Beyond the rand flora pattern: Phylogeny and biogeographical history of Volutaria (compositae)

Abstract

The Rand Flora describes the disjunct phytogeographical pattern of a wide range of taxa distributed at the periphery of Africa and adjacent islands, as well as the Arabian Peninsula. We focused on Volutaria (Compositae: Cardueae: Centaurineae), a genus of ca. 18 species that conforms to the Rand Flora pattern; yet its phylogeny, interspecific relationships, and biogeographical history remain poorly known. We aim to construct a robust phylogeny that will allow us to interpret the biogeography and the diversification of this genus, together with its present distribution pattern, and to ascertain whether the latter arose by single or multiple gradual expansion processes, vicariance, or long-distance dispersal events. We sampled all extant Volutaria species, covering its entire geographical range, and generated sequences of nuclear-ribosomal DNA and three plastid regions, which were analyzed by Bayesian inference and maximum parsimony. Samples yielding more than one sequence in direct sequencing were cloned. Historical biogeographical analysis was performed using BioGeoBEARS based on a phylogeny dated using a relaxed molecular clock calibrated with a previous dating undertaken for the tribe Cardueae. Volutaria is a monophyletic taxon, having an Asian ancestor, and its present diversity is represented by four main clades that emerged in the Miocene. The earliest taxa of Volutaria to diverge are preserved in North Africa, whereas clades that diverged more recently have done so on both sides of the Sahara. This process involved both ancient and recent interspecific introgression and hybridization events, as indicated by incongruities between plastid and nuclear results, and by cloning of the ITS region. The distribution of Volutaria around two diversity poles conforms to the Rand Flora pattern, but this did not arise by a single event or process. Instead, the historical biogeography of Volutaria involves at least one migration wave from east to west, followed by the extinction of ancestral taxa and subsequent expansion and retraction events, together with speciation processes on both sides of the Sahara. The intense tectonic and climatic changes that occurred in North Africa and western Asia throughout the Neogene and Pleistocene periods might explain the present diversity and distribution pattern of the genus.