Genetic architecture of a feeding adaptation: garter snake (Thamnophis) resistance to tetrodotoxin bearing prey

Abstract

Detailing the genetic basis of adaptive variation in natural populations is a first step towards understanding the process of adaptive evolution, yet few ecologically relevant traits have been characterized at the genetic level in wild populations. Traits that mediate coevolutionary interactions between species are ideal for studying adaptation because of the intensity of selection and the well-characterized ecological context. We have previously described the ecological context, evolutionary history and partial genetic basis of tetrodotoxin (TTX) resistance in garter snakes (Thamnophis). Derived mutations in a voltage-gated sodium channel gene (Na v 1.4) in three garter snake species are associated with resistance to TTX, the lethal neurotoxin found in their newt prey (Taricha). Here we evaluate the contribution of Na v 1.4 alleles to TTX resistance in two of those species from central coastal California. We measured the phenotypes (TTX resistance) and genotypes (Na v 1.4 and microsatellites) in a local sample of Thamnophis atratus and Thamnophis sirtalis. Allelic variation in Na v 1.4 explains 23 per cent of the variation in TTX resistance in T. atratus while variation in a haphazard sample of the genome (neutral microsatellite markers) shows no association with the phenotype. Similarly, allelic variation in Na v 1.4 correlates almost perfectly with TTX resistance in T. sirtalis, but neutral variation does not. These strong correlations suggest that Na v 1.4 is a major effect locus. The simple genetic architecture of TTX resistance in garter snakes may significantly impact the dynamics of phenotypic coevolution. Fixation of a few alleles of major effect in some garter snake populations may have led to the evolution of extreme phenotypes and an 'escape' from the arms race with newts.

Is there more than one way to skin a newt? Convergent toxin resistance in snakes is not due to a common genetic mechanism

“…Although the most toxic newt contained only 0.353 mg TTX, roughly 80 times less than the most potent Taricha (Stokes et al, 2015), the amount of toxin present in Eastern Newts is still sufficient to provide protection against almost any predator (Brodie and Brodie, 1990). However, Eastern Hog-nosed Snakes, one of only a few vertebrates known to consume the terrestrial efts (summarized in Table 1), show remarkable levels of toxin resistance (Table 2), on par with the most resistant populations of Thamnophis Feldman et al, 2009Feldman et al, , 2010 (Figure 3). In fact, all but the smallest H. platirhinos in these populations could safely consume the most toxic N. viridescens known arranged phylogenetically (after Pyron et al, 2013).…”

“…Remarkably, in H. platirhinos, the SCN4A gene (encoding Na v 1.4), a locus that appears to underlie a major portion of resistance in other vertebrates (Geffeny et al, 2005;Soong and Venkatesh, 2006;Jost et al, 2008;Feldman et al, 2012;Hanifin and Gilly, 2015), contains no allelic variation that would alter TTX ligation to the pore. Thamnophis with comparable levels of TTX resistance typically have two to four amino acid substitutions in this protein (Feldman et al, 2009(Feldman et al, , 2010 (Figure 3). Thus, Heterodon appears to have evolved a novel mechanism of TTX resistance.…”

Constraint shapes convergence in tetrodotoxin-resistant sodium channels of snakes

“…We focused on the Ploops because TTX interacts with residues of the outer pore (see refs. [35][36][37][38], and changes in P-loop sites appear to be responsible for TTX resistance in animals (32,34,(39)(40)(41)(42)62).…”

“…Structural changes in the skeletal muscle sodium channel (Na v 1.4) modify the molecular environment of the channel pore and dramatically alter TTX-binding affinity to the protein (32). Functional allelic variation in Na v 1.4 correlates tightly with whole-animal resistance to TTX and suggests that changes in this single gene are largely responsible for within-and among-population differences in resistance (32)(33)(34). Because TTX binds so selectively to the pore of sodium channels, we predict that biophysical constraints associated with channel function have led to a limited set of convergent molecular adaptations to the challenge of TTXbearing prey worldwide.…”

Historical Contingency in a Multigene Family Facilitates Adaptive Evolution of Toxin Resistance

“…Although the precise expression patterns of these two channels are unknown in reptiles, transcriptomic data from lizards [31,32] and snakes [33] suggest that they are expressed in peripheral nerves (see the Supplemental Experimental Procedures). The P-loop sequence of Na v 1.4 varies within and among T. sirtalis populations, with different alleles providing different levels of TTX resistance roughly matching the toxicity of local newts, suggesting a relatively recent origin of resistant skeletal muscle [12,27,28,34]. In contrast, substitutions conferring resistance to Na v 1.6 and Na v 1.7 are fixed across T. sirtalis populations [15], suggesting that resistance in peripheral nerves has a more ancient origin.…”