A toxin-antidote system contributes to interspecific reproductive isolation in rice

Breakdown of reproductive isolation facilitates flow of useful trait genes into crop plants from their wild relatives. Hybrid sterility, a major form of reproductive isolation exists between cultivated rice ( Oryza sativa ) and wild rice ( O. meridionalis , Mer ). Here, we report the cloning of qHMS1 , a quantitative trait locus controlling hybrid male sterility between these two species. Like qHMS7 , another locus we cloned previously, qHMS1 encodes a toxin-antidote system, but differs in the encoded proteins, their evolutionary origin, and action time point during pollen development. In plants heterozygous at qHMS1 , ~ 50% of pollens carrying qHMS1- D (an allele from cultivated rice) are selectively killed. In plants heterozygous at both qHMS1 and qHMS7, ~ 75% pollens without co-presence of qHMS1-Mer and qHMS7- D are selectively killed, indicating that the antidotes function in a toxin-dependent manner. Our results indicate that different toxin-antidote systems provide stacked reproductive isolation for maintaining species identity and shed light on breakdown of hybrid male sterility.