Unveiling historical connections between island and continental floras: Phylogeographic evidence from Macaronesian bryophytes.

Since the traditional perception of islands as sinks of continental biodiversity, the relationships between island and mainland floras have been controversial. In particular, the larger carrying capacity and connectivity of islands during glacial periods has suggested a potentially important role of islands for continental diversity, especially in organisms with high dispersal capacities. Focusing on bryophytes, we determine the historical relationships between island and mainland populations and test the hypothesis that island colonization prevailed during glacial periods. Competing scenarios regarding the biogeographic origin and timing of colonization of islands were tested from analyses of DNA sequence data in 9 amphi-Atlantic species disjunct between Macaronesia and continental regions on both sides of the North Atlantic in a coalescence framework. The Macaronesian bryophyte flora results from a complex mix of origins, with the archipelagos acting as a source or a sink for mainland species from various continents, highlighting the complexity of its assembly mechanisms. No clear pattern emerged regarding the timing of colonization events. Our analyses indicate that extra-European regions have contributed to the diversity of Macaronesian bryophytes much beyond what the 90% of floristic similarity between Europe and Macaronesia would suggest. In groups with high long-distance dispersal capacities like bryophytes, floristic similarity hence appears as a poor indicator of biogeographic origin. The large contribution of extra-European regions to the Macaronesian bryophyte flora also explains why the endemic element has a primarily extra-European origin.