| Quercus L.shows the highest species diversity and has the widest geographical distribution among genera of the family Fagaceae.Querus sensu lato is divided into two subgenera as subgenus Quercus and subg.Cyclobalanopsis.Species of Quercus are mainly distributed in the Northern Hemisphere as common dominants in forests,which provide crucially ecomomic and ecological values.Thus,species of subg.Quercus are treated as tree models for ecological speciation aspects,and have numerous researches in relation to molecular genetics,ecology and species conservation.In China,high diversity and endemism of oaks in Quercus are found;this area is also considered as one of the origin and evolutionary centers of the global oaks.However,current molecular phylogeny and evolutionary studies have barely included the Chinese oaks;researches related to molecular genetics on Chinese oaks are also poor,which may lead to biased and stochastic conclusions.Accordingly,it is important to perform phylogenetic and molecular genetic studies on species of subg.Quercus in China.In this study,almost all species resources of the Chinese oaks in subg.Quercus were retrieved and collected to estimate the current diversity status and predict hotspot centers for Chinese oak species.Given the unestimated phylogeny of the Chinese oaks,a comparative analysis between the chloroplast genomes of two closely related species in Fagaceae was adopted to develop universal chloroplast DNA(cp DNA)markers.Five selected cp DNA markers and two nuclear genes were then used to construct the framework phylogeny and lineage evolutionary history,as well as in a DNA barcoding analysis to investigate genetic differentiation and relationships among the Chinese oaks of subg.Quercus.Additionally,the divergent history of two closely related deciduous oaks distributed in northern China were estimated to verify conclusions of the phylogenetic work.The main results of this research were listed as follows:1.Based on the information of species distribution,36 oak species were collected,representing the five morphology-based sections(Quercus,Aegilops,Heterobalanus,Engleriana and Echinolepides)of the Chinese oaks in subg.Quercus.The five morphological sections of Chinese oaks showed different distribution ranges and centers;within each section,most closely related species also showed diverse distributions,and exhibited varying ability of species adapatation on two ecological(temperature and precipitation)factors among their distribution ranges.Among the five morphological sections,Sections Quercus and Aegilops had the broadest distribution areas,while ranges of Sections Heterobalanus,Engleriana and Echinolepides were limited.Species of the subg.Quercus as well as of most sections were saturated,suggesting well adaptations of the Chinese oaks to current geographical and ecological conditions.The Hengduan Mountains and its adjacent areas,Qinling-Bashan Mountains as well as the eastern Tianmu Mountains were predicted as the species hotspot centers for the Chinese oaks,however,these areas were fragmented and isolated.2.A total of 47 cp DNA markers were designed using the comparative analysis between two chloroplast genomes of Quercus rubra(Quercus)and Castanea mollissima(Castanea).Based on 14 Chinese oaks,nine cp DNA barcode candidates and 12 markers that could generate high intraspecific genetic variations were selected for the Chinese oak species.All these selected cp DNA markers showed high rates of amplification success among the Chinese oaks,which may be used as universal DNA markers for subsequent researches related to molecular genetics in Quercus as well as in other genera of Fagaceae.3.Five selected cp DNA barcoding candidates(psb A-trn H,mat K-trn K,ycf3-trn S,mat K and ycf1)and two nuclear genes(ITS and SAP)showed varying species identification rates in the DNA barcoding analysis.The four DNA barcoding methods utilized also had different powers on species discrimination;the character-based method showed the best species identification power among the four DNA barcoding methods.Among the seven barcoding markers,ITS and psb A-trn H showed relatively high species discrimination rate(both 30.30%),and were recommended as core DNA markers for barcoding the Chinese oaks.The combination of six DNA barcoding markers(psb A-trn H + mat K-trn K + mat K + ycf1 + ITS + SAP)revealed the highest rate of species identification(84.85%)using the character-based method.Among the identified species,most of them were found in the morphology-based section Engleriana,while individuals of species in Sections Quercus and Heterobalanus were highly mixed,suggesting complex genetic relationships and evolutionary history among these closely related oaks.4.The phylogenetic research of the Chinese oak species in subg.Quercus strongly supported three main clades(Groups Quercus,Cerris and Ilex)of the phylogeny of Quercus.By contrast,group boundaries among the five morphology-based sections of the Chinese oaks,especially the three evergreen sections(Heterobalanus,Engleriana and Echinolepides)were rather ambiguous.Within these three major groups,individuals of most closely related species were mixed,indicating unclear species boundaries.The molecular evolutionary analysis using fossil calibrations indicated that the infrageneric divergence of major groups of subg.Quercus in China began at about 32-34 Ma,closely followed by species diversification within the evergreen Ilex group around 30-33 Ma.Species of Section Aegilops(Group Cerris)diverged at about 17-18 Ma,while species differentiation of Group Quercus was late with the mean time of 8-13 Ma.Among the three major groups of subg.Quercus in China,Group Quercus had a higher species diversification rate(0.1725/Myr)than that of the remaining groups,suggesting that a potential evolutionary shift of this group have happened during the process of lineage diversification,which evolved as a distinct evolutionary regime among the Chinese oaks.5.The comparative phylogeographic and species divergent research on two closely related oaks(Quercus liaotungensis and Q.mongolica)from Section Quercus in northern China indicated that the two oak species split at about 1-2.15 Ma in the presence of asymmetric and bidirectional gene flow.Q.liaotungensis and Q.mongolica possessed varying core distribution areas,ancestral gene pools and spatial genetic structures,and showed distinct population dynamics and ecological adaptations during the climatic fluctuations of glacial-interglacial periods in Pleistocene. |
