Island biogeography of the Macaronesian Gesnouinia and Mediterranean Soleirolia (Parietarieae, Urticaceae) with implications for the evolution of insular woodiness

Abstract

Urticaceae tribe Parietarieae serves as an excellent example to study hypotheses on Macaronesian‐​Mediterranean island biogeography. Parietarieae is distributed in both of these floristically closely related regions and contains two island endemic genera. Gesnouinia is endemic to Macaronesia and was considered a Tertiary relict from a European paleotropical vegetation. This, however, may contradict the general idea of insular woodiness also proposed for this genus. For the origin of the western Mediterranean island endemic Soleirolia, a vicariance scenario, i.e., the split of the Hercynian massif in southern Europe has been suggested. To evaluate these hypotheses and to provide a time frame for island and inter‐island colonization, we applied molecular phylogenetics and dating based on two plastid and one nuclear DNA marker. Additionally, we performed ancestral area reconstruction, analyzed the anatomy of the stem and reconstructed the ancestral character states of woodiness. Our results indicate that Gesnouinia colonized Macaronesia during the Miocene via long‐distance dispersal and may, therefore, be a Tertiary laurel forest relict. Diversification between the laurel forest species Gesnouinia arborea and the rupicolous and more xeric G. filamentosa occurred within Macaronesia during the Pleistocene, possibly due to climate fluctuations. Therefore, G. arborea is not a relict from the Tertiary. Stem anatomy suggests that although woodiness in Gesnouinia likely is derived, it may have evolved prior to island colonization. Soleirolia originated in the Mediterranean during the Miocene. This rules out vicariance from the split of the Hercynian massif in southern Europe during the Oligocene. The divergence of Soleirolia from Majorca, Corsica, Sardinia and the Italian mainland dates to the Pleistocene. This indicates long‐distance dispersal as predominant colonization mode, not ruling out that migration between these areas was possibly facilitated by land bridges during the Pleistocene glaciation cycles.