzed together with its original variety Toyonishiki. The semi-sterility in hybrids between Toyonishiki and

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Table 3. Alleles at HSGLi and markers in tester varieties

*The isozyme allele systems quoted from Morishima and Glaszmann 1991.

** Seven hybrid sterility loci are shown under the marker loci.

( ): allele at the hybrid sterility locus; (-): no data.

IR36 was caused only by an allelic interaction of S-5i/S-5j. Whereas, the semi-sterility in F1 hybrids between Miyukimochi and IR36 was attributed to allelic interactions by both S-5i/S-5j and S-7i/S-7j. Thus, the neutral allele S-7n in Toyonishiki was found to be mutated into S-7'jby irradiation with 60^Co (Wan and Ikehashi 1996b).

An experimental line, 02428 was found to possesses the S-5n allele. This line was selected from a progeny population of a cross between Pangxiegu and Jibangdao, both of which were irradiated with 60 ^Co and crossed at their M1 stage (Zou et al. 1992). Pangxiegu and Jibangdao were proved to possess S-5j. Thus, the allele S-5n identified in 02428 mutated from S-5j (Wan and Ikehashi 1996b). The fact that an allele at a hybrid sterility locus mutated into another would explain the origin of hybrid sterility among differnt groups of rice.

Three alleles were identified at S-5 locus. In the course of further analyses, however, allelic differentiation at HSGLi was shown to form a number of alleles at a

Ikehashi, H. and H. Araki, 1986. Genetics of F1 sterility in remote crosses of rice. In "Rice Genetics", IRRI,

Ishikawa, R., H. Morishima., K. Mori and T. Kinoshita, 1989. Chromosomal analysis of isozyme loci and the

Toda, M., 1982. The breeding of four new mutant varieties by gamma-rays in rice. Report of symposium

Wan, J. and H. Ikehashi, 1995b. A new locus for hybrid sterility in remote crosses of cultivated rice (Oryza

Wan. J., Y. Yamaguchi., H. Kato and H. Ikehashi, 1996a. Two new loci for hybrid sterility in rice (Oryza

Wan, J. and H. Ikehashi, 1996b. Evidence for mutational origin of hybrid sterility genes in rice (Oryza sativa

Yuan, L. P., 1992. The strategy of the development of hybrid rice breeding. In "Current Status of Two Line

Zheng, K., B. Shen, H. Qian and J. Wang, 1992. Tagging genes for wide compatibility in rice via linkage to