Two new loci for hybrid sterility in cultivated rice (Oryza sativa L)

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 allelic 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-5(i) and S-5(j) alleles respectively and Javanicas, such as Ketan Nangka, have a neutral allele S-5(n). The S-5(i)/S-5(j) genotype is semi-sterile due to partial abortion of female gametes carrying S-5(j), but both the S-5(n)/S-5(i) and S-5(n)/S-5(j) genotypes are fertile. The S-5(n) allele is thus a ''wide-compatibility gene'' (WCG), and parents homozygous for this allele are called wide-compatible varieties (WCV). Such parents when crossed with Indica or Japonica varieties do not show F-1 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 IRRI line, IR2061-628-1-6-4-3(IR2061-628). By genetic analyses using isozyme markers, a new locus causing hybrid sterility in crosses between Ketan Nangka and the Indicas was located near isozyme loci Est-1 and Mal-1 on chromosome 4, and was designated as S-9. Another new locus for hybrid sterility in the crosses between Dular and the IR2061-628 was identified and was found linked to four isozyme loci, Sdh-1, Pox-2, Acp-1 and Acp-2, on chromosome 12. It was designated as S-15. On the basis of allelic interactions causing female-gamete abortion, two alleles were found at S-9, S-9(kn) in Ketan Nangka and S-9(i) in N22 and Jaya. In the heterozygote, S-9(kn)/S-9(i), which was semisterile, female gametes carrying S-9(kn) were aborted. The hybrid of Dular and IR2061-628, with a genetic constitution of S-15(Du)/S-15(i), was semi-sterile and the female gametes carrying S-15(Du) were aborted. A Japonica tester variety, Akihikari, and an Indica variety, IR36, were found to have neutral alleles, S-9(n) and S-15(n), at these loci, in addition to S-7(n) and at S-7. The accumulation of three neutral alleles into a breeding line should help solve the hybrid sterility problem in wide crosses of rice.