Phylogeography Of Fagus Longipetiolata: Insights From Nuclear DNA Microsatellites And Chloroplast DNA Variation

Phylogeography studies the principles and processes governing the geographical distributions of genealogical lineages,which emphasizes historical aspects of the contemporary spatial distributions of gene lineages.Compared with biparentally inherited nuclear markers,the cpDNA variation provides special markers for phylogeny reconstruction at the population level and the geographic structure of variation,which provide insights into the phylogeograpy of the species. The contrasted patterns of inheritance of organelle and nuclear genes can be used to unravel the complexity of gene flow in plants,as they are predicted to result in very different distribution of genetic diversity within and among populations.In the last two decades,extensive phylogeographic studies have been conducted on taxa from Europe and North America,where were covered by huge glaciers during the Quaternary.Some species only survived in a few refugia and recolonized after glaciation in Europe and North America.Contrasting to Europe and North America,East Asia had not been covered by huge glaciers during the Quaternary glaciation. However,the temperature had decreased about 8-12℃.Did distribution ranges of the species in East Asia also retract like those in European and North America? Where did they survive from the cold temperature and how did they recolonize after the Quaternary glacial ages?In order to answer these questions,we choose Fagus longipetiolata as the focal species. Fagus(beech)is among the most abundant and economically important genera of broadleaved trees in northern hemisphere temperate forests.The genus Fagus comprises 10 to 14 species of monoecious trees that are disjunctly distributed in Europe and south-western Asia(1-5 species), eastern Northern America and Mexico(1-2 species)and eastern Asia(7-12 species).F. longipetiolata distributes widely in subtropical southern China,which distributed much further north during the late Tertiary in China.During the early Quaternary,it declined and disappeared in northern China,but expanded in subtropical southern China.And its distribution range is from Xichou County,Yunnan Province,to Tiantai Mountain,Zhejiang Province.F.longipetiolata is a monoecious,long-lived woody angiosperm.It is pollinated by wind and dispersed by gravity,but rodents may serve as the secondary dispersers.Nuclear microsatellites and cpDNA variations among populations of F.longipetiolata were investigated in the study.The main results are as following.(1)A total of 576 individuals were collected from 27 populations covering most distribution range of E longipetiolata.We used 7 pairs of microsatellite primers,among them 6 were screened from congeneric species,E sylvatica and E crenata,and one,locus FL05,was developed by ourself.Low genetic diversity was found within populations.The genetic differentiation among populations was high(F_ST=0.23).Gene flow for 21 populations(Nm)was estimated to be 0.83.Different genetic patterns were found in populations of eastern and western range. Populations of western range had higher genetic diversity than those of eastern range.And genetic differentiation was small among populations.However,genetic diversity was much more diverse among populations of eastern range.Higher genetic diversity was found within large populations than small populations.Populations of northern and southern part had lower genetic diversity than that in central populations in eastern China.Recent bottleneck effect was found in a southern population,MD population,and a northern population,SH population.(2)Two pairs of universal cpDNA primers,trnS-trnG and psbB-psbF,were used.A total of 201 individuals from 26 populations were sequenced successfully.15 polymorphic sites over 1664bp and 13 haplotypes were observed.However,within-population haplotype diversity was very low.Only 4 populations had more than one haplotypes.About 93.197 percentage of genetic differentiation was among populations.The genetic differentiation value was high(G_ST=0.900, 21 populations).There were 3 haplotypes shared with F.lucida indicating the early introgression between these two species or they shared a common ancestor.Significant geographic structure of haplotypes was detected.Populations of western and eastern ranges had own unique haplotypes.More ancestral haplotypes were found and distributed widely in populations of western range.Dispersion of the same haplotype in western range happened in short distance.However the same haplotype in eastern China was found in a long distance,meaning that it was distributed from south refugia to north mountains in eastern China.(3)Based on Ennos' model,the pollen-seed ratio(r)was estimated to be 34.228,indicating that pollen flow had played a more important role in the maintenance of genetic structure than seed flow.However this value was much smaller than that in other species of Fagaceae,due to the decreased pollen flow among populations by natural fragmentation,given similar cpDNA-based G_STwith other Fagus species.(4)From this study,two contrasting pattern in glacial refugia and recolonization routes were found in eastern and western distribution range. Southwestern China including Guangxi Province and Guizhou Province provided refugia for F.longipetiolata during Quaternary glaciations.When it became warm,its postglacial populations migrated from one region to the other along the lower mountains.It was short-distance disseminated for populations of south part in this refugia because of high mountains.While there were Sichuan Basin in north part of this refugia,population migrated northward along the mountains surrounding Sichuan Basin.An exceptional case of long-distance dissemination,the modern distribution of haplotype H1 was ascribed to a postglacial colonization route starting from north part of this refugia,eastward crossing the Sichuan Basin,central of China and reaching East China.Wuyishan-Nanling served as the other refugia.This refugia was separated into two parts: south and north,by Wuyishan.After glaciations,populations in the south part migrated northeastward along Wuyishan.In western Zhejiang,E longipetiolata migrated to the north and east parts of Zhejiang Province.Populations survived in the north Wuyishan refugia migrated to the north as well as northeastward along the north ridge of Wuyishan and then mixed with the migrators of south part in western Zhejiang Province.Therefore,high and mixed haplotype diversity was found in western Zhejiang Province.Given the closely phylogenetic relationship based on haplotype and SSRs between populations of Fujian Province and populations of western China,populations of eastern range might colonize from western China before the Quaternary glaciations.