Repeated asynchronous evolution of single-species endemics of ivies (Hedera L.) in Macaronesian archipelagos

Abstract

Aim: Evolutionary studies of oceanic island endemics are usually focused on lineages that have experienced in situ radiation, while those that speciated once on the island or archipelago but have not further diversified (single-species endemics) remain understudied. The Macaronesian archipelagos, in the Atlantic Ocean, are home to significant numbers of single-species endemics. The genus Hedera L. (12 species) includes three single-species endemics from three Macaronesian archipelagos with putatively independent origins. Here, we tested the role of phylogenetic niche conservatism in their evolution. To that end, we (1) reconstructed the spatio-temporal origin of Macaronesian ivies using phylogenomics, and (2) assessed the role of climatic niche during their colonization and speciation. Location: Azores, Canary Islands, Madeira, western Mediterranean. Taxon: Hedera spp. Methods: We used 166 samples representing all Hedera species to generate genotyping-by-sequencing (GBS) libraries and performed time-calibrated phylogenomic and biogeographic analyses. Climatic preferences and climatic niche evolution were assessed using a geo-referenced database of the western ivy species (2,297 records). Results: Independent and asynchronous colonization and speciation were estimated for the three Macaronesian ivies, resulting in different degrees of phylogenetic and climatic niche differentiation: H. canariensis displayed an early divergence (7.5–12 Ma) and high phylogenetic and niche isolation; H. azorica had intermediate phylogenetic isolation and niche divergence from its closest relative H. helix (4.4–6.8 Ma) and H. maderensis was embedded within the climatically similar H. iberica (2.8–4.6 Ma). A strong phylogenetic signal was suggested for climatic niche in the western clade of Hedera, where the three Macaronesian ivies are placed. Main Conclusions: The three Macaronesian ivies represent the evolutionary stages leading to the emergence of single-species island endemics. Climatic niche conservatism appears to be involved in the evolution of single-species endemics, first by facilitating island colonization, and then by limiting in situ diversification.