Understanding and breaking down the reproductive barrier between Asian and African cultivated rice: a new start for hybrid rice breeding

Identification of a novel hybrid sterility locus S67 between temperate japonica subgroup and basmati subgroup in Oryza sativa L

Asian cultivated rice (Oryza sativa L.) is the most important cultivated species in the AA genome species of the genus Oryza. basmati is a special and famous subgroup in Asian cultivated rice, and temperate japonica is one of the most important cultivated subgroup, too. However, hybrid sterility hinders the introgression of favorable traits and the utilization of hybrid vigour between the two subgroups. The genetic basis of intraspecific hybrid sterility between temperate japonica and basmati remained elusive. In this study, a novel hybrid sterility locus S67 was identified, which caused hybrid male sterility in hybrids between the temperate japonica rice variety Dianjingyou 1 (DJY1) and the basmati rice variety Dom-sufid. Initial mapping with BC1F1, BC4F1, BC4F2 populations and DNA markers located S67 between RM5362 and K1-40.6 on the long arm of chromosome 1. Genetic analysis confirmed that S67 caused a transmission advantage for the temperate japonica rice S67-te allele in the hybrid offsprings. This result not only fills the gap in the research on hybrid sterility between basmati and temperate japonica, but also lays a good foundation for the systematic study of the genetic nature of hybrid sterility between basmati and other subgroups, as well as the full exploration and utilization of this subgroup through the creation of wide or specific compatibility lines to overcome hybrid sterility. In addition, this result can also help us broaden our understanding of genetic differentiation within Asian cultivated rice and hybrid sterility between inter-subgroups.

Understanding the genetic and molecular constitutions of heterosis for developing hybrid rice

The wide adoption of hybrid rice has greatly increased rice yield in the last several decades. The utilization of heterosis facilitated by male sterility has been a common strategy for hybrid rice development. Here, we summarize our efforts in the genetic and molecular understanding of heterosis and male sterility together with the related progress from other research groups. Analyses of F₁ diallel crosses show that strong heterosis widely exists in hybrids of diverse germplasms, and inter-subspecific hybrids often display higher heterosis. Using the elite hybrid Shanyou 63 as a model, an immortalized F₂ population design is conducted for systematic characterization of the biological mechanism of heterosis, with identification of loci controlling heterosis of yield and yield component traits. Dominance, overdominance, and epistasis all play important roles in the genetic basis of heterosis; quantitative assessment of these components well addressed the three classical genetic hypotheses for heterosis. Environment-sensitive genic male sterility (EGMS) enables the development of two-line hybrids, and long noncoding RNAs often function as regulators of EGMS. Inter-subspecific hybrids show greatly reduced fertility; the identification and molecular characterization of hybrid sterility genes offer strategies for overcoming inter-subspecific hybrid sterility. These developments have significant implications for future hybrid rice improvement using genomic breeding.

Identification of QTL Combinations that Cause Spikelet Sterility in Rice Derived from Interspecific Crosses

BACKGROUND

The exploitation of useful genes through interspecific and intersubspecific crosses has been an important strategy for the genetic improvement of rice. Postzygotic reproductive isolation routinely occurs to hinder the growth of pollen or embryo sacs during the reproductive development of the wide crosses.

RESULT

In this study, we investigated the genetic relationship between the hybrid breakdown of the population and transferred resistance genes derived from wide crosses using a near-isogenic population composed of 225 lines. Five loci (qSS12, qSS8, qSS11, ePS6-1, and ePS6-2) associated with spikelet fertility (SF) were identified by QTL and epistatic analysis, and two out of five epistasis interactions were found between the three QTLs (qSS12, qSS8 and qSS11) and background marker loci (ePS6-1 and ePS6-2) on chromosome 6. The results of the QTL combinations suggested a genetic model that explains most of the interactions between spikelet fertility and the detected loci with positive or negative effects. Moreover, the major-effect QTLs, qSS12 and qSS8, which exhibited additive gene effects, were narrowed down to 82- and 200-kb regions on chromosomes 12 and 8, respectively. Of the 13 ORFs present in the target regions, Os12g0589400 and Os12g0589898 for qSS12 and OS8g0298700 for qSS8 induced significantly different expression levels of the candidate genes in rice at the young panicle stage.

CONCLUSION

The results will be useful for obtaining a further understanding of the mechanism causing the hybrid breakdown of a wide cross and will provide new information for developing rice cultivars with wide compatibility.

Recurrent breakdown and rebalance of segregation distortion in the genomes: battle for the transmission advantage

Mendel's laws state that each of the two alleles would segregate during gamete formation and show the same transmission ratio in the next generation. However, an unexpected biased allele transmission was first detected in Drosophila a century ago, and was subsequently observed in other animals, plants, and microorganisms. Such segregation distortion (SD) shows substantial effects in population structure and fitness of the progenies, which would ultimately lead to reproductive isolation and speciation. Here, we trace the early investigations on the violation of Mendelian genetic principle, which appears as a wide-existence phenomenon rather than a case of exception. The occurence of SD in the whole genome was observed in a number of plant species at the single- and multi-locus level. Biased transmission ratio might occur at meiosis stage due to asymmetric movement of the chromosome; transmission ratio advantage is also caused by interaction and battle between the alleles from respective genomes at the genetic and molecular level. The origin of a SD system is likely to be determined by coevolution of the killer and protector via recurrent breakdown or rebalance loop. These updated understandings also promote genetic improvement of hybrid crops.

Advances in genome-wide association studies of complex traits in rice

Genome-wide association studies (GWAS), genetic surveys of the whole genome to detect variants associated with a trait in natural populations, are a powerful approach for dissecting complex traits. This genetic mapping approach has been applied in rice over the last 10 years. During the last decade, GWAS was used to identify the loci underlying tens of rice traits, and several important genes were detected in GWAS and further confirmed in follow-up functional experiments. In this review, we present an overview of the whole process in a typical GWAS, including population design, genotyping, phenotyping and analysis methods. Recent advances in rice GWAS are also provided, including several examples of the functional characterization of candidate genes. The possible breakthroughs of rice GWAS in the next decade are discussed with regard to their application in breeding, the consideration of epistatic interactions and in-depth functional annotations of DNA elements and genetic variants throughout the rice genome.