Out of and in East Asia: Phylogeny, biogeography and diversification of Thalictroideae (Ranunculaceae) in the Northern Hemisphere.

Background and aims: Understanding the biogeographical patterns and processes underlying the distribution of diversity within the Northern Hemisphere has fascinated botanists and biogeographers for over a century. However, as a well-known centre of species diversity in the Northern Hemisphere, whether East Asia acted as a source and/or a sink of plant diversity of the Northern Hemisphere remains unclear. Here, we used Thalictroideae, a subfamily widely distributed in the Northern Hemisphere with the majority of species in East Asia, to investigate the role of East Asia in shaping the biogeographical patterns of the Northern Hemisphere and to test whether East Asia acted as a museum or a cradle for herbaceous taxa.

Methods: Based on six plastid and one nuclear DNA regions, we generated the most comprehensive phylogeny for Thalictroideae including 217 taxa (ca. 66% species) from all ten of the currently recognized genera. Within this phylogenetic framework, we then estimated divergence times, ancestral ranges, and diversification rates.

Key results: The monophyletic Thalictroideae contains three major clades. All genera with more than one species are strongly supported as monophyletic except for Isopyrum, which is nested in Enemion. The most recent common ancestor of Thalictroideae occurred in East Asia in the late Eocene (ca. 36 Ma). From the Miocene onwards, at least 46 dispersal events were inferred to be responsible for the current distribution of this subfamily. East Asian Thalictroideae lineages experienced a rapid accumulation at ca. 10 Ma.

Conclusions: The biogeographical patterns of Thalictroideae support the "out of and in East Asia" hypothesis, i.e., East Asia is both a source and a sink of biodiversity of the Northern Hemisphere. The global cooling after the middle Miocene Climatic Optimum, combined with the exposed land bridges due to sea-level decline, might have jointly caused the bidirectional plant exchanges between East Asia and other Northern Hemisphere regions. East Asia serves as evolutionary museums and cradles for the diversity of Thalictroideae and likely for other herbaceous lineages.