The origin of plants endemic to the Mediterranean and Irano-Turanian floristic regions: case studies from the Citrus family (Rutaceae)

Abstract

The present thesis deals with aspects of biogeography, phylogenetics, systematics, and evolution. Its goal was to investigate patterns of biotic assembly in the Mediterranean region, with special emphasis on the effect of earth processes on species origins and distribution, the origin of species endemic to islands, and the biogeographic links between the Mediterranean and Irano-Turanian regions. To address these issues, two genera of Rutaceae (citrus family) that met several requirements were selected as model systems: Ruta and Haplophyllum. The former, the type genus of the family, is restricted to the Mediterranean region and comprises species endemic to both continental fragment (Corsica and Sardinia) and oceanic (Canary Islands) islands. The latter, one of the most species-rich genera of the family, has been used to characterize the flora of the Irano-Turanian region, where it reaches maximum species diversity, but also includes species endemic to the Mediterranean region. In Chapter I we generated molecular phylogenies for Ruta and closely related taxa, essential to provide a robust framework for subsequent biogeographic analyses. Moreover, we tested conflicting taxonomic treatments of Ruta and affiliated taxa based on different classes of characters. The analyses supported the current circumscription of Ruta and showed that the genus can only be diagnosed by using a suite of homoplasious, plesiomorphic, and autapomorphic morphological character states. Conflict between molecular and phytochemical datasets was ascribed to convergence in secondary chemical compounds. In Chapter II we carried out biogeographic analyses of Ruta aimed at elucidating the time frame and sequence of range expansion events associated with its origin and diversification, focusing mainly on the island endemics. Biogeographic scenarios were proposed by integrating information from phylogeny, molecular dating, and ancestral range reconstruction methods that incorporate palaeo-geographic models. The analyses showed that Ruta invaded the Mediterranean region from the north before the onset of the current Mediterranean climate. Land migration through a temporary connection between the Corso- Sardinian and Apulian microplates, followed by vicariance, was inferred as the process underlying the origin of the Corso-Sardinian endemic lineage. The origin and diversification of the clade restricted to the Canary Islands was explained by means of a single colonization event of the archipelago, driven by long-distance dispersal from North Africa, followed by inter-island speciation and parallel invasion of similar ecological zones. In Chapter III we carried out molecular phylogenetic analyses of Haplophyllum in order to explore the biogeographic links between the Mediterranean and Irano-Turanian regions. The analyses identified many instances of species non-monophyly, but also cases of strongly-supported species monophyly. Optimization of morphological characters on the molecular phylogeny indicated that several species of the genus, especially those with a widespread distribution, can only be diagnosed by combinations of homoplasious character states. Preliminary biogeographic patterns suggested that the Mediterranean representatives of the genus arrived from the east multiple times, corroborating the hypothesis that the Irano- Turanian region served as a key source for the colonization of the Mediterranean region.