Alternatively spliced killer-protector system confers S19-mediated hybrid male sterility in rice

Hybrid sterility limits the use of strong interspecific heterosis and S19 is a major locus that confers hybrid sterility between Oryza sativa (Asian cultivated rice) and Oryza glaberrima (African cultivated rice). However, the S19 is not yet cloned and its underlying mechanism remains elusive. In this study, we identify two closely linked genes (S19A1 and S19A7) specific to African rice allele that encode a killer-protector module at the S19 locus. Two alternatively spliced transcripts expressed from the killer gene S19A1 (S19A1.1 and S19A1.2) encode mitochondria-targeted cytotoxic proteins that cause toxicity diversity for somatic and/or gametic cell death, respectively. However, S19A7 interacts with S19A1.1 and S19A1.2, blocking their cytotoxic effect. Because the Asian rice S19 allele lacks S19A1 and S19A7, male gametes that carry this allele are selectively aborted in Asian-African F₁ hybrids. Knockout of S19A1 can overcome S19-mediated hybrid sterility. Haplotype analysis reveals that the functional S19 allele is absent in non-AA-genome Oryza species and likely emerged in the O. barthii-O. glaberrima lineage through a multi-step evolutionary process. Our findings provide insight into the genetic mechanisms responsible for hybrid sterility between Asian and African rice and suggest genetic and biotechnological strategies for the use of interspecific heterosis in rice.