A fossil-calibrated relaxed clock for Ephedra indicates an Oligocene age for the divergence of Asian and New World clades and Miocene dispersal into South America

摘要

Ephedra comprises approximately 50 species,which are roughly equally distributed between the Old and New World deserts,but not in the intervening regions(amphitropical range).Great heterogeneity in the substitution rates of Gnetales(Ephedra,Gnetum,and Welwitschia) has made it difficult to infer the ages of the major divergence events in Ephedra,such as the timing of the Beringian disjunction in the genus and the entry into South America.Here,we use data from as many Gnetales species and genes as available from GenBank and from a recent study to investigate the timing of the major divergence events.Because of the tradeoff between the amount of missing data and taxon/gene sampling,we reduced the initial matrix of 265 accessions and 12 loci to 95 accessions and 10 loci,and further to 42 species(and 7736 aligned nucleotides) to achieve stationary distributions in the Bayesian molecular clock runs.Results from a relaxed clock with an uncorrelated rates model and fossil-based calibration reveal that New World species are monophyletic and diverged from their mostly Asian sister clade some 30 mya,fitting with many other Beringian disjunctions.The split between the single North American and the single South American clade occurred approximately 25 mya,well before the closure of the Panamanian Isthmus.Overall,the biogeographic history of Ephedra appears dominated by long-distance dispersal,but finer-scale studies are needed to test this hypothesis.