Understanding the genetic basis of reproductive barriers between species has been a central issue in evolutionary biology. The locus in rice causes hybrid sterility and is a major reproductive barrier between two rice species, and The-derived allele (denoted ) on the locus causes preferential abortion of gametes with its allelic alternative (denoted ) in/ heterozygotes. Here, we used mutagenesis and screening of fertile hybrid plants to isolate a mutant with an allele, , which does not confer sterility in the/ and / hybrids. We found that the causal mutation of the allele was a deletion in the peptidase-coding gene (denoted "") in the locus of No orthologous genes of were found in the genome. Transformation experiments indicated that the introduction of in carriers of the allele did not induce sterility. In / heterozygotes, the insertion of led to sterility, suggesting that 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 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.

BackgroundTo investigate plant hybrid sterility, we studied interspecific hybrids of two cultivated rice species, Asian rice (Oryza sativa) and African rice (O. glaberrima). Male gametes of these hybrids display complete sterility owing to a dozen of hybrid sterility loci, termed HS loci, but this complicated genetic system remains poorly understood.ResultsMicrospores from these interspecific hybrids form sterile pollen but are viable at the immature stage. Application of the anther culture (AC) method caused these immature microspores to induce callus. The segregation distortion of 11 among 13 known HS loci was assessed in the callus population. Using many individual calli, fine mapping of the HS loci was attempted based on heterozygotes produced from chromosome segment substitution lines (CSSLs). Transmission ratio distortion (TRD) from microspores was detected at 6 of 11 HS loci in the callus population. The fine mapping of S1 and S19 loci using CSSLs revealed precise distances of markers from the positions of HS loci exhibiting excessive TRD.ConclusionsWe demonstrated that AC to generate callus populations derived from immature microspores is a useful methodology for genetic study. The callus population facilitated detection of TRD at multiple HS loci and dramatically shortened the process for mapping hybrid sterility genes.Electronic supplementary materialThe online version of this article (10.1186/s13007-018-0370-z) contains supplementary material, which is available to authorized users.

Title 7Diploid male gametes circumvent hybrid sterility between Asian and African rice 8 species 9

Spontaneous mutations are stochastic phenomena that occur in every population. However, deleterious mutated allele present in seeds distributed to farmers must be detected and removed. Here, we eliminated undesirable mutations from the parent population in one generation through a strategy based on next-generation sequencing (NGS). This study dealt with a spontaneous albino mutant in the 'Hinohikari' rice variety grown at the Miyazaki Comprehensive Agricultural Experiment Station, Japan. The incidence of albinism in the population was 1.36%. NGS analysis revealed the genomic basis for differences between green and albino phenotypes. Every albino plant had a C insertion in the Snow-White Leaf1 (SWL1) gene on chromosome 4 causing a frameshift mutation. Selfing plants heterozygous for the mutant allele, swl1-R332P, resulted in a 3:1 green/albino ratio, confirming that a single recessive gene controls albinism. Ultrastructural leaf features in the swl1-R332P mutants displayed deformed chlorophyll-associated organelles in albino plants that were similar to those of previously described swl1 mutants. Detection of the causative gene and its confirmation using heterozygous progenies were completed within a year. The NGS technique outlined here facilitates rapid identification of spontaneous mutations that can occur in breeder seeds.

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