The biogeography of Hoffmannseggia (Leguminosae, Caesalpinioideae, Caesalpinieae): a tale of many travels

Aim The flowering plant genus Hoffmannseggia consists of 21 species distributed amphitropically between the arid regions of the south-western United States and adjacent Mexico, and west-central South America. This pattern of geographical disjunction is shared by numerous other angiosperm genera and has been the subject of discussions for more than a century with various authors advocating a northern origin for particular taxa and others advocating a southern origin. This study uses a well-supported phylogeny of a genus with numerous species in each area to address the issues of a northern or southern origin and the facility with which organisms move between the two continents. Location South-western United States and northern Mexico, northern Chile and Argentina, southern Bolivia, and western Peru. Methods Using DNA sequence data from the nuclear and chloroplast genomes, we generated a phylogenetic hypothesis for all species of Hoffmannseggia rooted with Zuccagnia and Balsamocarpon. Geographical data were optimized on the resultant tree to assess the probable continent of origin for the genus, the pattern of disjunctions between North and South America, and species radiations within the genus. Main conclusions Hoffmannseggia arose in South America and initially split into a suffrutescent (somewhat woody) and an herbaceous clade. Within each of these major clades, there have been at least two exchanges between North and South America. There are no data to support an ancestral pan-American range for Hoffmannseggia and we therefore ascribe the amphitropical disjunctions to long-distance dispersal. The phylogeny clearly shows that all dispersals were from South to North America and they occurred at different times and thus the pattern is not the result of a single simultaneous set of dispersals.