Phylogeny and biogeography of Sassafras (Lauraceae) disjunct between eastern Asia and eastern North America

Sassafras (Lauraceae) consists of three species disjunct between eastern Asia (S. tzumu and S. randaiense) and eastern North America (S. albidum). Phylogenetic analysis based on sequences of nuclear ribosomal ITS and three chloroplast non-coding regions (rpl16, trnL-F, and psbA-trnH) showed that Sassafras is monophyletic and that the eastern North American S. albidum is sister to the clade of its two eastern Asian counterparts. Their intercontinental divergence was estimated to be 13.80 +/- 2.29-16.69 +/- 2.52 million years ago (mya) using the penalized likelihood method with the ITS and three chloroplast markers. Biogeographic analyses combined with fossil evidence suggest that Sassafras has a relict distribution in the Northern Hemisphere without a Gondwanan link. The divergence time of the two eastern Asian species (the continental Chinese Sassafras tzumu and S. randaiense endemic to Taiwan) is estimated to be 0.61 +/- 0.75-2.23 +/- 0.76 mya. Sassafras randaiense from Taiwan was most likely derived from an ancestor from continental China.