Evolution of androdioecy in Berchemia (Rhamnaceae) revealed by phylogenomics and ancestral character reconstruction

Androdioecy is an extremely rare sexual system in angiosperms, and its evolutionary origins and potential mechanisms remain poorly understood. The presence of androdioecy in Berchemia provides a valuable model for studying the evolution of plant reproductive strategies. In this study, we reconstructed a high-resolution phylogeny of Berchemia based on whole-genome resequencing data from 114 Rhamnaceae samples, confirming that the core Berchemia forms a monophyletic group and resolving four well-supported clades (Berchemia I-IV). Divergence time analysis indicated that the genus originated in the early Eocene (∼55.55 Ma), with key diversification events coinciding with the Eocene-Oligocene transition (EOT). Ancestral state reconstruction suggested that hermaphroditism and same-year fruit ripening were ancestral traits, whereas androdioecy and second-year fruit ripening evolved once and twice within the genus, respectively. Notably, all species in Berchemia IV exhibit the combined traits of androdioecy and second-year fruit ripening. We infer that the emergence of androdioecy may represent an adaptation to reproductive resource trade-offs imposed by second-year fruit ripening, which itself potentially emerged in a climatic context of EOT cooling. The results of this study suggest that historical climatic events and reproductive resource trade-offs may be associated with shifts in sexual systems and reproductive phenology in Berchemia, offering new insights into the origins of androdioecy and its ecological correlates.