Biogeographic History Of Betulaceae In The Northern Hemisphere

Betulaceae is a well-defined family of Fagales,including six living genera and more than 167 modern species mostly found from the Northern Hemisphere,with only a few extending to the Andes Mountains and northern Africa.Furthermore,the abundant fossils of the Betulaceae in the Northern Hemisphere also makes it as the most ideal taxa to study biogeographic history of plants in the Northern Hemisphere.In this study,phylogenetic relationship of Betulaceae was reconstructed by adding taxonomic samples and gene fragments.We also collected and assembled the fossil records to discuss the origin and evolution of the Betulaceae.Estimation of the divergence time of the Betulaceae and reconstruction its ancestral origin,further explore the origin of the Betulaceae and biogeographical evolution in the Northern Hemisphere.Phylogenetic studies were conducted using sequences of nuclear DNA(Pig C,ITS,GBSSI)and chloroplast sequences(trn L-trn F,rps16,rbc L,rpl16,psb A-trn H,mat K,atp B-rbc L).The combined analyses of the three nuclear regions support that the traditional two subfamilies,Betuloideae(Alnus,Betula)and Coryloideae(Carpinus,Corylus,Ostrya and Ostryopsis),except that the Ostrya is nested in Carpinus.But the combined analyses of the chloroplast regions support that Betuloideae is paraphyletic,and Carpinus is nested in Ostrya.For relationships within genera,results from nuclear genes are consistent with morphological traits,while those of chloroplast genes are more related to their distributions.By combining nuclear genes and chloroplast genes to reconstruct the phylogenetic relationship,the results support that the two traditional subfamilies of the Betulaceae are monophyly,as well as each genera,respectively.Betulaceae fossils include five extant genera(Alnus,Betula,Corylus,Carpinus,Ostrya)and nine extinct genera(Palaeocarpinus,Corylites,etc)from a total of 889 records.Many species of Betulaceae are widely distributed in the Northern Hemisphere,such as Asia,Europe and North America since Cretaceous.According to the fossil records,eastern Asia was the center of Betulaceae in the Late Cretaceous;Betulaceae had diversified and expanded in Europe and North America during Paleocene to Eocene and continued to spread in Asia,Europe and North America during Oligocene with some genera extinct.Due to changes in climate and geological events during Miocene,Betulaceae had the highest diversity during that period with a number of fossil and diversity fallen sharply in Pliocene-Pleistocene.Present distribution pattern and evolution of Betulaceae are closely related to geological changes,Quaternary glaciers,paleoclimate and paleogeography.Trees were rooted with Ticodendraceae,and divergence times were inferred under relaxed molecular clocks,using alternative fossil constraints.Ancestral areas were reconstructed using Bio Geo Bears.The results show that the Betulaceae originated from the Campanian-Maastrichtian stage of the Late Cretaceous,about 70.94 million years ago(Mya).The differentiation time of the two subfamilies was about 51.90 mya in the early Paleocene.The differentiation of the genus Alnus occurred in the Miocene(about28.66 Mya),and the other genera all occurred in the middle of the Miocene.Betulaceae experienced massive extinction events during the late Eocene-middle Oligocene,leaving only the "survivors" of the eastern Asia,and the modern distribution is the result from the re-radiative expansion of the "survivors" during the Miocene.Our study shows that an integrative approach to historical biogeography—that combines sources of evidence including paleontology,paleoclimate,geologic events and phylogenetics –could help us obtain more accurate reconstructions of ancient evolutionary history.It also reveals that groups with high extinction rates may lead to inferring wrong results of the ancestral origin if only modern information was used.