Molecular Phylogenetic Reconstruction And The Evolution History Of The Genus Populus L

Salicaceae family species are widely distribution pant groups which include the only two genera,Populus and Salix,since Linnaeus established the family.The genus Populus is divided into 6 sections based on the classical morphological classification method.The phylogenetic relationship among 6 sections has been lack of systematic research due to insufficient phylogenetic information of molecular markers or the limitation of sampling in the early studies,especially the origin,evolutionary history and the time framework of Populus are all matter of concern to systemologists.Based on the above problems,Ricinus communis firstly used as outgroup in this study,fifty-four representative species of 11 genus from Salicaceae sensu lato were selected,and using the combined matK and rbcL sequences dataset reconstruct the molecular phylogenetic relationships among genus of the family through maximum parsimony(MP)and Bayesian Inference analyses(BI),and the aim was to provide the theoretical basis for choosing outgroups in the Populus and Salix phylogenic study.Secondly,we selected four representative species and three relatives species(Idesia polycarpa,Poliothyrsis sinensis and Itoa orientalis that formerly included in Flacourtiaceae)used for the outgroups,and selected 34 representative species distribute Eurasian and North American which covered the six sections of Populus.Twenty-three single-copy nuclear gene fragments and 34 chloroplast fragments were sequenced for 41 species,and phylogenetic tree were constructed by combining fragments respectively.Then the relaxed molecular clock method and ancient fossil data were used to estimate the divergence time among nodes to analyze and discuss the origin,evolution history and the mechanism of modern geographical distribution pattern of Populus.The main results are as follows:(1)The same topology of the MP tree and BI tree of Salicaceae sensu lato and with high support values based on the combined matK and rbcL sequences,Idesia and Bennettiodendron were the closest relatives with the genus Populus-Salix,which were the sister of the genus and with higher support value(0.99/70),followed by the Poliothyrsis(0.97/64).All species of Populus and of Salix clustered a monophyletic group respectively,with high support values(1/99),wich were sister to each other and sharing the recent common ancestor.All outgroups of Salicaceae only extant in Asia,especially the Salicaceae relative to Idesia and Bennettiodendron,which can further confirm that Salicaceae originated in Asia.(2)The sections of Turanga,Abaso and Leuce were all formed a robust monophyly by the comparison of the cpDNA tree and the nuclear gene tree,therefore,it is reasonable to divide into three independent sections.In addition,Members of the sections in Tacamahaca,Aigeiros and Leucoides can hybridize with each other that led to the topological conflict between the nuclear and chloroplast phylogenies and revealed reticulate evolutionary patterns,and the intersection boundary was still not clear.(3)Based on fossil data and cpDNA time tree,we speculated that one of ancestors of Populus was first to arrive in North America by the Bering Land Bridge(BLB)or the North Atlantic Land Bridge(NALB)after the Salicaceae differentiation from the late mid-Paleocene to Eocene early and then resulting in species differentiation and diversity.With the climate cooled progressively in the Eocene,especially the abrupt decline in the Oligocene,leading to the extinction of all the earliest Populus lineages from Eocene,while the existing North America branches from the ancient Mediterranean lineages during Miocene(14.26Ma).Based on this,we proposed that Populus species in North America have twice(respectively in the Eocene and in the Miocene)colonization from the Eurasian ancestral lineage species in the geological history,and all the first colonized species have been extinct because of the global climate cooled rapidly in the Oligocene(34-23Ma).(4)Based on the cpDNA time tree,the ancestors of the present Populus originated from the ancient Mediterranean(14.26Ma)that is a surviving Asian lineages after the Oligocene extinction event,and caused the expansion of the ancestral descent because of the warm phase peaked in the Mid-Miocene Climatic Optimum(MMCO,17-15Ma),followed by a gradual cooling that the Populus was divided into two clades.The first large clade consists of B and A branches.The B branch in which Turanga species that as basal taxa;A branches consists of A1,A2 and A3,The A1 branch includes three Asian species that is P.afghanica from central Asia,northern P.simonii and southern P.lasiocarpa in China,which P.afghanica as the basal taxa;The Eurasian branch of A2 consists of Leuce(P.alba,P.adenopoda and P.tomentosa),Aigeiros(P.nigra)and Tacamahaca(P.cathayana and P.przewalskii);The Eurasian-North America branch of A3 includes the other species in Leuce(P.davidiana,P.hopeiensis and P.tremula where distributed in the Eurasian and P.tremuloides and P.grandidentata that distributed in North America).Obviously,both branch A and branch B are all originated in Eurasian continent.The other large clade includes Asian branch C and North America branch D The study found that after the North American branch D and Asian branch C diverge in 12.58 Ma and then entered North America,but species diversification occurred in 6.68 Ma and these species form a monophyletic clade.Populus balsamifera and P.trichocarpa occupied the basal position of the D branch which is the original species of North America lineage D.We speculated that the climatic changes may have served as a major driving force in the speciation of Populus species in North America and geographical distribution pattern based on the monophyletic origin of North America lineage D.(5)Both between Eurasian P.davidiana and P.tremula in Leuce and North American P.tremuloides and P.grandidentata in Tacamahaca that exhibit Eurasian-North American disjunct distribution pattern,the divergence time of the two clades was 5.34 Ma,which is the result of vicariance by BLB breakup.The time of the ancestors of Leuce species entered North America at 6.13 Ma,while the ancestral lineages of Aigeiros,Abaso,Tacamahaca and P.heterophylla in Leucoides entered North America at 12.58 Ma,that much earlier than Leuce‘ lineages.(6)We found that an transcontinental hybridization event between Eurasia and North America occurred in the late Pliocene,and then resulted in the hybrid speciation of between A1 branch species(P.afghanica,P.lasiocarpa and P.simonii)and A2 branch species(P.nigra,P.przewalskii and P.cathayana),and resulted in the topological conflict between the cpDNA and nuclear phylogenies.We have initially confirmed that the parent lineage of the hybrid events stem from North American clade D and may be completed through the NALB,which further speculated that the NALB and BLB broke down are all possible dated at 5.5-5.4Ma.(7)Further studies are warranted to resolve about the speciation of P.mexicana species and even the monophyletic North American D branch whether the product of cpDNA capture from survival of ?ghost lineages‘ from an extinct poplars taxon or vicariance caused by climate driven.