Intersubspecific hybrid sterility is a common form of reproductive isolation in rice (Oryza sativa L.), which significantly hampers the utilization of heterosis between indica and japonica varieties. Here, we elucidated the mechanism of S7, which specially causes Aus-japonica/indica hybrid female sterility, through cytological and genetic analysis, map-based cloning, and transformation experiments. Abnormal positioning of polar nuclei and smaller embryo sac were observed in F 1 compared with male and female parents. Female gametes carrying S7 cp and S7 i were aborted in S7 ai /S7 cp and S7 ai /S7 i , respectively, whereas they were normal in both N22 and Dular possessing a neutral allele, S7 n . S7 was fine mapped to a 139-kb region in the centromere region on chromosome 7, where the recombination was remarkably suppressed due to aggregation of retrotransposons. Among 16 putative open reading frames (ORFs) localized in the mapping region, ORF3 encoding a tetratricopeptide repeat domain containing protein was highly expressed in the pistil. Transformation experiments demonstrated that ORF3 is the candidate gene: downregulated expression of ORF3 restored spikelet fertility and eliminated absolutely preferential transmission of S7 ai in heterozygote S7 ai /S7 cp ; sterility occurred in the transformants Cpslo17-S7 ai . Our results may provide implications for overcoming hybrid embryo sac sterility in intersubspecific hybrid rice and utilization of hybrid heterosis for cultivated rice improvement.KEYWORDS hybrid sterility; female gamete; tetratricopeptide repeat (TPR); transgenic; rice (Oryza sativa L.) H YBRIDIZATION between two different species can lead to a distinct phenotype, which can also be fitter than the parental lineage. However, reproductive isolation maintains the integrity of a species over time, reducing or directly impeding gene flow between individuals of different species (Mayr 1942;Grant 1981;Coyne and Orr 2004;Widmer et al. 2009;Baack et al. 2015). The mechanisms of reproductive isolation were classified into two broad categories: prezygotic and postzygotic isolation mechanisms (Mayr 1963;Levin 1978;Sweigart and Willis 2012;Chen et al. 2014). According to the classical Dobzhansky-Muller model, postzygotic isolation results from a deleterious interaction between functionally diverged genes from the hybridizing species (Dobzhansky 1937;Ting et al. 1998;Barbash et al. 2003;Presgraves et al. 2003;Brideau et al. 2006;Bayes and Malik 2009;Ferree and Barbash 2009;Phadnis and Orr 2009;Tang and Presgraves 2009;White et al. 2011). Genes for hybrid sterility, a common pattern of postzygotic isolation, have been reported in several organisms, including fungi, animals, and plants (Brideau et al. 2006;Lee et al. 2008;Bikard et al. 2009;De Vienne et al. 2009).Major progress has been made in rice and the interspecific and intersubspecific hybrid sterilities are perhaps the best known examples (Chen et al. 2008;Long et al. 2008 Wan and Ikehashi 1995;Zhu et al. 2005;Li et al. 2007;Zhao et al. 2007;Chen et...
Hybrid sterility locus S37 between Oryza glaberrima and Oryza sativa results in both pollen and embryo sac sterility. Interspecific crossing between African cultivated rice Oryza glaberrima and Oryza sativa cultivars is hindered by hybrid sterility. To dissect the mechanism of interspecific hybrid sterility, we developed a near-isogenic line (NIL)-S37 using Dianjingyou1 (DJY1) as the recipient parent and an African cultivated rice variety as the donor parent. Empty pollen and embryo sac sterility were observed in F1 hybrids between DJY1 and NIL-S37. Cytological analyses showed that pollen abortion in the F1 hybrids occurred at the late binucleate stage due to a failure of starch accumulation in pollen grains. In addition, partial abortion of the embryo sac in the F1 hybrid was observed during function megaspore developing into mature embryo sac. Molecular analysis revealed that the semi-sterility was largely caused by the abortion of male and female gametophytes carrying the S37 allele from DJY1. A population of 25,600 plants derived from the hybrid DJY1/NIL-S37 was developed to fine map S37. Based on the physical location of molecular markers, S37 locus was finally delimited to a region of 205 kb on the short arm of chromosome 1 in terms of reference sequences of cv. Nipponbare. Interestingly, an about 97-kb DNA segment was deleted in the NIL-S37 based on BAC clone information of O. glaberrima. Fifty-four open reading frames (ORF) were predicted in this 205-kb region of DJY1, whereas only 31 ORFs were in that of NIL-S37. These results are valuable for cloning of S37 gene and further breaking reproductive isolation between Oryza glaberrima and Oryza sativa cultivars, as well as marker-assisted transferring of the corresponding neutral allele in rice breeding programs.
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