Phylogeny and historical biogeography of the genus Platanus as inferred from nuclear and chloroplast DNA

Phylogenetic analyses of DNA sequences of six species of Platanus were conducted to estimate species relationships and analyze biogeographic history. On the basis of a broader analysis of the third exon of the nuclear gene LEAFY, the root node for the genus was confirmed to fall between subg. Castaneophyllum (P kerrii) and the species of subg. Platanus. Separate phylogenetic analyses of the nuclear ribosomal ITS region, the 3' region of the second intron of LEAFY, and the chloroplast region trnT-trnL intergenic spacer provided various levels of resolution, and the combined data yielded a fully resolved set of relationships within subg. Platanus. Two major clades were identified: one with species from Europe (EUR) and western North America (WNA) (R orientalis, P racemosa s.l.), the other with species from eastern North America (ENA; US and Canada) and eastern Mexico (EMEX) (P mexicana, P occidentalis, and R rzedowskii). Within subg. Platanus, six subclades corresponded to previously recognized taxa, and one accession may be of hybrid origin. The historical biogeography of Platanus was interpreted using phylogenetic pattern, estimates of divergence times, the fossil record, and climate reconstructions. The pattern of relationships was consistent with a hypothesis of vicariance and the oldest divergence between taxa within the set of area relationships of ((WNA + EUR), (ENA + MEX)) suggested an initial barrier affecting taxa that are now mostly confined to North America. The second oldest divergence within subg. Platamis involves the intercontinental disjunction of semi-arid species from WNA + EUR, which diverged by at least 15 MYBP, consistent with the Madrean-Tethyan hypothesis. Calibrated phylogenies were used to estimate divergence times for five more recent intracontinental disjunctions. These times correlated with the timing of geological events in Southwestern North America and northern Latin America.