Patterns Of Genetic Variation And Their Underlying Evolutionary Model Simulation Of Toona Ciliata (Meliaceae) In China

Toona ciliata is a deciduous or semi-deciduous tree of the family Meliaceae and genus Toona.As one of the precious timber species,it is also a key protected wild endangered plant in China.It declines owing to natural habitat fragmentation,deforestation and over exploitation.The genetic protection of natural provenances of T.ciliata is now a focus of our current research.In my thesis,I applied both molecular markers and theoretical modeling to analyze genetic variation in natural distribution of T.ciliata in China.In the part of study with molecular markers,both nuclear ITS（Internal transcribed spacer）and mitochondrial DNA（mt DNA）markers were selected to investigate the genetic variation of T.ciliata.The ITS markers were amplified by designed ITS5a-ITS4 primers,and the mt DNA markers were screened and selected by two pairs of primers（26S-r RNA＿t RNA-Leu and 26S-r RNA＿t RNA-Leu）.Twenty-nine populations across the whole natural distribution of the species in China were studied.Results showed that a high level of population genetic differentiation occurred among 29 populations,with the estimate of F_stbeing 0.854-0.914 for mt DNA markers and 0.615-0.766 for ITS markers.Genetic differentiation measured by N_st was significantly greater than that measured by G_st,indicating the presence of phylogeographical structure in natural distribution of the species.This pattern was also confirmed by the isolation-by-distance（IBD）tests.Genetic diversity analysis showed that a relatively high level of genetic diversity existed in Baoshan,Yunnan province in Southwest China.Cluster analysis based on ITS sequence differences indicated two distinct regions（eastern and western part）with significant IBD effects,the same as previous reports assayed with different nuclear markers.However,the analysis based on mt DNA markers indicated three distinct regions with significant IBD effects where two smaller regions were subdivided from the western part derived from ITS marker analysis.Such differences were likely related to the difference between ITS and mt DNA markers,including the mode of inheritance,mutation rate and genetic drift effects.Further analysis from comparing F_stvalues derived from mt DNA vs.ITS markers implied that gene flow was contributed mainly from pollen flow in the western region but was comparable from both seed and pollen flow in the eastern part of the natural distribution of T.ciliata.The neutrality test indicated that ITS markers were non neutral,with a significantly negative value of Tajima’s D（-1.973,P-value<0.05）calculated with all populations.Most populations tended to expansion,with negative values of Tajima’s D.A few SNP loci had the expected heterozygosity that was negatively correlated with longitude but positively correlated with the frost-free period and annual average temperature,and exhibited high expected heterozygosity in areas with lower longitude and latitude but higher temperature in Southwest China.Sequence mismatch analysis indicated that a few local populations exhibited expansion,including population Xilin（XL）in Guangxi and Jingxian（JX）in Anhui Province.The observed heterozygosity was greater than the expected heterozygosity at most SNP loci in each population.Test of bottleneck effects detected that almost all populations had past reduction in population size.Both theoretical analysis and simulations were conducted to analyze the influences of mating system on gene spread in one-dimensional homogeneous space.Both theoretical and simulation results show that selfing or inbreeding system can impede to some extent the spread of nuclear adaptive genes or cytoplasmic adaptive genes.Outcrossing system favors the wave of advance of advantageous genes.Neutral gene spread can be impeded owing to the genetic linkage disequilibrium generated by selfing or inbreeding when the neutral gene is tightly linked with the adaptive gene.However,the genetic hitchhiking effects break down by recombination,and the wave of advance of neutral genes goes down.The above theoretic results predict that the mixed mating system with predominant outcrossing in T.ciliata enhances its expansion in space.Taken together,we need to pay attention to the areas with high genetic diversity in Southwest China in genetic conservation and resource management of the threatened species in China.Besides,attention is needed to pay to other influences on genetic diversity,including the population expansion in the future,a potential trend of expansion to the northeast,the change of environmental factors（e.g.,the temperature in the native distribution areas）,and the genetic mixture due to gene flow from artificial forests.