Domain Unknown Function DUF1668-Containing Genes in Multiple Lineages Are Responsible for F1 Pollen Sterility in Rice.

Postzygotic reproductive isolation maintains species integrity and uniformity and contributes to speciation by restricting the free gene flow between divergent species. In this study we identify causal genes of two Mendelian factors S22A and S22B on rice chromosome 2 inducing F1 pollen sterility in hybrids between Oryza sativa japonica-type cultivar Taichung 65 (T65) and a wild relative of rice species Oryza glumaepatula. The causal gene of S22B in T65 encodes a protein containing DUF1668 and gametophytically expressed in the anthers, designated S22B_j. The O. glumaepatula allele S22B-g, allelic to S22B_j, possesses three non-synonymous substitutions and a 2-bp deletion, leading to a frameshifted translation at the S22B C-terminal region. Transcription level of S22B-j and/or S22B_g did not solely determine the fertility of pollen grains by genotypes at S22B. Western blotting of S22B found that one major band with approximately 46 kDa appeared only at the mature stage and was reduced on semi-sterile heterozygotes at S22B, implying that the 46 kDa band may associated in hybrid sterility. In addition, causal genes of S22A in T65 were found to be S22A_j1 and S22A_j3 encoding DUF1668-containing protein. The allele of a wild rice species Oryza meridionalis Ng at S22B, designated S22B_m, is a loss-of-function allele probably due to large deletion of the gene lacking DUF1668 domain and evolved from the different lineage of O. glumaepatula. Phylogenetic analysis of DUF1668 suggested that many gene duplications occurred before the divergence of current crops in Poaceae, and loss-of-function mutations of DUF1668-containing genes represent the candidate causal genetic events contributing to hybrid incompatibilities. The duplicated DUF1668-domain gene may provide genetic potential to induce hybrid incompatibility by consequent mutations after divergence.

Development and evaluation of rice giant embryo mutants for high oil content originated from a high-yielding cultivar 'Mizuhochikara'.

Rice bran oil is a byproduct of the milling of rice (Oryza sativa L.). It offers various health benefits and has a beneficial fatty acid composition. To increase the amount of rice bran as a sink for triacylglycerol (TAG), we developed and characterized new breeding materials with giant embryos. To induce mutants, we treated fertilized egg cells of the high-yielding cultivar 'Mizuhochikara' with N-methyl-N-nitrosourea (MNU). By screening M2 seeds, we isolated four giant embryo mutant lines. Genetic analysis revealed that the causative loci in lines MGE12 and MGE13 were allelic to giant embryo (ge) on chromosome 7, and had base changes in the causal gene Os07g0603700. On the other hand, the causative loci in lines MGE8 and MGE14 were not allelic to ge, and both were newly mapped on chromosome 3. The TAG contents of all four mutant lines increased relative to their wild type, 'Mizuhochikara'. MGE13 was agronomically similar to 'Mizuhochikara' and would be useful for breeding for improved oil content.

Two linked genes on rice chromosome 2 for F1 pollen sterility in a hybrid between Oryza sativa and O. glumaepatula.

Hybrid incompatibility plays an important role in establishment of post-zygotic reproductive isolation. To unveil genetic basis of hybrid incompatibilities between diverged species of genus Oryza AA genome species, we conducted genetic dissection of hybrid sterility loci, S22(t), which had been identified in backcross progeny derived from Oryza sativa ssp. japonica (recurrent parent) and South American wild rice O. glumaepatula near the end of the short arm of chromosome 2. The S22(t) region was found to be composed of two loci, designated S22A and S22B, that independently induce F1 pollen sterility. Pollen grains containing either of the sterile alleles (S22A-glum (s) or S22B-glum (s) ) were sterile if produced on a heterozygous plant. No transmission of the S22A-glum (s) allele via pollen was observed, whereas a low frequency of transmission of S22B-glum (s) was observed. Cytological analysis showed that the sterile pollen grains caused by S22A could reach the bicellular or tricellular stage, and the nearly-sterile pollen grains caused by S22B could reach the tricellular stage. Our genetic analysis showed repulsion linkage effect is possible to induce strong reproductive barrier by high pollen sterility based on recombination value and transmission ratio of hybrid sterility gene to the progeny was influenced by frequency of competitors on fertilization.