Koide Y, Ogino A, Yoshikawa T, Kitashima Y, Saito N, Kanaoka Y, Onishi K, Yoshitake Y, Tsukiyama T, Saito H, Teraishi M, Yamagata Y, Uemura A, Takagi H, Hayashi Y, Abe T, Fukuta Y, Okumoto Y, Kanazawa A.
Understanding the genetic basis of reproductive barriers between species has been a central issue in evolutionary biology. The S1
locus in rice causes hybrid sterility and is a major reproductive barrier between two rice species, Oryza sativa and Oryza glaberrima The O. glaberrima-derived allele (denoted S1g) on the S1
locus causes preferential abortion of gametes with its allelic alternative (denoted S1s) in S1
s heterozygotes. Here, we used mutagenesis and screening of fertile hybrid plants to isolate a mutant with an allele, S1
mut, which does not confer sterility in the S1
g and S1
s hybrids. We found that the causal mutation of the S1
mut allele was a deletion in the peptidase-coding gene (denoted "SSP") in the S1
locus of O. glaberrima No orthologous genes of SSP were found in the O. sativa genome. Transformation experiments indicated that the introduction of SSP in carriers of the S1
s allele did not induce sterility. In S1
s heterozygotes, the insertion of SSP led to sterility, suggesting that SSP complemented the loss of the functional phenotype of the mutant and that multiple factors are involved in the phenomenon. The polymorphisms caused by the lineage-specific acquisition or loss of the SSP gene were implicated in the generation of hybrid sterility. Our results demonstrated that artificial disruption of a single gene for the reproductive barrier creates a "neutral" allele, which facilitates interspecific hybridization for breeding programs.