| Summary: | Hybrid sterility is a reproductive barrier that prevents gene flow between species. In <i>Oryza</i> species, some hybrid sterility loci, which are classified as gamete eliminators, cause pollen and seed sterility and sex-independent transmission ratio distortion (<i>si</i>TRD) in hybrids. However, the molecular basis of <i>si</i>TRD has not been fully characterized because of lacking information on causative genes. Here, we analyze one of the hybrid sterility loci, <i>S</i><sub>2</sub>, which was reported more than forty years ago but has not been located on rice chromosomes. Hybrids between African rice (<i>Oryza glaberrima</i>) and a near-isogenic line that possesses introgressed chromosomal segments from Asian rice (<i>Oryza sativa</i>) showed sterility and <i>si</i>TRD, which confirms the presence of the <i>S</i><sub>2</sub> locus. Genome-wide SNP marker survey revealed that the near-isogenic line has an introgression on chromosome 4. Further substitution mapping located the <i>S</i><sub>2</sub> locus between 22.60 Mb and 23.54 Mb on this chromosome. Significant TRD in this chromosomal region was also observed in a calli population derived from cultured anther in hybrids of another cross combination of African and Asian rice species. This indicates that the pollen abortion caused by the <i>S</i><sub>2</sub> locus occurs before callus induction in anther culture. It also suggests the wide existence of the <i>S</i><sub>2</sub>-mediated <i>si</i>TRD in this interspecific cross combination. Chromosomal location of the <i>S</i><sub>2</sub> locus will be valuable for identifying causative genes and for understanding of the molecular basis of <i>si</i>TRD.
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