Convergent Substitutions in a Sodium Channel Suggest Multiple Origins of Toxin Resistance in Poison Frogs

Abstract

Complex phenotypes typically have a correspondingly multifaceted genetic component. However, the genotype-phenotype association between chemical defense and resistance is often simple: genetic changes in the binding site of a toxin alter how it affects its target. Some toxic organisms, such as poison frogs (Anura: Dendrobatidae), have defensive alkaloids that disrupt the function of ion channels, proteins that are crucial for nerve and muscle activity. Using protein-docking models, we predict that three major classes of poison frog alkaloids (histrionicotoxins, pumiliotoxins, and batrachotoxins) bind to similar sites in the highly conserved inner pore of the muscle voltage-gated sodium channel, Nav1.4. We predict that poison frogs are somewhat resistant to these compounds because they have six types of amino acid replacements in the Nav1.4 inner pore that are absent in all other frogs except for a distantly related alkaloid-defended frog from Madagascar, Mantella aurantiaca. Protein-docking models and comparative phylogenetics support the role of these replacements in alkaloid resistance. Taking into account the four independent origins of chemical defense in Dendrobatidae, phylogenetic patterns of the amino acid replacements suggest that 1) alkaloid resistance in Nav1.4 evolved independently at least seven times in these frogs, 2) variation in resistance-conferring replacements is likely a result of differences in alkaloid exposure across species, and 3) functional constraint shapes the evolution of the Nav1.4 inner pore. Our study is the first to demonstrate the genetic basis of autoresistance in frogs with alkaloid defenses.

Does batrachotoxin autoresistance coevolve with toxicity inPhyllobatespoison‐dart frogs?

“…Combining data from previous work (Tarvin et al. ; Yuan and Wang ) and newly generated sequences, we amassed a dataset of SCN4A sequences from 147 individuals of 45 species (35 Dendrobatoids and 10 outgroups; Tables and ), including the five known species of Phyllobates (36 samples; 2–14 per species). Alkaloid profiles are available for 30 of the 35 dendrobatoids used (Table ), which allows us to confidently assume that at least among the species sequenced BTX secretion originated at the base of Phyllobates .…”

“…; Santos and Cannatella ), and recent studies have identified amino acid substitutions on ion‐transport proteins targeted by these toxins that coincide phylogenetically with the origins of alkaloid sequestration (Tarvin et al. , ; Yuan and Wang ). Some of these changes have been shown to provide toxin resistance in vitro (Tarvin et al.…”

Interacting amino acid replacements allow poison frogs to evolve epibatidine resistance

“…It is unclear how common such pre-existing compensatory mutations are, although it appears that mutations providing incremental increases in resistance are quite common. In Danainae butterflies, newts, garter snakes, and poison frogs, toxin resistance tends to increase over evolutionary time via additional AA replacements that occur in parallel with increased concentrations of chemical defenses (15, 16, 34, 35). It is possible that pre-adaptive mutations that allow resistance to evolve with little cost are present in these organisms and simply have not been identified.…”

“…The Epipedobates, Ameerega , and Dendrobates ( Oophaga ) clades, which are evolutionarily young (6, 36), are an example of rapid and ongoing diversification possibly driven by the evolution of resistance to anti-predator toxins (15). We demonstrate that resistance to epibatidine involves finely tuning a highly conserved binding site without disrupting receptor function, providing insights into evolutionary pathways culminating in chemical defenses.…”

“…Toxic animals, including poison frogs, often evolve resistance to their toxins via amino acid (AA) replacements in toxin-binding sites (target-site insensitivity; 15, 16). The location of these replacements is constrained by protein function, leading to predictable and convergent mechanisms of resistance (17).…”