Marker-assisted breeding of a photoperiod-sensitive male sterile japonica rice with high cross-compatibility with indica rice

The incomplete fertility of japonica x indica rice hybrids has inhibited breeders' access to the substantial heterotic potential of these hybrids. As hybrid sterility is caused by an allelic interaction at a small number of loci, it is possible to overcome it by simple introgression at the major sterility loci. Here we report the use of marker-assisted backcrossing to transfer into the elite japonica cv. Zhendao88 a photoperiod-sensitive male sterility gene from cv. Lunhui422S (indica) and the yellow leaf gene from line Yellow249 (indica). The microsatellite markers RM276, RM455, RM141 and RM185 were used to tag the fertility genes S5, S8, S7 and S9, respectively. Line 509S is a true-breeding photoperiod-sensitive male sterile plant, which morphologically closely resembles the japonica type. Genotypic analysis showed that the genome of line 509S comprises about 92% japonica DNA. Nevertheless, hybrids between line 509S and japonica varieties suffer from a level of hybrid sterility, although the line is highly cross-compatible with indica types, with the resulting hybrids expressing a significant degree of heterosis. Together, these results suggest that segment substitution on fertility loci based on known information and marker-assisted selection are an effective approach for utilizing the heterosis of rice inter-subspecies.