Experimental evidence supports gradual evolution of alkaloid sequestration in poison frogs

The evolution of alkaloid sequestration and modification in poison frogs provides an opportunity to explore the emergence of complex traits, particularly the importance of intermediate phenotypes. The origin of alkaloids in lineages of frogs may, at first glance, appear to have undergone saltational evolution in amphibians. However, the recent detection of low levels of alkaloids in several species previously thought to be 'undefended' could represent an intermediate step between passive alkaloid retention and active sequestration. This study aims to address gaps in understanding the evolutionary pathway of alkaloid sequestration and modification in poison frogs. We conducted controlled alkaloid ingestion experiments in several frog species, including sequestering and putatively non-sequestering species, to assess their ability to accumulate and modify dietary alkaloids. Our results suggest that alkaloid sequestration and modification evolved gradually, with intermediate phenotypes representing transitional stages between non-sequestering species and the active sequestration and modification observed in some poison frogs. Given current understanding of alkaloid transport and metabolism, these results allow us to propose possible mechanisms that enabled the gradual adaptation of poison frogs to evolve from alkaloid resistance to active sequestration and modification.