Study On Formation And Mechanisms Of Heterosis In Chinese Cabbage

Brassica rapa is a widely cultivated vegetable in China and most commercial cultivars used for production are F₁ hybrids with high yield and good quality.However,the formation and mechanism of heterosis have not yet been elucidated.The selection of hybrids relies heavily on the experience of breeders and has a low breeding efficiency.Therefore,exploring the molecular mechanism of heterosis would have an important guiding significance for improving the breeding efficiency and clarifying the formation mechanism of heterosis in B.rapa.In this study,14 parental lines of B.rapa were crossed to produce hybrids,and the heterosis was measured for all the traits studied.To dissect the molecular mechanism of heterosis of yield-related traits in B.rapa,two representative F₁ hybrids were selected and self-pollinated to generate segregating F₂populations,then different analysis methods were used to identify plant weight QTL.Furthermore,we systematically identified candidate the heat tolerance genes by comparative transcriptome analysis of two contrasting B.rapa accessions in response to long-term heat stress.The results obtained are as follows:1.14 parental lines of B.rapa were studied in a half-diallel cross to produce 91 hybrids.We evaluated F₁ hybrids in the field to compare their major agronomic traits with those of the parents,then calculate the heterosis.Results showed that the significant maximum of high-parent heterosis among the 11 traits of 91 hybrids was observed in the biomass at 28 days（241.84%）,plant weight（118.14%）,head weight（120.69%）,and growth period（-207.79%）,which exhibited a yield improvement and reduced growth period in the breeding of B.rapa.2.2,444,676 high-quality SNPs were identified using whole-genome sequencing in 14 inbred lines.Genetic distance（GD）between the parents was calculated using two methods:the range of GD_total was0.222～0.379,which was calculated based on the difference of the whole genome,and the range of GD_homowas 0.211～0.365,which was calculated only considering the difference between the homozygous difference sites.The correlation analysis between the GD and heterosis indicated that GD_homo was positively correlated with the mid-parent heterosis for biomass（r=0.262）,head weight（r=0.214）,and the high-parent heterosis for biomass（r=0.234）.These results suggested that GD calculated by SNP markers could be used to predict heterosis to some extent.3.A strong dominant hybrid was selected and self-pollinated to generate segregating F₂ population.Then QTL-seq and Graded Pool-seq analysis were used to identify four plant weight QTLs in F₂population:q PW1.1,q PW5.1,q PW7.1,and q PW8.1.The traditional genetic linkage analysis with two years showed that q PW8.1,located in marker A08＿S45（18,172,719）and A08＿S85（18,196,752）,was mapped to a 23.5-kb genomic region.q PW8.1 explained 8.6%and 23.6%of the phenotypic variation in plant weight and the total number of head leaves and contained a heterozygous segment that might control the heterosis of plant weight.q PW1.1 had an 11.7%phenotypic contribution to plant weight.q PW7.1 was easily subject to environmental influence and explained 10.7%of the phenotypic variance.q PW5.1showed a significant signal and located in a genetic region near the centromere that shows high heterozygosity.Therefore,pseudo-overdominance effects and the synergistic allelic effect from parent‘XJD4’play an important role in heterosis for plant weight in B.rapa.4.For the hybrids with“YTBJ”as the parent,some traits like plant weight and head height were inherited from parent“YTBJ”among the F₁ and F₂populations,indicating that this parent was regarded as the superior parent and showed unique and strong heritability.The plant weight was determined as one of the most dominant traits in“YTBJ”.Then QTL-seq and Graded Pool-seq analysis were used to identify plant weight QTL and the interval was located on chromosome A09.5.Two B.rapa accessions,‘268’and‘334’,which showed significant differences in heat tolerance and recovery,were used for RNA sequencing analysis.We identified a total of 11,055 and 8,921differentially expressed genes（DEGs）in‘268’and‘334’,respectively.Weighted gene co-expression network analyses of all of the identified DEGs,revealed seven key modules and hub genes involved in long-term heat stress.Results showed that the autophagy pathway,glutathione metabolism,and ribosome biogenesis in eukaryotes were significantly up-regulated,whereas photosynthesis was down-regulated,in the heat resistance of B.rapa‘268’.Furthermore,when B.rapa‘334’was subjected to long-term high-temperature stress,heat stress caused significant changes in the expression of certain functional genes linked to protein processing in the endoplasmic reticulum and plant hormone signal transduction pathways.Several hub genes like HSP17.6,HSP17.6B,HSP70-8,CLPB1,PAP1,PYR1,ADC2,and GSTF11 were discussed in this study,which are potential candidates for further analyses of the response to long-term heat stress.