Population-based Genetic Improvement And Heterosis Analysis Of New-type Brassica Napus

Brassica napus is an important worldwide oilseed crop,and also is a young allotetraploid with a short evolutionary history and limited genetic diversity.To significantly broaden its genetic basis and create a novel heterotic population for sustainable rapeseed breeding,our group introgressed the Ar subgenomes from 122 accessions of B.rapa and Cc subgenomes from 74 accessions of B.carinata to B.napus by interspecific crossing and marker assistant selection,developed a new-type B.napus diverse population including hundreds of inbred lines,and created a new-type B.napus recurrent selection population(RS population)including thousands of lines with rich genetic variation through the introgression of dominant genic male sterility trait and four rounds of population improvement.Based on this RS population,this study continued the traits improvement,evaluated the effect of population improvement,estimated the population genetic diversity and genetic variations,analyzed and predicted the intersubgenomic heterosis,and tried to develop a new-type B.napus breeding population with improved traits and strong heterosis potential which could continuously supply breeding materials for the creation of new germplasm and utilization of intersubgenomic heterosis.Seven major objectives and results are as follows:1.The fifth round of improvement for the RS population was performed,then 240 lines were randomly selected from the first,third,and fifth round of RS population(80 lines of each population),to evaluate the effect of the population improvement.After five rounds of recurrent selection,average glucosinolate content and erucic acid content of the RS population reduced 40.6% and 89.4%,respectively,and oleic acid content increased 19.2%,but the total oil content increased slowly.For agronomic traits,seed weight,pod density and seed number per pod were improved.Three generations of the new-type B.napus were showed rich phenotypic variation,lines with elite traits,such as big seed,high oil content and high seed density were obtained.2.A total of 486 lines including 55 B.rapa accessions,55 B.carinata accessions,56 traditional B.napus accessions,160 plants of the third round of RS population,and 60 plants of the fifth round of RS population,were evaluated with 82 SSR&Indel markers.Genetic clustering analysis of the RS population and traditional B.napus showed a large genetic distance and genetic variation.147 loci were detected in newtype B.napus,59 of them contained the alleles specific introgressed from B.rapa and B.carianta,including 10 B.carinata B genomic alleles.Besides,eight possible new alleles were detected in new-type B.napus.The high heterozygosity of the new-type B.napus recurrent population indicated fully hybridization among the individuals which might result in rich recombination.3.Nearly a thousand new-type B.napus inbred/DH(double haploid)lines were obtained from the RS population through selection,selfcrossing and microspore culture.51 newtype B.napus DH lines and the founder parents of new-type B.napus(HS3)were genotyped by sequencing,and a total of 50,222 high quality markers were detected.By comparison with HS3,we observed that approximately 52% of the genomic components of the new-type B.napus DH lines were different with HS3.Besides,140 small B genomic introgression signals were detected in the new-type B.napus DH lines,except for a relatively large 160 kb region.4.Six traditional B.napus female parent lines,61 new-type B.napus male parent lines,and their 363 intersubgenomic hybrids were grown in three environments and tested for 10 traits.Seed yield of the intersubgenomic hybrids showed strong heterosis,and 15 hybrids that significantly exceeded the commercial hybrid control were selected.5.48,602 markers were obtained after sequenced the 67 tester parents.The analysis of the genetic variation components of mid-parent heterosis(MPH)showed that the dominance component explained the largest proportion(29%-41%)of MPH.33,47,18 and 24 significant heterosis loci were detected through GWAS with four data sets of cross-environments,Wuhan,Xiangyang and Jingtai,respectively,and one B.rapa specific heterosis loci was detected in Wuhan and Xiangyang,respectively.In addition,10 additive-by-dominance epistatic effects between B.carinata(Cc)loci and B.napus loci and 41 dominance-by-dominance epistatic effects between B.rapa/B.carinata(Ar/Cc)loci and B.napus loci were detected in WH,thus indicated that the introgressed exogenetic genomic components contributes to heterosis to some extent.6.We used genome-wide prediction model including different types of genetic effects to predict the MPH,the prediction ability with dominance alone in the model were range from 0.446 to 0.637,and just lower than the highest prediction ability with four effects in the model about 0.02-0.08,thus proved the importance of dominance in intersubgenomic heterosis again.The prediction ability in Wuhan was the lowest,and the prediction ability of the cross-environment BLUEs data was the highst,indicated that materials type and the numberof environment can inflence the prediction ability.