Genome-wide Association Analysis For The Traits Related To Feeding Value Of Rapeseed(Brassica Napus L.)

The rapid development of animal husbandry in China results in a shortage of forage grass, especially during winter and spring, which restricts its sustainable development to a certain extent. Because of its fast growth and high biological output, rape is not only an important oil crop, but also a kind of forage grass. Rape is suitable to be grown in winter and spring with its strong cold resistance, which can effectively alleviate the forage scarcity in such seasons. Growing forage rape can also improve soil fertility and obtain high planting benefit. In order to make efficient use of forage rape and to select and breed the kind with best nutrient content, it is important to analyze its nutrient content with its utilization efficiency in animals as well as feeding effect, and to grasp the genes that control the traits related to feeding value.Based on the population consisting of 300 kinds of rape(Brassica napus L.) with 279 kinds selected except for those which grew abnormally, the research measured and analyzed the content of moisture, CP, EE, NDF, ADF, ADL, AIA, hemicellulose, fiber and NFE with proximate analysis and Van Soest’s method. Analyze the population structure, kinship and linkage disequilibrium(LD) with quantities of SNP markers, then make genome-wide SNP association analysis based on the LD status of the population. The main results were summed up as follows:1. Phenotypic analysis of traits related to feeding value: The research population got extensive phenotypic variations in the 10 target traits related to feeding value, which was in line with the typical characteristics of quantitative traits. Between each two traits, significant or extremely significant phenotypic correlation was observed mostly.2. Analysis of population structure and relative kinship: The study divided the population of 300 materials into two groups named P1 and P2 with 3791 SNP markers. P1 group accounts for 26.2% with 79 materials while P2 73.8% with 221 materials. The results of kinship analysis showed that of all the materials, 57.63% got the kinship value of 0 and 71.78% got it less than 0.05. The kinship value within 0.5 for 99.15% materials in total indicated the weak relationship, which could cause little influence on the association analysis.3. Analysis of linkage disequilibrium: By calculating the r2 value of pairwise markers in the same chromosome, the study simulated the trend lines of LD and found that r2 value declined rapidly while the physical distance increased. It showed that when the threshold value of r2 set as 0.2, the decay of the LD is around 150 Kb in A genome while 750 Kb in C genome, indicating that Brassica rapa. was more widely used in Brassica napus L. breeding.4. Association analysis of the traits related to feeding value: Making a genome-wide SNP association analysis with the MLM model program in TASSEL5.1 software under the Q+K model, the study detected 50 SNP markers associated with the target traits significantly. The markers distribute on different chromosomes, of which, 2 were associated with CP content, 3 associated with EE content, 4 associated with NDF content, 4 associated with ADF content, 1 associated with ADL content, 6 associated with AIA content, 10 associated with hemicellulose content, 15 associated with fiber content and 5 associated with NFE content.5. Prediction of candidate genes: The study analyzed the homologous genome sequences of B. napus with the SNP markers annotated according to the genome information of Arabidopsis thaliana, selecting out 62 candidate genes. Among which, 7 genes i.e. GDH2, GRP17, ADC2, AT1G31150, AT5G06839, AT3G22980 and AT5G06800 are associated with CP content, influencing gene transcription, translation and the biosynthesis of amino acids thus affecting protein synthesis. 9 genes are associated with EE content i.e. OLEO1, AT1G54570, ACBP4, AT3G04360, LTL1, LPP3, AT1G56670, AT5G03610 and AT3G05180, encoding proteins which participate in the physiology activity of lipid to regulate lipid metabolism and accumulation. 11 genes are associated with CF content, in which, XND1, CESA6, CSLA07 and CESA820 affect fiber content by encoding cellulose synthase, other 7 genes i.e. MUCI10, AGP17, AGP12, AGP1, AT4G38940, AT1G55270 and VGT1 affect hemicellulose content through making an influence on the synthesis and transport of saccharides. 39 genes are associated with AIA content such as FSD3, FTRA1, FRO4, ZIP6, KT1, and they encode proteins that mainly adjust the content of ash by affecting the absorption of mineral elements. 6 genes are associated with NFE content i.e. RCA, EULS3, SWEET9, PGP1, AT2G39470 and AT5G54800, encoding proteins which influence the content of NFE by the means of affecting photosynthesis and the synthesis of sugar and starch. The selected candidate genes will lay the foundation of further study in molecular breeding and gene cloning for the traits related to feeding value of rape.