Cladistic biogeography of Gleditsia (Leguminosae) based on ndhF and rpl16 chloroplast gene sequences.

We used cladistic analysis of chloroplast gene sequences (ndhF and rpl16) to test biogeographic hypotheses in the woody genus Gleditsia. Previous morphological comparisons suggested the presence of two eastern Asian-eastern North American species pairs among the 13 known species, as well as other intra- and inter-continental disjunctions. Results from phylogenetic analyses, interpreted in light of the amount of sequence divergence observed, led to the following conclusions. First, there is a fundamental division of the genus into three clades, only one of which contains both Asian and North American species. Second, the widespread and polymorphic Asian species, G. japonica, is sister to the two North American species, G. triacanthos and G. aquatica, which themselves are closely related inter se, but are both polymorphic and paraphyletic. Third, the lone South American Gleditsia species, G. amorphoides, forms a clade with two eastern Asian species. Gleditsia thus appears to have only one Asian-North American disjunction and no intercontinental species pairs. Low sequence divergence between G. amorphoides and its closest Asian relatives implicates long-distance dispersal in the origin of this unusual disjunction. Sequence divergence between Asian and North American Gleditsia is much lower than between Asian and North American species of its closest relative, Gymnocladus. Estimates of Asian-North American divergence times for Gymnocladus are in general accordance with fossil data, but estimates for Gleditsia suggest recent divergences that conflict with ages of known North American Gleditsia fossils.