Phylogeography Of Pinus Tabulaeformis Carr. (Pinaceae), A Dominant Species Of Coniferous Forest In Northern China

How coniferous trees in northern China changed their distribution ranges in response to Quaternary climatic oscillations remains largely unknown.Here we report a study of the phylogeography of Pinus tabulaeformis,an endemic and dominant species of coniferous forest in northern China.We examined sequence variation of maternally inherited,seed-dispersed mitochondrial(mt)DNA(nad5 intron 1 and nad4/3-4)and paternally inherited,pollen-dispersed chloroplast(cp)DNA(rpl16 and trnS-trnG) within and among 30 natural populations across the entire range of the species.Six mitotypes and five chlorotypes were recovered among 291 trees surveyed.Population divergence was high for mtDNA variation(G_ST=0.738,N_ST=0.771)indicating low levels of seed-based gene flow and significant phylogeographic structure(N_ST＞G_ST, P＜0.05).The spatial distribution of mitotypes suggests that five distinct population groups exist in the species;one in the west comprising 7 populations,a second with a north-central distribution comprising 15 populations,a third with a southern and easterly distribution comprising 5 populations,a fourth comprising one central and one western population,and a fifth comprising a single population located in the north-central part of the species' range.Each group apart from the fourth group is characterized by a distinct mitotype,with other mitotypes,if present,occurring at low frequency.It is suggested,therefore,that most members of each group apart from Group 4 are derived from ancestors that occupied different isolated refugia in a previous period of range fragmentation of the species,possibly at the time of the Last Glacial Maximum.Possible locations for these refugia are suggested.A comparison of mitotype diversity between northern and southern subgroups within the north-central group of populations(Group 2)showed much greater uniformity in the northem part of the range both within and between populations.This could indicate a northward migration of the species from a southern refugium in this region during the postglacial period,although alternative explanations cannot be ruled out.Two dominant chlorotypes were distributed across the geographic range of the species,resulting in lower levels of among-population chlorotype variation but significant phylogeographic structure(G_ST=0.188＜N_ST=0.471,P＜0.05).The geographical pattern of variation for all 5 chlorotypes provided some indication of the species surviving past glaciations in more than one refugium,although differentiation was much less marked,presumably due to the greater dispersal of cpDNA via pollen.