Biogeography Of Holcoglossum Schltr.(Orchidaceae)with Special Emphasis On The Diversification Of Aeridinae

There are 35 biodiversity hotspots in the world,and the formation and evolution of biodiversity in these hotspots have attracted the attention of many biogeographers.There are 11 biodiversity hotspots in Asia.However,the biogeographic and diversification pattern of epiphytes in Asia is still unclear.Holcoglossum Schltr.(Aeridinae,Orchidaceae)comprises 16 species and endemic to East Asia.Holcoglossum is monophyletic and can be divided into three clade: Tropical subclade,Intermediate subclade,Temperate subclade.The morphological characters especially flower of Holcoglossum vary greatly,and their pollination systems are diverse.However,the spatial and temporal evolution pattern of Holcoglossum is still unclear.We analyzed the historical biogeography of Holcoglossum and explored the causes of epiphytic diversity in Asia,which will help us to understand the biodiversity in Asia.The main findings are as follows:(1)Phylogenetic relationships within Holcoglossum: We sampled 15 species of Holcoglossum,and selected 11 chloroplast DNA markers(psb A-trn H?atp H-I?psb KI?mat K?rpo B?rbc L?rpo C1?trn L-F?rp S12-rp L20?trn S-G?trn S-f M)and one nuclear markers(ITS)in this study.Phylogenetic reconstruction was performed using maximum parsimony,Bayesian inference and maximum likelihood methods,respectively.The results show that: The phylogenetic topologies constructed by the three methods are consistent,all of which support that Holcoglossum is monophyletic,and divided into three clades with high supported values.Our results is consistent with the results of previous studies.Further,the interspecific relationships of the Holcoglossum are relatively clear.(2)Molecular dating and biogeography: we used a relaxed lognormal clock model to reconstruct a chronogram of Holcoglossum.The results show that:Holcoglossum originated at the late mid-Miocene(8.53 Ma,95% HPD: 4.19-14.02),diversification began at 6.33 Ma(95% HPD: 3.3-10.6).The results obtained by the two biogeographic reconstruction methods(S-DEC and S-DIVA)are consistent:Holcoglossum occurred in the palaeotropical region and underwent four dispersal events and two vicariance events: around 3.63 Ma(95% HPD: 1.64-6.24),a migration event from the Paleotropical region to the Sino-Himalayan region;about 2.7 Ma(95%HPD: 1.16-4.78),there was a migration from the Paleotropical and Sino-Himalayan regions to Taiwan and a migration from the Paleotropical and Sino-Himalayan regions to Sino-Japanese region;around 1.22 Ma(95% HPD: 0.5-2.36),a migration event from Paleotropical and Sino-Himalayan region to Sino-Japanese region;around 2.7 Ma(95%HPD: 1.16-4.78),Holcoglossum experienced a vicariance event between Paleotropical and Sino-Himalayan regions;the disjunct distribution between mainland China and Taiwan occurred at the late Pliocene(2.7 Ma,95% HPD: 1.16-4.78).The spatial and temporal evolution of Holcoglossum is closely related to the strengthening of East Asian monsoon,sea level fluctuation,Quternary glacial periods and uplift of Hengduan Mountain.(3)A preliminary study on the diversification of the Aeridinae: To further study the causes of epiphytic biodiversity in tropical Asia,we selected 362 species from 83 genera of Aeridinae,based on three chloroplast genes(mat K,rbc L and trn L-F)and one nuclear gene(ITS).The time estimation was performed relaxed-clock method,and the ancestral range reconstructed was used the S-DEC method.The results show that:Aeridinae originated during the mid-Oligocene in the palaeotropical region,and the tropical region is the center of its diversity.There were 79 migration events from the palaeotropical region to East Asia and 6 migration events from the palaeotropical region to Australia.Moreover,we performed diversification analysis for Aeridinae,the result of BAMM showed that Aeridinae has happened radiation evolution at the period of late Miocene,and the result of LTT showed that the lineages of Aeridinae is accumulating constantly,and a rate shifts happened at middle Miocene.We hypothesized that the niche conservatism of Aeridinae may have promoted its diversification.