Double-Neutral Genes, Sa-n and Sb-n, for Pollen Fertility in Rice to Overcome Indica x Japonica Hybrid Sterility

A major obstacle in the development of superior intersubspecific hybrids between subspecies (indica and japonica) of Asian cultivated rice (Oryza sativa L.) is partial sterility of the F-1 generation. Neutral genes, Sa-n, Sb-n, and Sc-n, for pollen fertility have the potential to overcome F-1 pollen sterility. The present study was planned to find double-neutral genes for pollen fertility and to evaluate their effect on pollen fertility of intersubspecific hybrids. Five genotypes were crossed with eight isogenic lines to mine neutral genes for pollen fertility. Pollen fertility of hybrids was observed in F-1 and F-2, and segregation ratios were recorded in F-2 to confirm the neutral genes. The results showed that DN18 and DN75 had double-neutral genes (Sa-n and Sb-n) whereas DN22 had three neutral genes Sa-n, Sb-n, and Sc-n at Sa, Sb, and Sc pollen sterility loci, respectively. Furthermore, we found a single neutral gene (Sc-n) for pollen fertility at the Sc locus and neutral gene for embryo sac fertility at the S5 locus in DN13. DN18 and DN75 were crossed with numerous typical japonica and indica cultivars. Results showed that double-neutral genes could overcome most of the intersubspecific hybrid pollen sterility. These lines could be used for the development of high yielding indica x japonica hybrids and neutral genes could be introgressed into elite cultivars of Asian cultivated rice for fertility restoration.