The phylogeny of Cercis (Fabaceae) was estimated with DNA sequences of the nuclear ribosomal ITS region and the 3′ end of the chloroplast gene ndhF. The combined analysis recovers three trees in which a well supported clade of North American and western Eurasian species is nested within a paraphyletic group of Chinese species. In the single most unambiguously resolved topology from these trees, C. canadensis from eastern North America is more closely related to C. siliquastrum from western Eurasia than to C. occidentalis from western North America. DIVA and character optimizations based on this topology suggest that the initial intercontinental divergence events in Cercis involved mesophytic ancestors. Subsequent inferred intercontinental divergence events involving xerophytic ancestors are consistent with the Madrean-Tethyan hypothesis, which postulates an early Tertiary floristic link between the arid regions of western North America and western Eurasia. Calibration of branch lengths with the fossil record suggests that the North American and western Eurasian lineages diverged between 6 and 32 million years ago. The oldest of these values is consistent with both the Madrean-Tethyan hypothesis (>23 million years ago) and dispersion across a North Atlantic land bridge connecting Europe to North America (>13 million years ago), whereas the youngest requires an explanation involving long-distance dispersal.
Communicating Editor: John V. Freudenstein