Meeting Abstract
The convergent evolution of tetrodotoxin (TTX) resistance in garter snakes (Thamnophis) has shown remarkable predictability. Patterns at both the phenotypic and genetic level have been consistent across multiple Thamnophis species throughout North America, implying that there may be strong molecular constraints involved in this adaptive trait. To further examine the predictability of TTX-resistance, we studied patterns of resistance in the Sierra Garter Snake (Thamnophis couchii). To characterize the phenotype, we quantified TTX resistance at both the whole animal and skeletal muscle levels. We then determined patterns of functional genetic variation in these snakes by genotyping three sodium channel genes (Nav1.4, Nav1.6, and Nav1.7) that are the molecular targets of TTX, and that display important amino acid replacements known to confer TTX-resistance in snakes. Finally, we measured expression levels in one of these genes (Nav1.4), known to be a locus of major effect in other Thamnophis species. We found that T. couchii demonstrate high variation in phenotypic TTX-resistance, despite possessing identical amino acid sequences in all three genes (that is, each gene is fixed for a single allele). Furthermore, we found no differences in gene expression in our candidate locus between TTX-resistant and TTX-sensitive snakes. Currently, resistance cannot be explained by a relationship between Nav genotype and phenotype. These results suggest that there are additional loci or other genetic mechanisms involved in TTX-resistance in T. couchii and that TTX-resistance may not be as predictable as previously thought.
