Phylogeny, origin, and biogeographic history of Aesculus L. (Sapindales) - an update from combined analysis of DNA sequences, morphology, and fossils

Conflicting phylogenies and biogeographic hypotheses for Aesculus L. were reevaluated using data integrating morphology, fossils, and DNA sequences. A more robust phylogeny of Aesculus was constructed by increasing taxon and character sampling to include 42 morphological characters and five DNA regions including rps16, trnHK, matK, ITS, and part of the LEAFY gene. Our analyses revealed three well-supported major lineages in Aesculus. These are sect. Calothyrsus excluding A. californica, sect. Macrothyrsus + A. californica, and sect. Aesculus + sect. Pavia + sect. Parryana. The relationship of sect. Aesculus to sect. Parryana + sect. Pavia is strongly supported and previously unreported. Relationships among the three major clades remained incompletely resolved. Biogeographic and divergence time analyses using DIVA and Multidivtime support a Paleocene origin of Aesculus in eastern Asia and western North America, and showed that the genus subsequently dispersed into eastern North America, Central America, and Europe. Our results support that Aesculus originated as an element of the Paleogene mesophilic flora in the Beringian region with multiple eastward migrations characterizing the early spatial history of this genus. This biogeographic history supports Trans-Beringian migration of plants in the very early Paleogene.