Development and evaluation of improved lines based on an elite rice variety 9311 for overcoming hybrid sterility in rice

Hybrid sterility is a major obstacle to the development of superior inter-subspecific hybrids between indica and japonica subspecies of Asian-cultivated rice. To overcome hybrid sterility, we introduced four japonica alleles (S-j) at Sa, Sc, f5, and S32 loci from Nipponbare into 9311 through marker-assisted backcross breeding, and four improved lines were obtained. Three wide-compatibility lines 9311(S5-n), 9311(f5-n), and 9311(S5-n+f5-n) previously constructed were used as control and parental line of hybridization. By crossing with 9311(S5-n+f5-n), two pyramiding lines 9311(Sa+S5-n+f5-n) and 9311(S32+Sa+S5-n+f5-n) were obtained. To evaluate the gene effect of pollen fertility and spikelet fertility, the near-isogenic lines (NILs) harboring S5-n, f5-n, Sa, Sc, f5, or S32 and three pyramiding lines with S5-n+f5-n, Sa+S5-n+f5-n, or S32+Sa+S5-n+f5-n were testcrossed to six japonica varieties. The results showed that single-gene f5, f5-n, S32, and S5-n could significantly improve spikelet fertility in majority of indica-japonica hybrids. Single-gene Sa and Sc could significantly improve spikelet fertility in part of indica-japonica hybrids. Moreover, the pyramiding lines exhibited significantly higher spikelet fertility than the control in indica-japonica hybrids. A significant increase in spikelet fertility (31.4–55%) was observed in three- or four-gene pyramiding lines. Our results provide an effective approach to exploiting heterosis between indica and japonica subspecies, which had a profound implication in rice breeding.