Meeting Abstract
Adaptive evolution may proceed down a predictable molecular path when functional constraints or pleiotropy restrict molecular evolution to a limited number of amino acid positions. We found convergent, adaptive changes in the sodium channel protein of the common garter snake (Thamnophis sirtalis) in response to its poisonous prey, the Pacific newt (Taricha spp.). These newts possess tetrodotoxin (TTX), a lethal neurotoxin that blocks nerve and muscle sodium channels and prevents the propagation of action potentials. The toxin deters most predators; however, key amino acid replacements in the pore region of the NaV1.4 skeletal muscle sodium channel of T. sirtalis inhibit toxin binding and confer large increases in TTX resistance. We constructed a NaV1.4 gene tree and surveyed TTX resistance alleles across western North America where the predator and prey co-occur. We found that TTX resistance likely evolved independently in two separate coevolutionary hotspots: central California and the Pacific Northwest. In each instance, adaptive amino acid replacements occurred in the same specific pore region of the NaV1.4 channel. Population frequencies of these TTX resistance alleles were also tightly correlated with sympatric levels of newt toxicity. The occurrence of repeated adaptive changes within a single species suggests that convergent molecular evolution may not be uncommon, particularly when the availability of beneficial mutations is limited by functional constraints like the maintenance of sodium channel function.
