Molecular Phylogeny Of Genus Povulus L. And Biogeography Of Three Aspen Species

Inferring phylogeny and speciation process of plant species is one of the mostfundamental problems in plant evolution biology. Reconstruction of phylogeny will dissectrelationships among species and understanding speciation process is a prerequisite to explainorigin and variation of species diversity. It is of great helpful to infer the divergence and originof species accurately in detecting the biogeography history. In the past decades, advance onboth biological techniques and theoretical methodologies have remarkably reshaped ourunderstanding of phylogeny, speciation and biogeography history among model organisms,such as Arabidopsis, maize and rice. In tree species, researches on the phylogeny, speciationand biogeography history of different lineages using the variance pattern of nucleotidesequence have accumulated, however, rareness has been reported in model tree species,Populus. In this study, using multiple nucleotide loci, the molecular phylogeny of Populus andthe speciation model and biogeography of three aspens in section Populus, P. davidiana, P.tremula and P. tremuloides were explored. The main results are as follows:1)24single-copy nuclear markers were developed through bioinformatics methods basedon whole genomic sequences of P. trichocarpa and Salix arbutifolia. Twenty individuals of P.davidiana and S. matsudana were used to evaluate the basic application of15loci with respectto marker length and diversity indices, respectively. Results indicated that all the15locicontained abundant high phylogenetic information and fitted the null hypothesis of neutralevolution well. The utility of single-copy nuclear markers will facilitate subsequent studies onphylogeny, population genetics and biogeography, as well as the evolutionary dynamics ofSalicaceae.2) The complex reticulate evolution (hybridization, introgression) and highly variableintraspecific morphological traits have posed difficulties in resolving the phylogeny of Populus.Based on a large data set of nuclear and chloroplast DNA sequences, we reconstructed robustphylogeny of Populus using maximum parsimony, maximum likelihood and Bayesian inference methods. The resulting phylogenetic trees showed better resolution at both inter-andintra-sectional level than previous studies. The results revealed monophyly of Populus.CpDNA based phylogenetic tree resulted in two main clades, one consisted section Populus,Leucoides and species of section Aigeiros and Tacamahaca, the other comprised of sectionTuranga and the remaining species of section Aigeiros and Tacamahaca. This topologysuggested an early divergence of the maternal progenitors of Populus. In the nuclear DNAbased phylogenetic tree, section Turanga occupied the basal position following sectionPopulus and Leucoides, section Aigeiros and Tacamahaca took the terminal position. SectionPopulus and Turanga were monophyly and section Aigeiros and Tacamahaca were polyphyly.The three advanced sections, Populus, Aigeiros and Tacamahaca may be hybridized origin.Comparative analysis of the cpDNA and nuclear DNA based phylogeny elucidated that somespecies were hybrid origin, such as P. tomentosa, P. nigra and P. deltoides. Realizing the hybridorigin of species in Populus will conspicuously promote our understanding the reticulateevolution in Populus, which may have played a significant role in the evolution history ofPopulus by facilitating rapid adaptive radiations into different environments.3) Morphological traits and relationships among P. davidiana, P. tremula and P.tremuloides were close, which caused huge controversy on the taxonomical delimitation of thethree aspens. Furthermore, speciation model and biogeography of these three aspens havenever been reported before. In this study, these questions were addressed using multilocusnucleotide sequences and multiple samples. All the three aspens showed high nucleotidediversity, which was comparable with that of annual plant species in some loci. Significantgenetic differentiation was detected among the three aspens and Structure results showed thebest clustering model when K=3. Furthermore, no shared cpDNA haplotype was probed. Basedon these findings, we proposed that previous taxonomic classification which classified P.davidiana as a subspecies of P. tremula or classified these three aspens as one species wasinappropriate. The three aspens should be delimitated as three single species. Phylogenetic treebased on cpDNA haplotypes and BEST species tree based on6nuclear loci showed that P.tremuloides was the more ancestral lineage in the three species. Divergence of the three species inferred from Beast relaxed molecular clock occurred in early Quaternary. The ancestrallineage dispersed from North America to Eurasia through Beringia land bridge (BLB) followedby fast expansion throughout Eurasia. The formation of Bering strait and the uplift of thenorthern Qinghai-Tibetan Plateau (QTP) and adjacent area in Quaternary isolated thedistribution of the three species, which have undergone a process of populationexpansion-contraction dynamics since divergence from the common ancestor. No obvious geneflow was detected which suggested that the present patterns of nucleotide variation was notshaped by migration among these species and that introgression was not pervasive in the groupof species. The speciation model of the three species was allopatric speciation caused byvicariance.