Transoceanic Stepping-stones between Cretaceous waterfalls? The enigmatic biogeography of pantropical Oocyclus cascade beetles.

Beetles have colonized freshwater habitats multiple times throughout their evolutionary history. Some of these aquatic lineages are associated exclusively with waterfall-like habitats, often with modified morphologies to cope with their unusual way of life. The historical biogeography of such cascade beetle lineages has been shown to strongly reflect ancient tectonic events. We focus on the pantropical genus Oocyclus of which species dwell in waterfalls and associated habitats. We infer the first molecular phylogeny of Oocyclus using a dataset of seven gene fragments. We recover a well resolved phylogenetic hypothesis, with a monophyletic Oocyclus divided in three genetically well-differentiated subclades which correspond to geography. Comparative dating analyses across Hydrophilidae based on ten fossil calibrations recover a Cretaceous origin for the genus. Based on a comprehensive suite of ancestral range analyses, we suggest a unique pattern with an origin in Southeast Asia followed by the successive colonization of India and the Neotropics via transoceanic stepping-stone dispersal. Diversification rate analyses support a scenario in which old Oocyclus lineages diversified slowly with a homogeneous rate regime. Waterfall beetle radiations are ancient and remarkably track Earth's paleogeological history, shedding light on intricate patterns of macroevolution.