| Populus simonii, indigenous to north China, is one of key forest resources for shelterbelt with a lot of characteristics, such as higher tolerance with drought, low temperature and sterile soil and other adaptability. It is propagating easily, and has long life and higher ability to reproduce with hybridization and other characteristics. Therefore, it is the main tree species in ecological protection and the important parent materials for cultivation of excellent resistance germplasm in "Three North" area of China. The genetic resource of P. simonii is being reducing dramatically owing to the reasons of natural disasters, including drought, water shortage, desertification, deforestation, and man-made factors. Thus, it is urgent to evaluate and conservate the genetic resource of natural population in P. simonii. For this purpose, in this thesis, a total of 528 individuals were collected from 16 provenances, covering 11 provinces or autonomous regions, and then established ex situ conservation pool of P. simonii. The genetic diversity of natural populations of P. simonii were systematically studied at the phenotype, physiology and biochemistry, and DNA levels, and the law of genetic variation of different sources and diversity of the state is revealed, while a sample of the core genetic resource conservation programs is constructed. The results provide the necessary theoretical basis and reference for the rescue and protection of existing P. simonii resources. Interestingly, the association between SSR genotypes and phenotypes of 25 economic traits were conducted by the linkage disequibrium mapping based on natural population in P. simonii. The SSR markers significantly associated with commercial traits provide the solid foundation for screening good alleles, discovering and developing genetic resource and making genetic of policies and strategies in P. simonii. The main research results were as follows:1. The genetic diversity of natural population of P. simonii was studied at the phenotypic level, and the result indicated that variation of 15 phenotypic traits of P. simonii is significant different among provenances and among clones within provenances. The phenotypic coefficients of variation of leaves, stems and roots were 22.91%,35.68% and 40.90%, respectively; In which, coefficient of variation for leaf area (48.79%) was the highest, followed by petiole length (45.32%), leaf thickness (12.89%) and the lateral veins angle (7.38%) were the lowest, and leaf growth characteristics is higher than the others; As for the phenotypic coefficient of variation, the largest one is located in Zhangjiakou (32.15%), and the second one is located in Henan Luoning (31.89%), the minimum one from Shandong Yishui (25.13%) and Qinghai Dulan (24.80%) provenances; the variation of measured characteristics is as following among provenances:North China where are the distribution center (31.92%) and Central China (29.99%) were the highest and other marginalized groups in the Northwest (28.36%), Northeast (28.25%) and Southwest (26.96%) were followed, East China (25.13%) was minimum. Repeatability values showed that leaf, stem and root phonotypical trait is mainly affected by fix environment and gene in moderate and high degree. The scope of variation is 0.456-0.798; the average of all repeatability is no substantial difference which ranged from 0.5 to 0.6. Phenotypic differentiation coefficient is 0.4518; variation (54.82%) among provenances is higher than variation (45.18%) among clones with in provenances, which indicates that clones within provenances, is the major source of genetic diversity of P. simonii. Correlation analysis showed those only 3 traits of leaf width and 4 physiological and ecological factors of elevation have significant relationships. Clustering of 16 P. simonii provenances can be divided into four main groups though UPGMA testing, Zhangjiakou in Hebei and Luochuan in Shanxi each form a separate group, Huzhu, Qilian and Dulan in Qinghai form the third group, and the remaining species constitute the fourth source group.2. The genetic diversity of natural population of P. simonii also detected at the physiological and biochemical levels, and the results revealed that the genetic variation was significant different among population and among clones within provenances for 10 physiological traits of P. simonii. Coefficients of variation of photosynthesis, respiration, and stress resistance was 22.94%,36.61% and 40.25% respectively, of which malondialdehyde (own coefficients of variation 56.47%) is the highest, phenylalanine ammonia lyase (54.48%) followed, chlorophyll A (22.01%) and carotenoids (20.31%) is minimum; various original average coefficient of variation is insignificant, the majority is between 20% and 30%, of which Luochuan (40.71%) and Fuxian (36.61%) in Shanxi province is the highest, Huzhu in Qinghai (24.78%) is minimum; Similarly, the coefficient of variation were the highest among North China which are the distribution center (35.63%), Central China (34.39%) and the northeast (34.59%), other marginalized groups followed, northwest (33.74%) was minimum. All the physiological indicators were mainly affected by fix environment and gene in high degree. The repeatability of each physiological indicator was 0.694-0.986; each original repeatability is no big difference which ranged from 0.8 to 0.9, of which Ningwu in Shanxi (0.929) and Kangding (0.925) in Sichuan was the highest, Tongliao (0.840) in Inner Mongolia and Chaoyang (0.826) in Liaoning is the lowest. Physiological differentiation coefficient was 23.93%,that was P. simonii physiological variation accounted for 76.07% among clones within population, indicating the genetic resource among clones within population is richer than among population. Correlation analysis showed that four physiological indices including catalase and the geographical and ecological factors excepting longitude were highly significant correlation. Clustering of 16 P. simonii provenances can be divided into four main groups though UPGMA testing:Ningwu in Shanxi and Chaoyang in Liaoning formed the first two groups, Huzhu and Qilian in Qinghai and Luochuan in Shaanxi formed the third group, other 13 kinds of provenances including Dulan in Qinghai and Diebu in Gansu was the fourth group; Mantel's tests show phenotype relationship is not significant between Euclidean distance and geographical distance3. The genetic diversity of natural population of P. simonii was studied at the molecular level. In total,306 polymorphic SSR loci were detected using 20 pairs of SSR primers, of which each SSR marker produced polymorphic loci alleles ranged from 8 to 28, and all sites observed number of alleles, effective number of alleles, observed heterozygosity, expected heterozygosity, polymorphism information content, Nei's gene diversity, Shannon information index, fixation index and inbreeding index were 6.981,3.877,0.511,0.691,0.637,0.677,1.443,0.348 and 0.244, respectively. It was very rich in genetic diversity within populations, the genetic parameters were Na = 6.981, Ne= 3.877, Ho= 0.511, He= 0.691, PIC= 0.637, h= 0.677,I= 1.443, Nm= 1.588. Analysis of molecular variance showed that differences in populations was the main source of genetic variation, variance within groups accounted for more than 85%, less than 15% between groups. Nei's unbiased genetic distance and genetic identity varied from 0.442-0.818 and 0.115-0.542 in each group; P. simonii was divided into three groups by clustering, Yishui in Shangdong provenance formed a group of individual, provenances of Qinghai, Fuxian and Ningwu in Shaanxi, were the second group, others formed the third; Mantel test showed that P. simonii genetic distance and geographic distance was no correlation.4. Though three levels of the research content of coupling for P. simonii in phenotype, physiology and DNA molecular markers, it was found that the threes had certain differences in the levels of genetic diversity, it only had a certain agreement that the population genetic structure and differentiation, genetic variation and population diversity cluster, etc. Phenotype of genetic diversity and physiological parameters are not correlated but the SSR markers; the difference among population was the major source of genetic variation, populations greater the genetic diversity within populations was more than among populations; there were abundant genetic variations and the highest genetic diversity in the middle of Shaanxi and Funiu mountains of the Northwest of Henan and so on. However, it opposites in Yimeng of Shandong, Dulan and Huzhu of Qinghai and so on. Huzhu of Qinghai, Qilian, Inner Mongolia and Tongyu of Jilin and so on were consistent on phenotype, physiological and clustering results of SSR marker diversity. The genetic diversity of three markers of the reflection had focused on the situation but complement each other.。5. Combining with the data of genetic diversity at the phenotypic, physiology and molecular levels, the sampling strategy for establishing core germplasm of P. simonii was plotted within populations and between the populations, i.e., samples of more than 12 groups were chose randomly in the whole distribution area in P. simonii, each group kept more than 36 plants. Meanwhile, the genetic resource was constructed for protecting the primary core offsite collection, which has important theoretical and practical significance in the full protecting P. simonii genetic diversity, preventing gene loss and screening of elite germplasm.6. Considering sub-structure of natural population of P. simonii, the association studies between SSR genotypes and phenotypic data of phenotype, physiological and biochemical traits were performed. The results showed that 17 SSR markers associated with 8 traits, including leaf length/width, etc. For each trait, the numbers of associated loci is 1 to 8, related loci on the phenotype of rate of explanation is 1.84%-23.39%; physiological trait association mapping only detected the 6 related loci of 4 trait including chlorophyll B, each trait loci is 1 to 3, explain rate of related loci on the physiological traits'change is large ranging from 11.33% to 26.46%; there was "pleiotropic effects" and "a result of multi-effect" phenomenon in association mapping.7. The alleles of SSR markers associated with phenotype were further analysis through comparing with the phenotype average date of null allele. Alleles potential phenotypic effects of increment (decrement) was estimated, and using the information estimated the average effect of alleles of the site efficiency (less effective). The results showed that there were different among loci positive and negative effects, the average of alleles; the alleles variation of multiple trait loci has their own phenotypic direction and size among different traits. Alleles variation on the direction of effects in the relevant traits, the size of the similarities and differences between characteristics explained the positive and negative correlation of genetic causes; study identified a number of phenotypic and physiological characteristics with excellent sites and the associated alleles and carrying outstanding alleles of the carrier material.
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