Studies On Population Genetics And Phylogeography Of Davidia Involucrata(Davidiaceae)

Davidia involucrata Baill., an ornamental also named "Chinese dove tree" has been known as one of the "living fossils" in the world. It is a Tertiary paleotropical relict plant and has been classified as one of the first-grade-protected plants in our country. The questions about its possible refugia area and genetic structure have attracted the interest of many botanists and biogeographers. However, answers to the questions remain elusive because of limited population sampling and genetic markers used in previous studies. This study focused on the population genetic structure and phylogeography of Davidia using 22 natural populations from across the geographic range of the plant, plastid (cpDNA) trnS-trnG and trnL-rpl32F intergenic spacers, and 7 pairs of SSR loci. In addition, we evaluated the taxonomic position of Davidia involucrata var. vilmoriniana based on data from both morphology and molecular biology. The main conclusions are as follows:1. Analysis based on cpDNA haplotypeTwenty-two populations with 231 individuals were analyzed with cpDNA trnS-trnG and trnL-rpl32F intergenic spacer. A total of 12 variation sites and 17 cpDNA haplotypes were detected. Nucleotide diversity and haplotype diversity are n = 1.92 x 10-3 and h= 0.886, respectively. In the Western region,10 haplotypes were found, in which 7 are unique. In the Eastern region,7 haplotypes were found, in which 2 are unique. AMOVA analysis revealed that most of the variation (39.29%) existed among populations, the level of genetic differentiation is very high (Fst= 0.932).Phylogenetic, TCS analysis and the distribution of cpDNA haplotypes revealed that there are two geographic groups:west and east. Mismatch distribution supported postglacial population expansion in the Eastern region. As for the Western region, significant expansion didn't happen, and the current distribution patterns of this region may be due to the survival in situ of several populations after the glacial period. 2. Development of SSR markersUsing both the modified magnetic beads hybridization method and the dual-suppression-PCR method, we isolated and characterized 12 polymorphic micro satellite loci, obtaining 159 allelic genes from 134 individuals in 5 populations of southwest China. The number of alleles per locus ranged from 6 to 21 (mean= 10.8). The expected heterozygosity per locus ranged from 0.404 to 0.918 and the observed heterozygosity ranged from 0.015 to 0.821. Our results indicated that these microsatellite markers were highly polymorphic and the primer sets developed by the dual-suppression-PCR method possessed higher polymorphic than those by the magnetic beads hybridization method. There was no significant linkage disequilibrium (LD) in the populations. Most loci in the 5 populations are significantly deviated from the HWE due to homozygote excess.Above results shows that the microsatellite markers developed for D. involucrata can be used as genetic markers and have laid a solid foundation for further studies of the population genetics of this famous dove tree.3. Population genetic analysis based on SSR dataA total of 475 individuals from 22 populations in China were investigated using 7 pairs of SSR loci. AMOVA revealed that 64.38% variation exists within populations. Genetic differentiation among populations was detected with Fst= 0.356. Similar with the cpDNA analysis, structure analysis also revealed two major geographical groups: Western and Eastern lineage. The populations (JS, KK, YY) near the junction area of eastern and western regions display mixture of the western and eastern gene pool.Genetic differentiation among populations detected by SSR markers (Fst=0.356) are much lower than the cpDNA level (Fst=0.93228). The reason for such situation may be the nuclear markers tend to eliminate the genetic differentiation compared to cpDNA markers.4. The taxonomy treatment of Davidia involucrata var. vilmoriniana (Dode) Wanger.The morphological analysis shows the fact that the hair varieties of Davidia leaves exist along a continuum. The different group of Davidia individuals with hairs or not can't be distinguished by molecular analysis. Together with the geography distribution and the field investigation results we concluded that Davidia involucrata var. vilmoriniana (Dode) Wanger. should be merge into Davidia involucrata Baill.5. Conservation strategiesFor the protection of Davidia, preserving of the habitat is the first priority. Since there is no gnetic differentiation between Davidia involucrata and Davidia involucrata var. vilmoriniana, we must consider all of them en bloc. According to the genenetic structure detected in this study, we suggested the populations in the weastern and eastern region should be treated differently.