Convergent adaptation to dangerous prey proceeds through the same first-step mutation in the garter snake Thamnophis sirtalis

Convergent phenotypes often result from similar underlying genetics, but recent work suggests convergence may also occur in the historical order of substitutions en route to an adaptive outcome. We characterized convergence in the mutational steps to two independent outcomes of tetrodotoxin (TTX) resistance in separate geographic lineages of the common garter snake (Thamnophis sirtalis) that coevolved with toxic newts. Resistance is largely conferred by amino acid changes in the skeletal muscle sodium channel (NaV1.4) that interfere with TTX-binding. We sampled variation in NaV1.4 throughout western North America and found clear evidence that TTX-resistant changes in both lineages began with the same isoleucine-valine mutation (I1561V) within the outer pore of NaV1.4. Other point mutations in the pore, shown to confer much greater resistance, accumulate later in the evolutionary progression and always occur together with the initial I1561V change. A gene tree of NaV1.4 suggests the I156...