Are Pacific Chorus Frogs (Pseudacris regilla) Resistant to Tetrodotoxin (TTX)? Characterizing Potential TTX Exposure and Resistance in an Ecological Associate of Pacific Newts (Taricha)

Abstract

Animals that frequently encounter toxins often evolve mechanisms of toxin resistance. Both predators that consume toxic prey and organisms in physical contact with a toxin or pollutant in their environment may experience natural selection for resistance. Based on observations that Pacific Chorus Frogs (Pseudacris regilla) sometimes eat and mistakenly amplect tetrodotoxin (TTX)-defended Taricha newts, we hypothesized that P. regilla may possess resistance to TTX. We tested this prediction by comparing the amino acid sequences of the domain IV pore loop of the muscle voltage-gated sodium channel gene SCN4A (Nav1.4 protein) in populations of P. regilla that are sympatric and allopatric with Taricha. We identified a single substitution in the domain IV pore loop of Nav1.4 in P. regilla that has been hypothesized to provide TTX resistance based on computational modeling, though its role in resistance has been challenged. Nonetheless, both allopatric and sympatric P. regilla had this substitution, suggesting that it may be unrelated to TTX exposure from Taricha. Thus, there is no conclusive evidence that P. regilla has evolved TTX resistance encoded by amino acid substitutions in this domain. In addition, California occurrence data from the last 50 years indicate that Taricha activity peaks in January while the activity of P. regilla peaks in April. These relatively distinct activity patterns suggest that P. regilla may not be exposed to levels of TTX from Taricha that are high enough to require mutations in SCN4A. Nevertheless, other unidentified mechanisms of TTX resistance could be present in P. regilla and other species that are sympatric with toxic newts.