| Tea (Camellia sinensis (L. ) O. Kuntze) is an important economic and beverage crop in China. Tea plant is originated in the southwest area of China. Tea has wide genetic variation owing to the long time of spreading from the original center to different ecological environments through natural selection and human intervention. As a result, diverse tea germplasm were developed. Moreover, long-term allogamy and self-incompatibility contribute to high heterogeneity in tea plant. It takes a long time for the successful breeding of a new tea cultivar. It thus becomes important to improve the efficiency of early appraisal of individual selection and accurate evaluation of target traits in a breeding programme. In order to accelerate improvement of tea, therefore, a good understanding of the genetic diversity and population structure of tea germplasm is a prerequisite. Besides, germplasm collection, preservation, special gene discovery and cultivar selection are based on genetic diversity and population structure studies of genetic resources. Association analysis based on the linkage disequilibrium offer a new method for identification of loci controlling traits of interest. The approach explores the association between molecular markers and key economic traits. This study revealed tea genetic diversity and population structure through phenotypic markers and EST-SSR markers. The main objective of the study was to identify the associated loci with phenotypic variation, which offer a new method for marker assisted selection in tea cultivar selection. The main results are summarized as follows:1. Genetic diversity evaluation of tea germplasm based on phenotypic markersVariation analyses were carried out among tea leaf, two and a bud and biochemical components. Results showed that the level variation of numerical traits was higher than non-numerical traits. The shannon-weave index (H') of phenotypic traits was from 0.73 to 2.13. The mean H'for non-numerical traits was 0.91, while that for numerical traits was 1.89. There was obvious and diverse variation among phenotypic traits. Coefficient of variation varied from 16.06 to 195.70, in which catechin was the highest but caffeine was lowest. Principal component analysis showed that the contribution of each trait was multi-directional and the growth of cumulative contribution rate was not obvious. Based on the phenotypic variation data, tea germplasm were clustered into 5 groups. For tea accessions in group I and II, leaf and shoot traits were larger and the content of biochemical components was higher, especially the ratio of non-ester catechins was higher. However, the size of tea leaf, shoot and the content of biochemical components were lower for tea accessions in fifth group. The mean of variation coefficient of resources from different provinces varied from 21.5% to 39.5%, which Hunan was highest and Henan was lowest. The genetic diversity of Guangxi, Yunnan and Guangdong was higher, the mean of H'higher than 1.5, Henan, Anhui and Hunan were lower, the mean of which were 1.1, 1.1 and 1.2, separately. Variation and diversity analyses were carried out among wild germplasm, landraces and improved cultivars. The level of genetic diversity and variation among wild tea was higher than that among landraces, improved cultivars. 2. Genetic diversity research of tea germplasm based on EST-SSR markersAll selected 110 primers were polymorphic and 457 alleles were amplified with 4 alleles per primer pair on an average. The mean gene diversity of all primers among the tested germplasm was 0.58, ranging from 0.03 to 0.91, TM146 was lowest, while TM210 was highest. The polymorphic content (PIC) varied from 0.03 to 0.91, with an average of 0.55. Tea germplasm were divided into three groups based on Nei's genetic distance. The tea germplasm in the first group were from Yunnan, Sichuan, and Hubei, while the second and third groups were from Guizhou, Chongqing, Guangxi, Guangdong and Fujian, Zhejiang Hunan, Anhui, Jiangsu, Henan, Jiangxi, separately. The genetic diversity of Yunnan, Guangxi, and Guangdong was higher, the mean of Nei diversity index were 0.46, 0.45 and 0.45; Hunan, Jiangsu, Anhui and Henan were lower, the mean of which were 0.32, 0.34, 0.34 and 0.34, separately. The diversity was similar to the results based on phenotypic markers. The level of genetic diversity and heterogeneity among wild tea was higher than that among landraces, improved cultivars and breeding lines, which was same to the result based on phenotypic markers.3. Association analysis between EST-SSR markers and target traitsTea genome linkage disequibrium was estimated by EST-SSR markers. Four thousand and six hundred fifty-six pairs of loci were detected LD among the 110 EST-SSR loci, 301 of which include high level LD (D'>0.5). The mean of r2 between loci was 0.01 and D'was 0.28. A model-based population structure analysis subdivided the tea germplasm into five populations. Association analysis was carried out between the target traits and EST-SSR markers by Q-value (k=5) of each individual as covariates. A total of 71 loci were detected to be significantly (p<0.01)associated with 21 accumulated traits, out of which 39 loci significant at a 0.001 level associated with 15 accumulated traits. |
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