Construction And Evaluation Of Core Collections Of Chinese Cabbage[Brassica Campestris L. Ssp. Pekinensis (Lour) Olsson]

Chinese cabbage [Brassica campestris L. ssp. pekinensis (Lour) Olsson] originated in China. There are extremely abundant germplasm resources of Chinese cabbage in China. The national medium-term genebank of vegetable germplasm resources has conserved the 1691 (including replications) accessions of Chinese cabbage germplasm resources, on one hand, such a large collection is beneficial for study and breeding, on the other hand, it is not good for further to germplasm collection, evaluation, research and utilization. Therefore, it is very important to study the construction and evaluation of core collection of Chinese cabbage and analyze the distribution of genetic diversity and genetic relationship of Chinese cabbage germplasm resources in theory and practice for the further collection of resources, origin and classification study, genetic resources mining and genetic improvement of Chinese cabbage varieties. This study included construction, evaluation and improvement of the core collection of Chinese cabbage germplasm resources based on phenotypic data, and optimized method, construction of the core collection based on molecular data and phenotypic + molecular data, and analysis of the genetic diversity the integrated core collection of Chinese cabbage germplasm resources, the main results are as follows:1. In the first part, 1651 accessions (eliminating replications) of Chinese cabbage germplasm were used. According to the classification system of Chinese cabbage, all germplasm were divided into 6 groups. Based on 43 phenotypic data, 4 kinds of sampling ratio methods in group, 6 kinds overall sampling scales and 2 kinds of sampling methods were compared. The results showed that: the diversity ratio method in group could make more balance of the distribution of samples and better conservation of variability from the original collection. The genetic diversity index of core collection constructed was the largest and the proportion of the phenotype retained could reach 98 percent according to the overall sampling scale of 15 percent and the diversity ratio method. While the overall sampling scale increased to more than 20 percent, the genetic diversity index of core collection rapidly decreased although the proportion of phenotypic retained was closed to 100 percent. Therefore, 15 percent of the overall sampling scale was more appropriate. In certain sampling ratio method in group and overall sampling scale, I, RPR and CV of the core collection constructed by cluster sampling were higher than those by the random sampling. According to the optimized sampling stratege, a core collection of 248 accesions of Chinese cabbage germplasm based on phenotypic dates was established.2. In the second part, genetic representativeness of the 248 phenotypic core collection of Chinese cabbage germplasm to the original collection was evaluated by aid of phenotypic genetic diversity index, phenotypic reservation proportion, accordance rate of the range, average and standard deviation. The results showed that: the phenotypic genetic diversity index of the core collection and six groups (cylinderical-type, flat-type, oval-type, flower core, semi-heading and non-heading Chinese cabbage, flower Chinese cabbage and semi-heading Chinese cabbage) were 0.7114, 0.6464, 0.3010, 0.6979, 0.6012, 0.6418 and 0.6287 respectively, which were higher than those of the original collection. The average phenotypic reservation proportion of 24 characters was 91.67 percent. Accordance rate of the range, average and standard deviation of 18 characters could all reach more than 85 percent. Therefore, the abundance ratio and evenness of variation were both better than those of the original collection. The core collection was a good representation of the original collection in genetic diversity. In respect of germplasm geographical distribution, all accessions of the original collection came from 27 provinces, autonomous regions and municipalities. Only Anhui, Chongqing and Tibet had no contributed to the core collection distribution, because there were few accessions in these three regions. Geographical coverage rate reached 88.89 percent. According to the evaluation results of the phenotype and geographical representativeness, the core collection was added to 253 accessions. According to the geographical source of germplasm in the core collection, 4 Chinese cabbage production areas were classified: (1) North China and Northeast production areas with the major type of cylinder-type and non-heading Chinese cabbage. (2) East China production areas with the oval-type Chinese cabbage. (3) Sino-South China production areas with heading Chinese cabbage. (4) Southwest China production areas with flower core Chinese cabbage and semi-heading Chinese cabbage.3. In the third part, a total of 255 bands were detected with 45 pairs of SSR primer in the primary core collection of 501 accessions of Chinese cabbage germplasm. Among them, 232 bands (90.98 percent) were polymorphic, 5.66 bands and 5.16 polymorphic bands were amplified per primer. On this basis, 8 pre-core collections were formed in the light of 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% sampling size. Through analysis, it was showed that the ratio of allele variations retained and phenotypes retained increased with the increase of sampling scale, but increased greatly before 50 percent and increased slowly after that. Shannon-weaver diversity index and phenotype genetic diversity achieved maximum value at the 50 percent of sampling scale. Therefore, 50 percent of the sampling scale was believed as the best compression ratio. A molecular core collection of 251 accessions of Chinese cabbage was constructed, which accounted for 15.2 percent of the original collection.4. The primary core collection of 501 accessions was used as studied materials, after that, 2 candidates core collections (C3 and C4) were constructed, based on the phenotype + SSR data, and clustering compression sampling and completely random sampling. Using the Nei's allele diversity index, the number of alleles observed, the effective number of alleles, Nei's genetic diversity index and Shannon's information index to compare the phenotype core collection (C1), the molecule core collection (C2) and before 2 candidates core collection (C3, C4), the results showed that: C1, C2 and C3 were ranked at the first place in the light of genetic diversity, C4 in the second place. In other words, based on the phenotypic data, molecular marker data and phenotypic dats + molecular marker dates, clustering compression all could achieve very good results. The phenotypic data reflects the effect differences of the outside environmental conditions to some extent, and the molecular marker data reveals the differences at the plant genome DNA level. So when constructing of Chinese cabbage core collection, the environmental and genetic interaction effect should be taken into account. At last, the comprehensive core collection of 251 accessions of Chinese cabbage germplasm had been selected as the ultimate core collection, based on the phenotypic data + molecular marker data. 5. The molecular diversity of 251 the comprehensive core collection of Chinese cabbage which was based on SSR + phenotype data was analyzed. The results indicated that: the number of alleles (Na) was 1.8627 ~1.9059, the number of effective allele (Ne) was 1.5859~1.6167, Nei's genetic diversity was 0.3303~0.3478, Shannon's information index was 0.4860~0.5087, the overall expected heterozygosity (Expes Het) was 0.3185, its range was 0.3036~0.3204, the range of Simpson index and the Shannon - weaver index were 0.0926~0.4763 and 0.0986~0.6860. The value of these various indicators of Chinese cabbage, non-heading Chinese cabbage was the highest, followed by heading Chinese cabbage, flower core Chinese cabbage and semi-heading Chinese cabbage. Therefore, the degree of abundance of effective allele variations descended in the following order: non-heading Chinese cabbage > heading Chinese cabbage > flower Chinese cabbage > semi-heading Chinese cabbage. Non-heading Chinese cabbage was better than other three groups in both the degree of abundance ratio and evennessuniformity of variation, so it could be thought that non-heading Chinese cabbage is the most diverse in genetic differences germplasm.Finally, by cluster analysis based on the Nei's genetic distance and Nei's genetic consistency of 4 Chinese cabbage groups, after cluster analysis, the results showed that: semi-heading Chinese cabbage and non-heading Chinese cabbage were classified as a group respectively, the former had the farthest in genetic distance and the lowest genetic consistency to the rest two groups, the latter was the second distant; Heading Chinese cabbage and flower core Chinese cabbage had the closely genetic relations. There were similar morphological and geographical types of germplasm resources in aevery group which were usually similar in morphology and geographics, but there were also different types of germplasm in the same group.