Phylogeny and biogeography of Croton alabamensis (Euphorbiaceae), a rare shrub from Texas and Alabama, using DNA sequence and AFLP data



    1. University of Wisconsin-Madison, Department of Botany, 430 Lincoln Drive, Madison, Wisconsin 53706, USA,
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    1. University of Wisconsin-Madison, Center for Sustainability and the Global Environment, Nelson Institute for Environmental Studies, 1710 University Avenue, Madison, Wisconsin 53726, USA,
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    1. The University of Michigan, Department of Ecology and Evolutionary Biology, Natural History Building (Kraus), 830 North University, Ann Arbor, Michigan 48109-1048, USA,
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    1. The Morton Arboretum, 4100 Illinois Route 53, Lisle, Illinois 60532, USA
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Benjamin W. van Ee, Fax: (608) 262 7509; E-mail:


Croton alabamensis (Euphorbiaceae s.s.) is a rare plant species known from several populations in Texas and Alabama that have been assigned to var. texensis and var. alabamensis, respectively. We performed maximum parsimony, maximum likelihood, and Bayesian analyses of DNA sequences from the nuclear ribosomal internal transcribed spacer (ITS) and 5.8S regions and chloroplast trnL-trnF regions from collections of the two varieties of C. alabamensis and from outgroup taxa. C. alabamensis emerges alone on a long branch that is sister to Croton section Corylocroton and the Cuban endemic genus Moacroton. Molecular clock analysis estimates the split of C. alabamensis from its closest relatives in sect. Corylocroton at 41 million years ago, whereas the split of the two varieties of C. alabamensis occurred sometime in the Quaternary. Amplified fragment length polymorphism (AFLP) analyses were performed using two selective primer pairs on a larger sampling of accessions (22 from Texas, 17 from Alabama) to further discriminate phylogenetic structure and quantify genetic diversity. Using both neighbour joining and minimum evolution, the populations from the Cahaba and Black Warrior watersheds in Alabama form two well-separated groups, and in Texas, geographically distinct populations are recovered from Fort Hood, Balcones Canyonlands, and Pace Bend Park. Most of the molecular variance is accounted for by variance within populations. Approximately equal variance is found among populations within states and between states (varieties). Genetic distance between the Texas populations is significantly less than genetic distance between the Alabama populations. Both sequence and AFLP data support the same relationships between the varieties of C. alabamensis and their outgroup, while the AFLP data provide better resolution among the different geographical regions where C. alabamensis occurs. The conservation implications of these findings are discussed.