Time-calibrated phylogeny of Daphne (Thymelaeaceae): Pre-mediterranean temporal origin of the sclerophyllous Daphne gnidium

Abstract

The sclerophyllous syndrome is characterized by well-defined traits (evergreen coriaceous leaves, inconspicuous flowers, and fleshy fruits). It has been hypothesized that lineages displaying the sclerophyllous syndrome show lower speciation rates than non-sclerophyllous lineages after the establishment of the mediterranean climate. Daphne gnidium displays sclerophyllous traits and some differentiation into three subspecies (gnidium, mauritanica, maritima), but the spatio-temporal origin of this taxonomic group is unknown due to the lack of a time-calibrated phylogeny of the whole genus. Here, we inferred phylogenetic relationships and divergence times of Daphne (32 species, 238 samples) and other genera of Thymelaeaceae (16 genera, 38 species, 34 samples) using the internal transcribed spacer (ITS), which revealed that the current circumscription of Daphne is profoundly polyphyletic because some species are nested within the genera Wikstroemia and Edgeworthia. In contrast, D. gnidium formed a well-supported clade as recognized in taxonomic accounts (subgenus Spachia). We found morphological and phylogenetic support for Daphne mauritanica as a monophyletic lineage sister to D. gnidium. Divergence between D. gnidium and D. mauritanica appears to have predated the establishment of seasonally dry conditions, which supports a pre-mediterranean temporal origin of the lineage. A phylogeographical analysis within D. gnidium based on 66 nrDNA (ITS) and 84 cpDNA (rps16, trnV) sequences agreed with the low differentiation of the species in the Pleistocene despite its large distribution range. Altogether, D. gnidium illustrates one more example of the sclerophyllous syndrome with no speciation after the onset of the mediterranean climate.