The phylogeny and biogeography of Osmanthus, Cercis, and Tilia based on ITS nuclear ribosomal, ndhF and trnL-F chloroplast DNA sequence data

The similarity between the forests of eastern North America and eastern Asia has intrigued botanists since the time of Linnaeus. This biogeographic pattern is currently recognized by the disjunct distribution of approximately 65 genera. To estimate relationship between disjunct species on American and Eurasian continents, a molecular systematic analysis was conducted on three Tertiary disjunct genera: Osmanthus (the wild olives), Cercis (the red buds), and Tilia (the lindens). DNA was extracted from leaves of species from each genus and DNA sequences were generated for the internal transcribed spacer (ITS) region of nuclear ribosomal DNA, ndhF and trnL-F spacer regions of chloroplast DNA. Percent sequence divergence for the species of Osmanthus ranged between 0.28% and 9.17%, for the species of Cercis between 0.57% and 3.59%, for the species of Tilia between 0.00% and 7.78%. Using the sequence data for each genus, I constructed phylogenies (evolutionary hypotheses). The phylogenies generated by my work indicate that previously hypothesized intrageneric classification systems within Osmanthus, Cercis, and Tilia do not reflect the relationships for their respective species. Previously proposed Osmanthus taxonomy does coincide with my phylogenetic hypothesis. However, my data does not support the previous classification of Tilia by Engler 1909. Area cladograms were constructed for each genus by mapping the modern distributions of these plants onto phylogenetic trees. My data does not support a simple general explanation of a biogeographic pattern that can be applied to all taxa included in this analysis. It suggests a complex biogeographic pattern involving vicariance, long distance dispersal, and repeated migrations through Bering land bridge.