The Origin and Phylogenetic Relationships of the Californian Chaparral ‘Paleoendemic’ Pickeringia (Leguminosae)

Abstract—

Pickeringia, a monotypic genus of the Leguminosae endemic to the xerophytic sclerophyllous chaparral vegetation of the California Floristic Province, has been considered a “relict” of legume diversity in the North American flora and an example of the classic “paleoendemics” in the flora of California. Evidence is presented for the phylogenetic relationships of this genus, inferred from nucleotide sequence variation in the internal transcribed spacers of nuclear ribosomal DNA and the plastid matK gene. Phylogenies derived from maximum parsimony and Bayesian analyses both strongly support a close relationship of Pickeringia to the temperate to subtropical, deciduous genera Cladrastis and Styphnolobium of tribe Sophoreae consistent with morphological and cytogenetic evidence rather than to members of tribe Thermopsideae where the genus has been treated taxonomically. Cladrastis is resolved as paraphyletic while Styphnolobium is strongly supported as a monophyletic group. These results, plus an estimated age of ~31 million years for the genus, further substantiate the hypothesis that Pickeringia is geographically isolated in the flora of western North America, an old and phylogenetically distinct lineage of an early diverging group of papilionoid legumes that were much more widely distributed throughout temperate North America and Asia during the Tertiary but whose modern relatives are restricted to southern North America, Central America, and eastern Asia. These findings have implications not only for the evolutionary history of Pickeringia but also for the age and development of chaparral vegetation in the Californian flora.