Evolutionary History of the Palearctic genus Helianthemum (Cistaceae)

Resum

Evolutionary radiations are responsible for much of the diversity on Earth, but the mechanisms leading to their rapid diversification are not always clear. Ascertaining phylogenetic relationships among taxa is central to understand diversification patterns as well as to reconstruct the evolution of niches and traits. However, reconstructing the relationships of closely related species is a challenging task because of methodological (e.g. lack of resolution in specific DNA regions) and biological (e.g. incomplete lineage sorting or gene flow among incipient lineages) reasons. The genus Helianthemum, whose centre of diversity is located in the Western Mediterranean area, is the most diverse (ca. 100 species) and widespread of the family Cistaceae, ranging from Macaronesia to central Asia throughout the Mediterranean, Saharo-Arabian e Irano-Turanian Floristic Regions inhabiting a diversity of habitats from subtropical xerophytic to alpine environments. The species differ in life form and floral traits intimately related to mate and breeding system (from self-incompatibility to complete selfing by cleistogamy) and its complex systematic and taxonomy is supposedly due to a rapid and recent diversification. In this thesis, molecular data (DNA regions based on Sanger sequencing, Genotyping by sequencing and microsatellites) is combined with biogeographical, environmental and morphological information in order to reconstruct the evolutionary history of the genus Helianthemum and infer the macro and microevolutionary processes that have led to the species-rich diversity of this lineage. Our results suggest that the most recent common ancestor of the genus may occurred in a pre-mediterranean type ecosystem of Northern Africa during the Middle Miocene, and its evolution has probably entailed three evolutionary radiations corresponding to its three largest sections (Eriocarpum, Pseudocistus and Helianthemum). These radiations may have been initially driven by the paleo-climatic and geological events that impacted the Mediterranean Basin during the last 6 Mya, and maintained by recent events of hybridization, convergence, isolation by distance and local adaptation, whilst strong environmental niche and floral conservatism may have prevailed along its rapid diversification. Overall, the robust spatio-temporal framework of the genus Helianthemum here retrieved represents a window into the past biogeography of the Mediterranean Basin, and highlights the essential role played by this area as a cradle of diversity and an evolutionary hub, facilitating transitions between environmental niches and contributing to the building up of this important biodiversity hotspot on the Earth.