Phylogeny, biogeography, and chromosome evolution of the amphitropical genus Grindelia (Asteraceae) inferred from nuclear ribosomal and chloroplast sequence data

Grindelia is among the most taxonomically challenging groups of North American composites. The genus as a whole has an amphitropical distribution, with approximately half of the species native to North America and Mexico and the remainder native to South America. We used DNA sequence data from the nuclear ribosomal ITS and EFS and chloroplast psal-accD regions to revisit hypotheses on biogeographic history across the genus. Grindelia as a whole is well-supported and is composed of two sister clades, one native to South America and the other native to North America including Mexico. The North American taxa constitute two clades that largely occur on different sides of the Continental Divide. The diverse radiation of Grindelia in the California Floristic Province appears to be most closely related to species from the Great Basin and Colorado Plateau and evidently descended from drought-adapted ancestors. Although Steyermark's hypotheses about the relationships of North American Grindelia are not all supported, we did recover a clade corresponding to his Pacific radiation and many of the Mexican and Texan species that he hypothesized to be basal in the genus represent early diverging lineages in our trees. Dunford's cytogenetic data on the North American species of Grindelia were also examined in a phylogenetic context.