Partial abortion of female gametes and the resulting semi-sterility of indica x japonica inter-subspecific rice hybrids have been ascribed to an allelic interaction, which can be avoided by the use of wide compatibility varieties. To further understand the genetic mechanism of hybrid sterility, we have constructed two sets of hybrids, using as male parent either the typical japonica variety Asominori, or the wide compatibility variety 02428; and as female, a set of 66 chromosome segment substitution lines in which various chromosomal segments from the indica variety IR24 have been introduced into a common genetic background of Asominori. Spikelet semi-sterility was observed in hybrid between CSSL34 and Asominori, which is known to carry the sterility gene S31 (Zhao et al. in Euphytica 151:331-337, 2006). Cytological analysis revealed that the semi-sterility of the CSSL34 x Asominori hybrid was caused primarily by partial abortion of the embryo sac at the stage of the mitosis of the functional megaspore. A population of 1,630 progeny of the three-way cross (CSSL34 x 02428) x Asominori was developed to map S31. Based on the physical location of linked molecular markers, S31 was thereby delimited to a 54-kb region on rice chromsome 5. This fragment contains eight predicted open reading frames, four of which encode known proteins and four putative proteins. These results are relevant to the map-based cloning of S31, and the development of marker-assisted transfer of non-sterility allele inducing alleles to breeding germplasm, to allow for a more efficient exploitation of heterosis in hybrid rice.

Fourteen new quantitative trait loci (QTLs) and ten favorable alleles were identified for lodging resistance traits in a natural population of rice. Parental combinations were designed to improve lodging resistance. Lodging is one of the most critical constraints to rice yield, and therefore, mining favorable alleles for lodging resistance traits is imperative for the advancement of cultivated rice and selection for market demand. This investigation was performed on a selected sample of 521 rice cultivars using 262 SSR markers in 2016 and 2017. Lodging resistance traits were evaluated by plant height (PH), stem length (SL), stem diameter (SD), anti-thrust per stem (AT/S), and stem index (SI), with AT/S, used as the lodging resistance index. A genome-wide association map was generated by combining phenotypic and genotypic data. Eight subpopulations were found by structure software, and the linkage disequilibrium (LD) ranged from 30 to 80 cM. Identification of 68 marker-trait associations (MTAs) linking in 64 SSR markers for five traits was done. QTL were detected, including 15 for PH, 14 for SL, 14 for SD, 7 for AT/S, and 18 for SI. A number of favorable alleles were also discovered, including 22, 24, 19, 12, and 28 alleles for PH, SL, SD, AT/S, and SI, respectively. These favorable alleles might be used to design parental combinations, and the predictable results found by relieving the favorable alleles per QTL. The accessions containing favorable alleles for lodging resistant traits mined in this study could be useful for breeding superior rice cultivars.

Stigma characteristics are important factors affecting the seed yield of hybrid rice per unit area. Natural variation of stigma characteristics has been reported in rice, but the genetic basis for this variation is largely unknown. We performed a genome-wide association study on three stigma characteristics in six environments using 1.3 million single-nucleotide polymorphism (SNPs) characterized in 353 diverse accessions of Oryza sativa. An abundance of phenotypic variation was present in the three stigma characteristics of these collections. We identified four significant SNPs associated with stigma length, 20 SNPs with style length (SYL), and 17 SNPs with the sum of stigma and style length, which were detected repeatedly in more than four environments. Of these SNPs, 28 were novel. We identified two causal gene loci for SYL, OsSYL3 and OsSYL2; OsSYL3 was co-localized with the grain size gene GS3. The SYL of accessions carrying allele OsSY-L3 AA was significantly longer than that of those carrying allele OsSYL3 CC. We also demonstrated that the outcrossing rate of female parents carrying allele OsSYL2 AA increased by 5.71% compared with that of the isogenic line carrying allele OsSYL2 CC in an F 1 hybrid seed production field. The allele frequencies of OsSY-L3 AA and OsSYL2 AA decreased gradually with an increase in latitude in the Northern Hemisphere. Our results should facilitate the improvement in stigma characteristics of parents of hybrid rice.

Hybrid sterility hinders the exploitation of the heterosis displayed by japonica × indica rice hybrids. The variation in pollen semi-sterility observed among hybrids between the japonica recipient cultivar and each of two sets of chromosome segment substitution lines involving introgression from an indica cultivar was due to a factor on chromosome 5 known to harbor the gene S24. S24 was fine mapped to a 42 kb segment by analyzing a large F(2) population bred from the cross S24-NIL × Asominori, while the semi-sterility shown by the F(1) hybrid was ascribable to mitotic failure at the early bicellular pollen stage. Interestingly, two other pollen sterility genes (f5-Du and Sb) map to the same region (Li et al. in Chin Sci Bull 51:675-680, 2006; Wang et al. in Theor Appl Genet 112:382-387, 2006), allowing a search for candidate genes in the 6.4 kb overlap between the three genes. By sequencing the overlapped fragment in wild rice, indica cultivars and japonica cultivars, a protein ankyrin-3 encoded by the ORF2 was identified as the molecular base for S24. A cultivar Dular was found to have a hybrid-sterility-neutral allele, S24-n, in which an insertion of 30 bp was confirmed. Thus, it was possible to add one more case of molecular bases for the hybrid sterility. No gamete abortion is caused on heterozygous maternal genotype with an impaired sequence from the hybrid-sterility-neutral genotype. This result will be useful in understanding of wide compatibility in rice breeding.