The presence of semi-sterile panicles in Indica-Japonica crosses of rice is due to an allelic interaction at a gamete abortion locus, S-5 on Chromosome 6. The S-5n allele is referred to as "wide compatibility gene" (WCG), since one of the parents carrying this allele does not show hybrid sterility in the F1 llowever, Ketan Nangka, which is a donor of S-5n, showed typical hybrid sterility when it was crossed to local varieties from Tai-hu Lake, such as Fengjingdao and Feilaifeng, as well as to those from Yun-nan Province, such as Dabai, Luanjiaolong and Xiaobaigu. A new locus for the sterility in the hybrid between Ketan Nangka and the local variceties was detected near the Est-5 locus on Chromosome 1, and designated as S-16 following our recent reports on S-7, S-8, S-9 and S-15 Three alleles were detected at S-16 as follows: Ketan Nangka and the Chinese varieties carried S-16kn and S-16j, respective-ly, and the hybrid genotye of S-16kn/S-16j showed semi-sterility due to partial abortion of the female gamete car-rying S-16j. A Japonica tester variely Akihikari and an Indica tester IR 36 were found to have a neutral allele S-16n at this locus. In addition to the unique allele at S-16, the native varieties from Tai-hu and Yunnan carried S-7ni at a relatively high frequency, while among the present varieties in the continent few varieties carried the allele, as indicated by the fertility of the hybrids between them and Ketan Nangka. These unique alleles in Chinese local varieties appear to reflect their common origin and isolation from major varietal grouns.

In hybrid rice breeding programs, genetic diverity between maintainers and restore, enhances heterosis. The objective was to measure the level of diversity between the two groups of rice (Oryza sativa L.) breeding lines or cultvars which were screened as potential maintainers or restnrers by test-cross to WA (wild abrtive)-cytoplasm. Allele frequencies at 15 isozyme loci were compared between 51 potential maintainers and 56 potential restorers. Differences between the two types were found at eight isozyme Iocl on chromosomes 4, 6, 7, 8,11, and 12. The majority of the potential maintaines exhibited japonica type alleles, whereas most of the potential restorers exhibited indica type alleles. This pattern is consdered to be due to retained associations between Rf (restorer)/rf (maintainer) genes and genetic markers on chromosomes other than those carrying Rflrf genes. The diversity as well as differentiation of indica and japonica type alleles between maintainers and restorers should contribute to enhanced heterosis in hybrid rice.

In rice breeding, genes for seed dormanoy as well as the markers for each of them are required of an incorporation of an adequate seed domancy into japonica cultivrs To screen genetic Ioci for seed dormancy in rice (Orvza saliva L.), two kinds of populations were tested: (i)an Fz population of a hybrid Milyang 23 I Todorokiwase. and (fi) Fl populations from a three-way cross, in which an indica donor of strong dormancy, IR 36, was crossed with non-dormant japonica cultivars, Nekken 2 and Miyukimochi, in the form of IR 36/ Nekken 2// Miyukimochi. The seeds predated on the F2 plants from the single cross and those on FI plants from the three-way cross were tested at three times: immediately after maturation, after 60 d storage at 4C, and after breaking dormancy (7 d at 50C). Of 24 marker Ioci assayed, at least fore loci linked to seed dormancy were detected. The strong seed dormancy of Milyang 23 was linked with three isozyme genes: Pgi-I1 on chrompsome 3, Amp-32 on chromosome 6, and Est92 on chrmosome 7, while the strong seed dormancy of IR 36 was linked with C+ (apiculus color) on chromosome 6, Est-92 on Chromosome 7, and Acp-21 an chromosome 12. The high level of seed dorloci. Differential responses of inch Iocl to a dormacy-breaking treatment were detected. An allele from IR 36 at a locus on chromosome 6 seemed to be responsible for strong seed dormany after maturation and to be easliy broken during storage. This allele may be incorporated into japonica ries by means of marker C for apiculus color.

Female gamete abortion in Indica-Japonica crosses of rice was earlier identified to be due to an allelic interaction at the S-5 locus on chromosome 6. Recently. in other crosses of rice, similar afielic interactions were found at loci designated as S-7 and S-8, located on chromosomes 7 and 6 respectively. All of them are independent of each other. At the S-5 locus, Indica and Japonica rice have S-5i and S-Y alleles respectively and Javanieas, such as Ketan Nangka, have a neutral allele S-5". The S-5i/S-5j genotype is senti-sterile due to partial abortion of female gametes carrying S-U, hul both the S 5"/S-5i and S-5"/S-5J genotypes are fertile. The S-5" allele is thus a "wide-compatibility gene" (WCG), and parents homozygous for this allele are called wide-com patible varieties (WCV), Such parents when crossed with Indica or Japonica varieties do not show F1 hybrid sterility. Wide compatible parents have been used to overcome sterility barriers in crosses between Indica and Japonica rice, However, a Javanica variety, Ketan Nangka (WCV), showed typical hybrid sterility when crossed to the Indian varieties N22 and Jaya. Further. Dular+ another WCV from India, showed typical hybrid sterility when crossed to an I RRI line, IR2061-628 1-6-4-3(IR2061-628). By genetic analyses using isozyme markers, a new locus causing hybrid stetihty in crosses between Ketan Nangka and the Indicas was located near isozyme loci Est-I and Mal-I on chromosome 4, and w as designated as S-9. An other new locus for hybrid sterility in the crosses between Du]ar and the IR2061 628 was identified and was found lin ked to four isozyme loci, Sdh-l, Pox-g, Aep-1 and Acp-2. on chromosome 12 It was designated as S-15. On the basis of allelic interac tlons causing female-gamete abortion, two alleles were found at S-9, S-9kn in Ketan Nangka and S-9iin N22 and Jaya. In the heterozygote, S-9kn/S-9i, which was semisterile, female gametes carrying S-9kn" were aborted. The hybrid of Dular and IR2061-628, with a genetic constitution of S-15Du/S-I5i, was semi-sterile and the female gametes carrying S-I 5Du were aborted. A Japonica tester variety, Akihikari, and an Indica variety, IR36, were found to have neutral alleles, S-9n and S-15n, at these loci, in addition to S-7" and at S-7. The accumulation of three neutral alleles into a breeding line should help solve the hybrid sterility problem in wide crosses office.