| As members of the class of “basal angiosperms” defined by plant phylogeny, plantsbelonging to the order Magnoliales have been keys to understanding the evolutionary history offlowering plants. The genus Liriodendron within the family Magnoliaceae, is primarilyrepresented by fossils from the late Cretaceous and early Tertiary of North American and CentralAsia. The only two extant members of Liriodendron are L. chinense Sarg. and L. tulipifera Linn.L. chinense and L. tulipifera are a relic species pair with a typical discontinuous distributionacross East Asia-North America. They have important scientific value for studying on plantpopulation genetics and molecular phylogeography. In this paper, microsatellite markers wereused to quantify the genetic variation, genetic structure, and genetic differentiation among naturalmature populations of L. chinense and L. tulipifera. The author also compared the level of geneticdiversity between natural mature populations of L. chinense and their corresponding offspringpopulations. Furthermore, the genetic variation pattern of L.chinense was dissected into threelevels of populations, families within population, and individuals within family. In addition, thegeographical distribution and possible refugia area in history were deduced using DNAsequences of3genes as chloroplast DNA （cpDNA） psbA-trnH and trnT-trnL intergenic spacersand the nuclear ribosomal DNA ITS region.The main conclusions are as follows:The population genetic structure of L. chinense. To better understand the genetic structureand differentiation among remnant populations of L. chinense, the author determined thegenotypes of14simple sequence repeats （SSRs） loci across318individuals from12naturalpopulations. It showed that L. chinense maintained high genetic diversity （He=0.7385） withinpopulations but moderate genetic differentiation （Fst=0.1956） and low gene flow （Nm=1.0283）between populations. This indicates that L. chinense may have mechanisms to maintain itsgenetic diversity. Moreover, significant bottlenecks in six populations were detected in theapplication of two-phased model of mutation （TPM）. A Mantel test revealed a statisticallysignificant correlation between the geographic distances and genetic distances betweenpopulations （r=0.5011, P=0.002）. Hence, the author presumes that geographical isolation andhabitat fragmentation might contribute jointly to current population structure of L. chinense. Theauthor also suggests that the populations from southern Yunnan can be regarded as a variatas of L.chinense, given their large deviation of phenotypic character and allelic variation from otherpopulations.The comparison on the level of genetic diversity between L. chinense and L. tulipifera. Theresults showed that plants in Liriodendron maintained a high level of geneticdiversity（He=0.7793）. The genetic diversities of natural population and provenances of L.tulipifera were higher than those of L. chinense. L. chinense maintained high levels of genetic differentiation （Fst=0.2328） and low levels of gene flow （Nm=0.8239） among populations,while L. tulipifera maintained low levels of genetic differentiation （Fst=0.0875） and high levelsof gene flow （Nm=2.6077） among populations. To elucidate the genetic relationships amongpopulations studied, the average genetic distances were used to generate a UPGMA tree. Thethirteen natural populations were grouped into two distinct clusters. Two populations （YN-JP andYN-MLP） from southern Yunnan formed a cluster, and the remaining eleven populations formedanother cluster. This supported that L. chinense from southern Yunnan have a large geneticdifferentiation to other populations.The comparison of genetic diversity between natural populations and their offspringpopulations in L. chinense. The genetic diversity of natural populations （Ne=4.14, He=0.74） washigher than that of their offspring populations （Ne=3.51, He=0.68）. The genetic differentiationcoefficient （Fst） among offspring populations was0.1722, which was significantly higher thanthat of natural populations （0.1254）. It indicated that the genetic variation of L. chinense mighttend to enlager. And, the level of gene flow （Nm=1.7432） among natural populations was largerthan that among offspring populations （Nm=1.2017）.The genetic variation pattern of L. chinense. To understand the genetic variation pattern of L.chinense, the overall genetic variation was dissected into three hierarchical levels of populations,families within population and individuals within family. In this section, five natural populationsof L. chinense were chosen as sample populations, in each population seeds from five maturetrees were harversted, thus five half-sib families each population were obtained.29-30seedlingseach family were sampled for the detection of14SSR loci and measurement of tree height.Population JX maintained the highest genetic diversity (Nei=0.63), while population XY had thelowest genetic diversity (Nei=0.41). To the class of families, XY26had the lowest geneticdiversity (Nei=0.2487), while ST27the highest (Nei=0.5642). As to the level of individuals,LP25-27had the highest genetic diversity (Nei=0.4456), XY3-2has the lowest (Nei=0). Most ofgenetic variations were contained within population. AMOVA analysis revealed that there wassignificant genetic differentiation existing among three hierarchical levels （populations, familieswithin population and individuals within family） in L. chinense, indicating limited gene flowamong populations. Among the overall genetic variations of L. chinense,16.47%,20.27%and62.76%variations were explained by populations, families within population and individualswithin family respectively. Moreover, the variation pattern on tree height was also analyzed, anda pattern similar to the AMOVA result was acquired.The molecular phylogeography of Liriodendron. The molecular phylogeography ofLiriodendron was investigated using allelic variation of three genes, two from chloroplast DNA（cpDNA）（psbA-trnH and trnT-trnL） intergenic spacers and one from the nuclear ribosomalDNA ITS region.Totally,148individuals from31populations with of Liriodendron were analyzed withcpDNA psbA-trnH and trnT-trnL intergenic spacer. The variation loci and nucleotide diversityof L. chinense （S=29; π=0.004135, θw=0.005305） were more than that of L. tulipifera （S=19;π=0.00204, θw=0.004125）.61.424%of the total genetic variation was distributed among populations of L. chinense, indicating that L. chinense contained a high level of genetic variation.While in L. tulipifera,13.855%of the total genetic variation was distributed among populations.Results of neutrality tests （Tajima’s D, Fu and Li’s D*and F*） supported the idea thatpopulations in Liriodendron underwent a process of expansion in histery. Analysis on TCS,phylogenetic and the distribution of cpDNA haplotypes revealed that both L. tulipifera and L.chinense underwent a process of population expansion, though their expansion routes werediverse. In the glacial period, two possible refugia of L. chinense were Ta-lou Mountains for thewestern populations, and south slope of Mount Wuyi for the eastern populations. The presenteastern populations of L. chinense might origin from the two populations of HB-EX andJX-WYS.Similar to most angiosperms plants, the length of ITS region in Liriodendron ranged from808bp to852bp, with larger variation ranges in L. chinense. A total of90allelic mutaions and53informative loci were detected in Liriodendron. Both the number of allelic mutaions andinformative loci of L. tulipifera （S=62, I=51） were higher than that of L. chinense（S=46, I=12）,similar to the nucleotide diversity. Neutrality tests （Tajima’s D, Fu and Li’s D*and F*） supportedthe viewpoint that both species in Liriodendron had undergone population expansion in history.Taking Magnolia denudata as outgroup plants, the phylogeography of Liriodendron was alsoinvestigated using nrDNA ITS sequences. All individuals in Liriodendron were basicallyclassified into two branches of L. chinense and L. tulipifera. The results suggested that the twospecies in Liriodendron, the North American species and the East Asia species, might haveundergone population expansion and evolved independently. |
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