Hybrid Mimics Development And Hybrid Vigor In Rice

Heterosis or hybrid vigor is a phenomenon that the hybrid F1 of two parents with different genetic backgrounds show improved and superior agronomical traits,like growth potential,viability,yield,quality,resistance and so on.Although the genetic mechanism of heterosis is no comprehensive understanding,heterosis have been used to produce highyielding crop varieties in crops such as rice and maize,and contribute to global food production.The utilization of heterosis is limited to the hybrid F1 generation due to F2 generation will lose hybrid vigor,which makes farmers have to buy hybrid seeds every year,increasing production costs.Developing stable and superior hybrid offspring has become the goal pursued by many breeders.In this study,two hybrid rice combinations,FLY1 and DY527,which were widely used in production,were used as materials.We have developed F6/F7 Hybrid Mimic Lines which were similar with F1 by recurrent selfing and selection.Through comparing the agronomic traits of hybrid mimic lines and F1 s to determine the similarity of them,and RNA-seq analysis of the gene expression basis of early germination dominant phenotypes.The main results are summarized as below:1.The hybrid mimic lines were similar with F1 hybrids.The main agronomic traits of the F7 mimic lines deriving from FLY1 and the F6 mimic lines from DY527 were similar with F1 hybrids at the mature stage.The mimic lines and F1 hybrid were at the same level of bacterial blight disease resistance,while the blast resistance of mimic lines was slightly weaker than that of the F1 hybrid.The results of the grain quality test indicated that mimic lines were not exactly the same as their F1 but most quality indicators of the mimic lines were at almost the same level as the F1.2.The genomes of the hybrid mimics are mostly homozygous.SNP analysis of two F6 FLY1 hybrid mimic lines demonstrated that the chromosomal segments of the mimic lines were highly homozygous with the genome-wide percentage of heterozygosity ranging from 11% to 17%,which proved the genetic stability of hybrid mimic lines.Compared across the two sets of sibling plants,there was no common heterozygous region indicating that heterozygosity was not essential for the vigor phenotype.3.Germination vigor of hybrids and Hybrid Mimics.From 3-4 to 6 DAS,the increase in fresh weight of F1 hybrids and hybrid mimic lines seedlings was significantly higher than that of their parents.4.RNA-seq analysis enriched the genes up-regulated in F1 and mimic lines in energy metabolism pathways.Transcriptome sequencing and analysis found that,compared to the mid-parent value,F1 and mimic lines had a lot of photosynthesis-related genes were up-regulated at 1 DAS.The expression levels of these genes were up-regulated to the level of F1 and mimic lines at 2,3 DAS.RT-qPCR certificated the reliability of RNAseq results,and a similar expression pattern existed in another combination,DY527 and its mimic lines.In addition to the photosynthesis pathway,the up-regulated Calvin cycle and down-regulated Tricarboxylic Acid cycle related genes suggesting there is a switch in metabolism from heterotrophy to auxotrophy earlier in the F1 and mimic lines than in the parents.The F1 hybrids and Mimics after germination produce energy through photosynthesis rather than consuming energy through catabolism of carbohydrates earlier than the parents.This vigor characteristic may contribute to the accumulation of energy in the germination stage and the later growth and development.