Evolutionary Patterns of Avian Tetrodotoxin Resistance

Abstract

Beyond Arms Races: Evolution of Tetrodotoxin-Binding Sites in Avian Naᵥ Channels Tetrodotoxin (TTX) is a potent neurotoxin that blocks voltage-gated sodium channels (Naᵥ), inhibiting nerve and muscle function. Some species possess TTX as an anti-predator defense, and some predator species have evolved resistance to TTX via evolutionary changes in Naᵥ channels. TTX resistance is well known in reptiles and amphibians, but its occurrence in birds has been relatively unexplored. Examining how Naᵥ channels have evolved in birds offers a valuable way to contrast lineages shaped by selection imposed by encountering TTX-bearing prey (such as newts) with those that evolved without such pressure and to make generalizations about molecular evolution in this gene family. This thesis explores patterns of TTX resistance across nine Naᵥ channel paralogs (SCN1A–SCN11A) in 107 bird species. Phylogenetic analyses reveal a patchy distribution of resistance across the avian tree, with multiple independent gains and losses of resistance concentrated in cardiac and peripheral nervous system channels. These patterns resemble those seen in resistant reptiles. Previous work in reptiles also suggested that a vertebrate diet may either directly or indirectly lead to the evolution of TTX resistance in some Naᵥ channels. In birds, we found that although the correlation between diet and Naᵥ resistance was not statistically significant, four-state ARD models suggested a trend toward more frequent gains of resistance in vertivorous lineages than in non-vertivores. We found that TTX resistance evolved multiple times in the avian cardiac channel gene (SCN5A), raising the question of whether Naᵥ resistance evolves as a side effect of selection of some aspect of cardiac physiology. To test this idea, we used a database of avian heart rates, in which we tested for an evolutionary association between SCN5A resistance resting heart rate. We found no significant evolutionary correlation, instead, resistance appears to arise sporadically, possibly in response to lineage-specific pressures. This work presents the first comprehensive survey of TTX resistance in birds, indicating that even in lineages with low or zero toxin exposure, sodium channel evolution can follow complex and convergent pathways. Our results show that resistance arises in scattered bird lineages, including species with little or no exposure to TTX. Bird Naᵥ paralogs contain many of the same resistance-associated substitutions seen in squamates, implying a shared evolutionary toolbox of allowable changes; in birds, these substitutions appear in a patchy, transition-rich pattern that is not easily attributable to TTX exposure, highlighting background constraints and recurrent substitutions rather than a toxin-driven arms race.