Biogeographic patterns and evolutionary history of Elatostema (Urticaceae).

Background: The paleotropics, home to half of Earth's rainforests, exhibit remarkable biodiversity and complex biogeographic patterns. Understanding the intercontinental distribution of plant taxa between Africa and Asia in this region is crucial for resolving longstanding debates on plant evolution and dispersal mechanisms. This study investigates the genus Elatostema, a widely distributed taxon found in subtropical and tropical Africa, Asia, and Australasia, aiming to elucidate the factors shaping its modern tropical disjunctions and evolutionary history.

Result: Using molecular dating and ancestral area reconstruction, we reconstructed the historical biogeographic pattern of Elatostema. Our results indicated that the genus originated in tropical Asia during the Eocene, likely associated with boreotropical floras. The evolutionary history of Elatostema involved multiple intercontinental dispersal events, including two independent colonizations of Africa from Asia. Diversification within the core Elatostema clade was primarily driven by events in Asia and Oceania, with key factors contributing to this diversification including reciprocal dispersal between Malesia and Australasia, eastward island hopping and karstification in China. Furthermore, a geographical phylogenetic structure was observed within the core Elatostema clade, possibly due to limited seed and pollen dispersal.

Conclusions: The present study provides the first comprehensive insights into the biogeography and evolution of Elatostema. The presence of numerous narrowly distributed endemics, relatively few widespread species, and geographical structures within Elatostema suggest that limited gene flow may be a crucial factor in speciation and evolutionary processes, similar to other species-rich genera.