Genetic Diversity Of Liquidambar Formosana In Northern Guangxi

Liguidambar fornosana is a deciduous broad-leaved tree of Liquidambar genus of Hamamelidaceae.It is one of the important native tree species in China.It is widely distributed in the south of the first line of the Huaihe River in Qinling Mountains.The altitude has high medicinal,ornamental and timber value.Based on the transcriptome sequencing of Liquidambar formosana SSR,18 pairs of ESR-SSR primers with good polymorphism and stable amplification were developed,and the following studies were carried out by using these18 pairs of SSR primers:The effect of habitat fragmentation on plant genetic diversity in Karst area;effect of altitude gradient on genetic diversity of Liquidambar formosana and genetic diversity level of Liquidambar seed orchard in northwest Guangxi.The results are as follows:（1）A total of 23777 SSR loci were found in Liquidambar formosana SSR Unigene based on transcriptome sequencing,and the proportion of mononucleotide repeat SSR loci was the highest（46.54%）.In terms of repetition frequency,SSR loci between 5 and 12 times accounted for the highest proportion（72.36%）.A total of262 SSR primers were developed,and the effective amplification rate was 53.1%.Finally,18primers with stable amplification and clear bands were screened.The results of polymorphism detection showed that all loci were polymorphic.The results of genetic diversity of natural populations showed that the variation ranges of allele number（Na）,effective allele number（Ne）,Shonnon information index（I）and observed heterozygosity（Ho）in the natural population were 2～4,1.1128～2.6096,0.2089～1.1127 and 0.2759～1.0000,respectively,and the average values were 2.3333,1.9574,0.7085 and 0.7226,respectively.In summary,the repeat types and motifs of SSR loci in Liquidambar formosana were basically the same as those in other species.The 18 pairs of EST-SSR primers developed in this study can meet the needs of population genetics research of Liquidambar formosana,and provide abundant marker primers for subsequent genetic diversity research of Liquidambar formosana.（2）Long-term human production activities made the fragile habitat fragmentation in karst area of northwest Guangxi,which hindered the gene exchange between natural populations of Liquidambar formosana in this area,resulting in the decrease of genetic diversity of each subpopulation and large genetic differences between subpopulations.A total of 119 alleles were detected by 18 pairs of SSR primers of Liquidambar formosana in the genomic DNA of420 samples from 14 subpopulations in karst area of northwest Guangxi,with an average of6.61 alleles per pair of primers.The genetic diversity of 4 subpopulations in Pingmen continuous habitat and barley continuous habitat was higher than that of 10 subpopulations in fragmental habitat.There were significant differences in expected heterozygosity（He）（P=0.045<0.05）and Nei’s diversity index（H）（P=0.044<0.05）between subpopulations in the region.The inter-subpopulation gene flow（Nm=1.13）in Pingmen and the inter-subpopulation gene flow（Nm=0.41）in barley were higher than those in 10 subpopulations（Nm=0.26）and14 subpopulations（Nm=0.29）in whole karst area.Habitat fragmentation resulted in significant genetic differentiation among Liquidambar formosana populations in karst area(G_ST=0.69).（3）The genetic diversity of natural populations of Liquidambar formosana at different altitudes was compared between Jiuwan Mountain and Fenghuang Mountain in northwest Guangxi.It was found that the altitude gradient had no significant effect on the genetic diversity of Liquidambar formosana,but it was negatively correlated with the altitude gradient to a certain extent,but did not reach the level of aboriginality.However,geographic isolation and human disturbance can lead to large genetic differentiation of natural populations of Liquidambar formosana at different altitudes(Jiuwan mountains:G_ST=0.79,Nm=0.27;Fenghuang Mountain System:G_ST=0.66,Nm=0.36),especially the maple subpopulation at lower altitude was most seriously disturbed by human disturbance.（4）The total population of Liquidambar formosana in northwest Guangxi still maintained a high level of genetic diversity（Na=7.39,Ne=4.56,He=0.73,I=1.61,H=0.73,PPB=100%）.There was a large gene flow（Nm=3.91）and a certain degree of genetic differentiation(G_ST=0.25)among the subpopulations of Liquidambar formosana in northwest Guangxi.The large gene flow offsets the decrease of genetic diversity caused by genetic drift to a certain extent,and reduces the genetic differentiation between subpopulations through pollen or seed transmission,which is the basis to determine the genetic diversity within and between populations.（5）Compared with the total population of Liquidambar formosana in northwest Guangxi,the genetic diversity parameters of seed orchard population decreased.A total of 70 alleles were amplified by 18 pairs of SSR primers from 122 clones in the seed orchard,which inherited about 56.5%of the overall genetic diversity of Liquidambar formosana in the region.The main reason is that the establishment of seed orchard is based on the selection of superior trees,and the selection process is mostly based on phenotypic data,so it will inevitably lead to the loss of some alleles.On the other hand,it was found through field investigation that the number of natural populations of Liquidambar formosana in this region was rapidly declining due to artificial logging and habitat destruction,which was also one of the important reasons for the loss of alleles.Therefore,in the later stage,the geographical range of selection should be expanded and the number of clones in the seed orchard should be increased to improve the genetic diversity level in the seed orchard,so as to provide help for the preservation and utilization of high-quality resources of Liquidambar formosana.