Detection Of QTL Loci For Agronomic Traits In Flowering Chinese Cabbage And Analysis Of Candidate Genes

Brassica campestris L.ssp.chinensis var.utilis Tsen et Lee,also known as Brassica campestris,is one of the largest planted Brassica vegetables in South China.It has a long history of traditional breeding,and varieties with high yield,good quality,and stable traits have always been the main goal of Brassica campestris breeding.However,most of its traits are complex,namely quantitative traits.In the past,conventional genetic improvement methods were susceptible to various factors such as genotype,environment,and genotype in the breeding process,resulting in a very slow breeding process for its cabbage variety.In recent years,quantitative trait loci（QTLs）mapping based on linkage analysis has become an important means of quantitative trait mapping in crop molecular breeding.The construction of the population used in linkage analysis is based on linkage recombination,so its internal genetic background is relatively clear.In addition,with the advancement of genomics technology and the decrease in sequencing costs,genome sequencing has gradually become an important component of molecular marker assisted breeding.Developing molecular markers based on genome sequencing,constructing high-density genetic linkage maps,and mining related genes have gradually become an important means of studying crop genetic laws and molecular breeding.There are currently reports on QTL mapping of agronomic traits in flowering Chinese cabbage,but there are issues such as low density of the constructed genetic map,which may interfere with the accuracy of subsequent QTL mapping.Therefore,this study selected cabbage germplasm"3T6"and"49-19"with significantly different agronomic traits as parents to construct 101 F₇cabbage RIL populations.Simple genome sequencing（GBS）technology was used to genotype them,multiple SNP markers were excavated in the cabbage RIL population,and high-density genetic linkage maps were established.QTL fine mapping of cabbage agronomic traits was performed,and candidate genes were predicted in the QTL region.The main work and achievements of this study are:1.A high-density genetic map of flowering Chinese cabbage has been established.Using GBS simplified genome sequencing technology,103 flowering Chinese cabbage materials,including parents"3T6"and"49-19"as well as RIL populations,were genotyped.29953polymorphic SNP loci were detected,and 3734 polymorphic SNP loci that met the composition requirements were screened.A genetic map of flowering Chinese cabbage with10 linkage groups,a total length of 844.174 c M,and 1266 SNP loci was constructed using Join Map4.0 software.The average distance of the map was 0.667 c M/marker,and the length of each linkage group ranged from 68.725 to 108.907 c M,with SNPs ranging from 88 to 205In addition,a total of 31 QTL loci were found to exhibit one cause multiple effects in 2or more traits,respectively.2.Genetic analysis was conducted on 8 major agronomic traits.Statistics and analysis were conducted on the development degree,maximum leaf petiole length,maximum leaf width,maximum leaf length,bolt thickness,plant height,plant weight,and SPAD values of103 rapeseed materials planted in 2015 and 2016.The results further clarified the significant differences in agronomic traits between parents"3T6"and"49-19",and found that the correlation between agronomic traits in the RIL population did not reach a significant level,The results showed that most agronomic traits were controlled by multiple genes,but they were also sensitive to the external environment.In addition,it was found that the main agronomic traits of Brassica campestris were similar to normal distribution,which met the requirements of QTL mapping.3.QTL analysis was conducted on 8 main agronomic traits of flowering Chinese cabbage.Based on the constructed high-density genetic map of flowering Chinese cabbage and two-year R1L population agronomic trait data,a composite interval mapping（CIM）method was used to QTL map the agronomic traits of flowering Chinese cabbage.A total of 420 QTL loci related to flowering Chinese cabbage agronomic traits were detected with a LOD of 2.5as the threshold.The LOD ranged from 2.5 to 12.21,and the phenotype interpretation rate ranged from 2.62%to 24.05%.Two QTL loci q SPAD64 and q SPAD102 related to SPAD values were repeatedly detected on the sixth and tenth linkage groups for two consecutive years.4.Predicted candidate genes within the QTL region of 8 agronomic traits in flowering Chinese cabbage.By comparing the QTL interval sequence of agronomic traits in cabbage with the reference genome of cabbage,candidate genes with GO annotation were screened,and then GO enrichment analysis was conducted for functional classification.It was found that these candidate genes were annotated into 31 subcategories of GO,including biological processes,cellular components,and molecular functions;The hypergeometric distribution was used for GO enrichment significance analysis.The results showed that 67 and 58 GO entries were significantly enriched in the agronomic traits of flowering Chinese cabbage in2015 and 2016,respectively.Among them,there were 5 and 1 GO entry in the two year repetitions of development degree and maximum leaf petiole length.In 2015 and 2016,2109and 124,6071 and 133 candidate genes were obtained,respectively.Among them,the proportion of GO entry genes in the cytoplasm and single biological process enriched by development degree was close to 40%and 24%,respectively.KO functional significance enrichment analysis showed that 51 and 36 pathways were enriched in 2015 and 2016,respectively.Additionally,2,1,and 1 pathways were repeatedly detected in terms of development degree,maximum leaf length,and maximum leaf petiole length,respectively.