Origin and diversification of Thymelaea (Thymelaeaceae): inferences from a phylogenetic study based on ITS (rDNA) sequences

Abstract

Thymelaea is a Mediterranean genus belonging to a primarily tropical and subtropical family. This genus is here presented as a particular case on which the hypothesis of an in situ evolution of the Mediterranean flora from a Tertiary subtropical stock can be phylogenetically tested. To better understand the evolutionary history of Thymelaea, molecular phylogenies based on ITS (rDNA) sequences are estimated. Phylogenetic analyses strongly support the monophyly of Thymelaea and the sister position of Daphne. Unexpected relationships between sequences of T. granatensis, T. pubescens and T. sanamunda are found and explained as a case of cryptic nuclear introgression. Phylogenetic analyses also reveal the polyphyletic character of Diarthron, sensu Tan, suggesting that Dendrostellera and Stelleropsis should be reinstated as independent genera. Based on the resulting phylogenetic hypothesis and using statistical tests it is suggested that Thymelaea experienced an initial rapid diversification. Time estimates for the origin and radiation of Thymelaea based on independent sources of evidence—paleontological, paleoclimatic and paleogeographic data, on one hand, and ITS substitution rates reported for other angiosperm groups, on the other—give strikingly similar results. The early radiation of Thymelaea is inferred to have occurred in the Upper Miocene, after this lineage diverged from that of Daphne in the Lower or Middle Miocene. Both evolutionary events are related to the overall climatic deterioration (declining temperature and increasing dryness) registered throughout the Miocene in Eurasia. The rapid and extensive diversification of Thymelaea is placed in the Iberian Peninsula or here and in the neighbouring North African territory.