Post-zygotic reproductive isolation (also called hybrid incompatibility) has been extensively studied in animals, plants and microorganisms that include hybrid sterility (HS) and hybrid necrosis or weakness. For example, the speciation locus YUP in mokeyflowers 1, the unilaterally incompatibility system found in Brassicaceae 2, 3, Zea mays 4, 5, 6, 7 and tomato 8, 9, 10 is the well-studied pre-zygotic mechanism. In plants, the pre-zygotic barriers include geographical isolation, differentiations in flower habit, failure in cell-to-cell recognition between the pollen and stigma, inability in pollen germination and pollen tube growth, and defects in cell fusion between sexual gametes. Depending on species, reproductive barriers can occur either before (pre-zygotic) or after (post-zygotic) fertilization. Reproductive isolation is a common phenomenon in nature and exists in different forms. 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. 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. In plants heterozygous at qHMS1, ~ 50% of pollens carrying qHMS1-D (an allele from cultivated rice) are selectively killed.

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. Here, we report the cloning of qHMS1, a quantitative trait locus controlling hybrid male sterility between these two species. Hybrid sterility, a major form of reproductive isolation exists between cultivated rice ( Oryza sativa) and wild rice ( O. Breakdown of reproductive isolation facilitates flow of useful trait genes into crop plants from their wild relatives.

Antidote asheville