An effective strategy for fertility improvement of indica-japonica hybrid rice by pyramiding S5-n, f5-n, and pf12-j

Though inter-subspecific hybrids from indica and japonica rice have gained great success for yield increases over the recent decade, there are severe sterility problems in such hybrids. To overcome hybrid sterility, we previously obtained the line PL-(S5-n + f5-n) in the genetic background of an elite indica restorer line 9311, via breeding strategy for construction of wide compatibility line, which was proven to be an effective strategy in improving the seed setting rate of indica-japonica hybrid rice. Here, a novel strategy for developing japonica-compatible indica lines was employed by introducing the japonica allele pf12-j into 9311. The obtained NIL-(pf12-j) could significantly improve pollen and embryo-sac fertility by 29.7% and 28.6% in indica-japonica hybrids, thus leading to 20.9% improvement of spikelet fertility. In addition, we achieved the pyramiding line PL-(S5-n + f5-n + pf12-j) by a combinational strategy using both wide compatibility line and japonica-compatible indica line. Compared to PL-(S5-n + f5-n) with 38.9% increase of spikelet fertility, the pyramiding line PL-(S5-n + f5-n + pf12-j) showed 49.7% increase of spikelet fertility, suggesting cumulative effect of wide compatibility alleles S5-n + f5-n and japonica allele pf12-j to shape normal fertility of inter-subspecies hybrid. Interestingly, these lines also showed compatibility to indica. Hence, our results demonstrate that the two strategies could be simultaneously applied for indica-japonica hybrid breeding, and S5-n + f5-n + pf12-j are the optional allelic combination for overcoming hybrid sterility. This finding will greatly enhance our understanding for breeding indica-japonica hybrid rice by molecular-assisted selection strategy.