On The Origin Of Tropical Forests In Hainan

Hainan is the only province in China with tropical island climate.The Island harbors complex and diverse vegetation and the main vegetation types include tropical rain forest monsoon rain forest and mangrove forest.Currently,there are two conflicting opinions on the origin of forest floristics of Hainan,one contends that the island was connected to the Beibu Gulf and ancient tectonic drift in the Eocene caused the separation of flora of Hainan from the mainland,the other maintains that the flora of Hainan originated recently on account of dispersal.Species of the family Lauraceae are common floristic elements in flora of Hainan,and endemic species are important in exploring the origin of tropical forest flora of the island.In this study,we assembled a datamatrix including 131 species from 16 genera of Asian lauraceae,reconstructed phylogenetic trees using both Maximum likelihood and Bayesian inference methods,estimated divergence time using MCMCtree,and reconstructed ancestral area of species with Bio Geo BEARS.The main results are as follows:1.The nr ITS,3 chloroplast fragments(rbc L,psb A-trn H and mat K)and plastome were obtained by experiments and NCBI database.In this study,special and endemic distribution of lauraceae from Hainan were selected for obtaining by experiments,including 17 species and 34sequences(chloroplast genomes and nr ITS).And 109 Asian lauraceae species were obtained from NCBI,including 101 chloroplast genomes and 91 nr ITS and chloroplast markers.A total of2 datasets were constructed for phylogenetic,molecular clock and biogeography.2.The phylogenetic tree based on nr ITS and three chloroplast fragments had a low support rate.The phylogenetic tree constructed from chloroplast genome data has a high support rate and is clear about the relationships among the main branches.3.The phylogenetic trees contains 6 clades,i.e.Neolitsea-Litsea clade,Cinmamomum-Camphora clade,Machilus-Dehaasia clade,Neocinnamomum clade,Cassytha clade and Beilschmiedia-Cryptocarya clade.4.The dating results show that the Lauraceae diverged from other Laurales in the Cretaceous(about 111Ma),and the crown group originated ca.108 Ma.The divergence of endemic species of Hainan island from their sister species in Indochina Peninsula and southwestern China was estimated to be during Oligocene.We thus favor the hypothesis that modern forest flora of Hainan island originated from recent dispersal from the mainland but negate the tectonic drift hypothesis.This study conducted phylogenetic reconstruction of Asian Lauraceae using plastome,and a few markers including nr ITS and rbc L,psb A-trn H,mat K,estimated the divergence time of species,and studied the ancestral distribution area,provided new evidence for understanding the origin of tropical forests and flora in Hainan.