On the origin of the Oriental plane tree (Platanus orientalis L.)

Oriental plane tree (Platanus orientalis) is native to the East Mediterranean region and sister to three western North American species, together forming the Pacific North American–European (PNA‐E) clade. Its sister clade, comprising several eastern North American–Mexican species, has been termed the Atlantic North American (ANA) clade. The origins of P. orientalis and the western North American–western Eurasian disjunction in the PNA‐E clade are poorly understood, with the North Atlantic and Bering land bridges being possible corridors for trans‐continental migration. Molecular phylogenetic studies suggested ancient hybridization between the ANA and PNA‐E clades prior to differentiation of modern species' lineages. We traced ANA‐ and PNA‐E‐specific leaf traits in the fossil record to locate areas of possible ancient hybridization. Leaf traits characteristic of the PNA‐E clade occurred in western North America (late Eocene of Montana, Early Miocene of Alaska) prior to appearing in the European fossil record. Fossil‐species with mixed PNA‐E–ANA leaf traits occurred in the Oligocene of Central Asia and Eocene and Miocene of western North America. In contrast, eastern North America and the Atlantic region hosted fossil‐species with leaf traits characteristic of modern ANA clade members. We propose that precursors of Platanus orientalis migrated to Europe via Beringia and through Central Asia. Initially, these Eurasian ancestors possessed ancestral PNA‐E clade leaf morphologies, which were gradually replaced by P. orientalis‐specific traits. Treated as a single fossil‐species, we document the evolution of P. academiae from predominately three‐lobed leaves in Miocene strata to narrowly five‐lobed leaves resembling modern P. orientalis in younger deposits of Greece.