Phylogeny and Biogeography of Cercis (Fabaceae): Evidence from Nuclear Ribosomal ITS and Chloroplast ndhF Sequence Data

The phylogeny of Cercis (Fabaceae) was estimated with DNA sequences of the nuclear ribosomal ITS region and the 39 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 MadreanTethyan 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 MadreanTethyan 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. Numerous groups of plants are known to be discontinuously distributed between two or more of the temperate mesophytic forested regions of the northern hemisphere. These regions include eastern and far western North America, parts of Europe and western Asia, and much of eastern Asia. There is now ample evidence that these disjunctions have originated largely through Tertiary range restrictions of formerly panhemispheric forest communities as a response to gradually cooling and drying climates (Wood 1972; Axelrod 1975; Tiffney 1985a, b, 2000; Boulter 1994; Collinson 1994; Manchester 1999; Wen 1999; Tiffney and Manchester 2001). Paleontological and molecular phylogenetic studies suggest that these disjuncts neither arose at the same time nor under the same set of geophysical and climatic conditions, but rather have multiple geographic origins, dispersion pathways, and divergence times (Tiffney 1985b; Manchester 1999; Wen 1999; Donoghue et al. 2001). The challenge that lies ahead for the study of Northern Hemisphere historical biogeography is to attempt to reconstruct an apparently complex history through the identification of subpatterns, perhaps associated with ecophysiological dif-