Snakes (Erythrolamprus spp.) with a complex toxic diet show convergent yet highly heterogeneous voltage-gated sodium channel evolution

Valeria Ramírez-Castañeda , Rebecca Tarvin, Roberto Marquez

Chemical defense plays a crucial role in shaping ecosystems through selection for toxin resistance and has evolved convergently across multiple lineages. Research on toxin resistance has been pivotal in understanding trait evolution, as it often evolves through a simple genetic mechanism, target-site resistance (TSR), where mutations in target genes confer resistance. However, in tropical ecosystems, multiple selective pressures from prey with different toxins create complex chemical scenarios for predators that require more nuanced research. Royal ground snakes (Erythrolamprus spp.) are significant but understudied predators of poisonous frogs (families Bufonidae and Dendrobatidae), whose toxins affect voltage-gated sodium channels (VGSCs) and other neuromuscular system proteins. This study introduces Erythrolamprus snakes as a model for studying toxin resistance. We investigated the evolution of TSR in VGSC genes in relation to toxic frog predation, tracing the phylogenetic origin and geographic distribution of TSR-conferring genotypes across six Erythrolamprus species and outgroups. Our findings reveal convergent yet highly heterogeneous TSR evolution in at least two species known to predate poisonous frogs. Amino acid changes at nine resistance-related positions across eight VGSC genes were identified, suggesting coordinated evolution across this gene family. Four of these changes are known to provide tetrodotoxin resistance in other species. We observed polymorphism in resistance-related sites across species, populations, and VGSC paralogs, possibly maintained through trade-offs among toxin resistance, VGSC function, and the snakes' wide geographic and ecological range. These findings offer new insights into adaptation mechanisms in predators with complex toxic diets, advancing our understanding of coevolution in tropical ecosystems.