Quaternary Radiation of Spring Ephemerals

The emergence of East Asian spring ephemerals and the unique ecosystem can be attributed primarily to vicariance, brought about by the Quaternary rifting of the Okinawa Trough, the formation of the East China Sea, and the isolation of the island chains of Ryukyu, Japan, and Taiwan from the Asian continent. The northern forests of Japan, dominated by Fagus crenata and the associated Quercus crispula, present a captivating display of spring-flowering ephemerals, including Erythronium japonicum, Viola bissetii, Anemone pseudoaltaica, and Corydalis fukuharae. Among these, Asarum is also considered part of the spring ephemerals. Our primary focus for phylogenetic analyses, which included constructing a haplotype network using PopART version 1.7 and molecular dating with BEAST v1.10.4, was on the genus Asarum. In the BEAST analysis, we set the Quaternary geological event calibration at 1.55 ± 0.15 Ma and applied pre-Quaternary fossil calibrations. When we input 1.55 Ma into BEAST, the analysis suggested that spring ephemerals underwent a simultaneous splitting and diversification event around that time, approximately 1.55 Ma, during the Quaternary period. The differentiation of these species is more likely a result of adaptive radiation rather than vicariance, particularly given their partially sympatric distribution and occurrence across islands. This radiation likely originated from the most recent common ancestor of the ingroup species of spring ephemerals and can be seen as a response to significant environmental changes associated with the formation of the East China Sea around 1.55 Ma. Notably, species such as Asarum megacalyx and Fagus crenata exhibit adaptations to heavy snowfall, further supporting this idea. The spring ephemerals in beech or oak forests in North America and Europe may have radiated and diversified as a result of Quaternary global climatic changes.