FRITSCH, PETER W.1*, CHARLES C. DAVIS2, JIANHUA LI3, and MICHAEL J. DONOGHUE4. 1Department of Botany, California Academy of Sciences, Golden Gate Park, San Francisco, CA 94118; 2Department of Organismic and Evolutionary Biology, Harvard University Herbaria, 22 Divinity Avenue, Cambridge, MA 02138; 3The Arnold Arboretum, Harvard University, 125 Arborway, Jamaica Plain, MA 02130; 4Department of Ecology and Evolutionary Biology, Yale University, P.O. Box 208106, New Haven, CT 06520. - The phylogeny and historical biogeography of Cercis: evidence from ITS and ndhF DNA sequences.
The genus Cercis (Fabaceae: Caesalpinioideae: Cercidae)
consists of approximately ten species distributed in mesic to arid
climates across the warm-temperate regions of both North America and
Eurasia. Variation in the shape, thickness, and upper surface of the
leaves of Cercis species is correlated with climate type, and
several biogeographers have invoked this pattern to support or refute
hypotheses bearing on the evolution of arid biomes across the Northern
Hemisphere. To employ Cercis in general biogeographical models,
however, presupposes that a rigorous phylogenetic estimate of the
genus exists, which has not been the case. We estimated the phylogeny
of Cercis with DNA sequences of the nuclear ribosomal ITS
region and a portion of the chloroplast gene ndhF. The
phylogenetic relationships inferred from each region accord with one
another and with results from the analysis of the combined data. The
combined analysis recovered a topology in which a well supported clade
of North American and western Eurasian species is nested within a
paraphyletic group of Chinese species. Cercis canadensis from
eastern North America is more closely related to C.
siliquastrum from western Eurasia than to C. occidentalis
from western North America. From DIVA and character optimization
analyses, we inferred that the initial intercontinental divergence
event in Cercis involved mesophytic ancestors. Subsequent
inferred intercontinental divergence events involving xerophytic
ancestors imply a Tertiary floristic connection between the arid
regions of western North America and western Eurasia, and secondary
migration to mesophytic habitats in eastern North America. Calibration
of branch lengths with the fossil record suggests that the North
American and western Eurasian lineages diverged between 9.1 and 32
million years ago. The oldest of these values is consistent with more
or less direct trans-Atlantic dispersion across a North Atlantic land
bridge (>13 million years ago), whereas the youngest requires an
explanation involving long distance dispersal.
Key words: Cercis, DIVA, historical biogeography, ITS, ndhF, phylogeny