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Summary: The genetic mechanisms of reproductive isolation have been widely investigated within Asian cultivated rice (Oryza sativa); however, relevant genes between diverged species have been in sighted rather less. Herein, a gene showing selfish behavior was discovered in hybrids between the distantly related rice species Oryza longistaminata and O. sativa. The selfish allele S13l in the S13 locus impaired male fertility, discriminately eliminating pollens containing the allele S13s from O. sativa in heterozygotes (S13s/S13l). Genetic analysis revealed that a gene encoding a chromatin-remodeling factor (CHR) is involved in this phenomenon and a variety of O. sativa owns the truncated gene OsCHR745, whereas its homologue OlCHR has a complete structure in O. longistaminata. CRISPR-Cas9-mediated loss of function mutants restored fertility in hybrids. African cultivated rice, which naturally lacks the OlCHR homologue, is compatible with both S13s and S13l carriers. These results suggest that OlCHR is a Killer gene, which leads to reproductive isolation.

Hybrid sterility genes define species identities, setting reproductive barriers between distantly related Oryza relatives. They induce allelic-specific selective gametic abnormalities by killing pollens, embryo sacs, or both, and thus resulting in the male specific transmission ratio distortion (mTRD), female specific transmission ratio distortion (f TRD), and/or sex-independent transmission ratio distortion (siTRD) in hybrids. Although more than 50 hybrid sterility genes have been reported, comprehensive analysis on the distributional pattern of TRD systems in Oryza species is limited. In this review, we surveyed the TRD systems and the underlying possible mechanisms in these species. In rice, pollen killers which cause mTRD are often observed in higher frequency than egg killers and gamete eliminators, which are factors affecting f TRD and siTRD, respectively. Due to the rather massive population of pollen grains, their reduction in the number caused by hybrid sterility possesses a smaller selective disadvantage to the hybrid individuals, in contrast to female gamete abortion. The pattern of TRD distribution displays less abundancy in siTRD. It suggests that fixation of siTRD might require a certain time rather than single sex-specific factors. The presence of linked sterility factors worked for mTRD and f TRD, and strength of their linkage in chromosomal regions might determine the type of sterility and TRD. The study of TRD systems has a potential to reveal the relationships between selfish genes and their functions for reproductive isolation.