A morphological phylogenetic analysis of Aesculus L. and Billia Peyr. (Sapindaceae)
The hippocastanaceous Sapindaceae (family Hippocastanaceae) consists of two genera: Aesculus and Billia. The genus Aesculus (buckeyes) is distributed throughout the northern hemisphere. Twelve of the thirteen species of Aesculus are found in eastern Asia and North America, and one is native to Europe. The two species of the genus Billia are found in South and Central America. Aesculus is traditionally divided into five sections. This analysis, based on morphological characters, suggests that the monotypic section Parryanae (Aesculus parryi Gray) is sister to the remainder of the genus, and the other species are divided into two clades. One clade comprises all species from southeastern North America with the monotypic section Macrothyrsus (Aesculus parviflora Walt.) as sister to section Pavia. The other clade consists of all species found in Eurasia included in sections Aesculus and Calothyrsus, with the only North American member of this clade (Aesculus californica (Spach) Nutall) as sister to the rest of the group. According to this analysis, section Calothyrsus is rendered paraphyletic by the inclusion of section Aesculus. In contrast to previous studies that suggest an Asian origin for the genus, our study suggests an American origin for Aesculus from an ancestor similar to Billia with a single migration to Eurasia via the Bering land bridge.Key words: Aesculus, Billia, character evolution, Hippocastanaceae, morphology, phylogeny, Sapindaceae.