The skin microbiome facilitates adaptive tetrodotoxin production in poisonous newts

<p>Rough-skinned newts (<em>Taricha granulosa</em>) use tetrodotoxin (TTX) to block voltage-gated sodium (Na_v) channels as a chemical defense against predation. Interestingly, newts exhibit extreme population-level variation in toxicity attributed to a coevolutionary arms race with TTX-resistant predatory snakes, but the source of TTX in newts is unknown. Here, we investigated whether symbiotic bacteria isolated from toxic newts could produce TTX. We characterized the skin-associated microbiota from a toxic and non-toxic population of newts and established pure cultures of isolated bacterial symbionts from toxic newts. We then screened bacterial culture media for TTX using LC-MS/MS and identified TTX-producing bacterial strains from four genera, including <em>Aeromonas</em>, <em>Pseudomonas</em>, <em>Shewanella</em>, and <em>Sphingopyxis</em>. Additionally, we sequenced the Na_v channel gene family in toxic newts and found that newts expressed Na_v channels with modified TTX binding sites, conferring extreme physiological resistance to TTX. This study highlights the complex interactions among adaptive physiology, animal-bacterial symbiosis, and ecological context. </p>