Analysis On Genetic Diversity, Heterosis, Combining Ability And Origin Of Tuber Mustard (brassica Juncea Var. Tumida Tsen Et Lee) Germplasm

Tuber mustard (Brassica juncea var.tumida Tsen et Lee), a variation in the species Brassica juncea belonging to the genus Brassica in the Cruciferae family, is an agriculturally and economically important crop merely cultivated along the Yangtze river in China. Fuling hot pickled tuber mustard, a product of mustard tuber, as well as European pickled vegetable and Japanese pickles, are hailed as the three most famous pickled vegetables in the world. In present study analysis of phenotypic characters and SSR molecular markers were made to invetigate the genetic diversity in 133 tuber mustard accessions. Fifteen tuber mustard inbred lines were used to detected their genetic diversity by SSR markers, among them, seven inbred lines (male) and eight sterile male inbred lines (female) were crossed to get 56 hybrids. The heterosis and combining ability were analyzed among these hybrids. And the low-copy nuclear Chs gene sequences of seventy-two brassica accessions were used to estimate the phylogenetic relationships among tuber mustard and its related genera. The main results are as follows:(1) The results showed that the variation coefficients of the twenty-one phenotypic characters changed in a small range except the number of bilobed leaf, depth of inter-nodule and stem weight per plant.(2) The principal component analysis of the twenty-one phenotypic characters showed that the first eight principal components can explain the genetic variation with 87.41% which can summarize the total information of the twenty-one phenotypic characters. From the first eight principal of the 133 tuber mustard accessions and the characteristic of each accessions, we can see that each tuber mustard accession has its characteristics correspondingly which could be used in the conservation and breeding of tuber mustard.(3) The results of system cluster analysis of 21 phenotypic characters showed that the genetic distances among the 133 tuber mustard accessions ranged from 0.01 to 7.77, with an average of 2.50. It was indicated that the genetic distance among the accessions raged in a small degree and the mean among them was small with 69.5% of all genetic distances were less than 4.00. Cluster analysis indicated that all the tuber mustard accession could be divided into eight Clusters. So litter Clusters were obtained and Cluster 3,6 and 8 included 81.20% of all accessions. It was indicated that the genetic distance was small and the genetic diversity was narrow among the 133 mustard tuber accessions.(4) Eighty-one pairs of SSR primers from 600 pairs of SSR primers in Brassica produced stable amplified bands and 810 bands were detected among the accessions with 724 polymorphic bands, accounting for 89.38%. The average number of bands per locus was 10.0 with a range from 5 to 16. The observed number of alleles was 2846 with 35.08% was effective number of alleles. The average number of observed number of alleles per locus was 35.14 with a range from 6 to 52. The values of Shannon's information index for each SSR locus varied from 0.52 to 3.72 with an average of 2.74. The coefficients of genetic similarity in the SSR marker patterns among the 133 accessions ranged from 0.77 to 0.91, with an average of 0.85. There were 5793 pairs of genetic similarity, in which 65.99% were 0.85 to 0.91. The cluster analysis showed that the accessions could be classified into six clusters when the genetic similarity was 0.83, with 90.23% of the accessions included in Cluster 5 and Cluster 6. It was also indicated that the similarity was high and the genetic diversity was narrow among the 133 mustard tuber accessions.(5) Principal component analysis was carried on based on the SSR data. The result showed that the first three principal components can explain the genetic variation with 85.47%,0.67% and 0.61%, and the 133 accessions could be divided into six Clusters according to the nearest phylogenetic relationship. Cluster 1 included the accessions 50 and 86. Cluster 2 included the accessions 8,11 and 64. Cluster 3 included the accessions 14,15 and 65. Cluster 4 included the accessions 9,26,99 and 125. The remains were included in Cluster 5 and 6. It was also indicated that the genetic diversity was narrow among the 133 mustard tuber accessions.(6) The accessions of mustard tuber in this study can be divided into up-Yangtze river group, mid-Yangtze river group and down-Yangtze river group based on their geographical distributions. According to analysis of phenotypic characters and SSR data, the order of genetic diversity level in the three groups was mid-Yangtze river group>up-Yangtze river group> down-Yangtze river group. The accessions of the mid-Yangtze river group was in the central region of the 3-D dendrogram by principal component analysis. It was presumed that the origin center or genetic diversity center of tuber mustard was the mid-Yangtze river valley, and the crop was transmited along the Yangtze River to it's upstream and downstream.(7) The analysis of combining ability indicated that the GCA relative effects among different parents in the same trait and among different traits in the same parent showed great different. The GCA relative effects of stem yield among different parents ranged from-29.88 to 13.48. There were five effects showed significant positive, accounting for 33.33%. Two male inbred lines (60 and 156), and three male sterile inbred lines (133-2A, 118-3A and 145-1A) had significant positive (P< 0.01) GCA relative effect. The SCA relative effects ranged from -35.86 to 39.93 with twenty-four effects were significant, accounting for 42.86%. The heterosis among parents was related to GCA and SCA. The hybrids with strong heterosis had excellent SCA or the parents had excellent GCA, or had both excellent SCA and GCA. During the using of heterrosis in tuber mustard, therefore, it should pay attention to both GCA and SCA.(8) The hight-parent heterosis of stem yield ranged from -46.35% to 77.22% with an average of 6.24%. There were 64.29% hybrids showed positive and significant hight-parent heterosis. It was indicated that there were significant heterosis of stem yield among parents. The results also showed some strong heterosis were existed, it indicated that two male inbred lines (19 and 60) and three female inbred lines (140-5A,122-3A and 133-2A) were the elit inbred lines which can be used to develop strong superiority hybridized hybrids. The correlation coefficient among the stem yield, hight-parent heterosis SCA were big and highly significant, which indicating that the heterosis can be reflected by SCA. The correlation coefficient of genetic distance based on phenotypic characters and SSR data with stem yield, hight-parent heterosis and SCA were small and not significant. It couldn't use the genetic distance based on phenotypic characters and SSR markers to predict the heterosis and combining ability in tuber mustard.(9) The sequences of the single-copy nuclear gene Chs were used to study the phylogenetic relationships among tuber mustard and its related plants. Sequence analysis indicated that the one hundred and seventeen Chs gene sequences could be classified into 11 branches. The split analysis indicated that there not only existed tree evolution, but also existed reticulate evolution among tuber mustard and its related plants. There were many reticulate evolution events during the evolutionary history of tuber mustard and its related plants. Median-joining(MJ) network analysis indicated that Brassica pekinensis was the parent donor of A genome in tuber mustard, while Brassica nigra was the other parent donor of B genome in tuber mustard. It was presumed that tuber mustard was origin from a natural mutation of Brassica juncea by artificial domestication and selecting, while the certain Brassica juncea was a crossing of Brassica pekinensis and Brassica nigra.