title: Parallel evolution of tetrodotoxin resistance in three voltage-gated sodium channel genes in the garter snake Thamnophis sirtalis.
abstract: Members of a gene family expressed in a single species often experience common selection pressures. Consequently, the molecular basis of complex adaptations may be expected to involve parallel evolutionary changes in multiple paralogs. Here, we use bacterial artificial chromosome library scans to investigate the evolution of the voltage-gated sodium channel (Nav) family in the garter snake Thamnophis sirtalis, a predator of highly toxic Taricha newts. Newts possess tetrodotoxin (TTX), which blocks Nav's, arresting action potentials in nerves and muscle. Some Thamnophis populations have evolved resistance to extremely high levels of TTX. Previous work has identified amino acid sites in the skeletal muscle sodium channel Nav1.4 that confer resistance to TTX and vary across populations. We identify parallel evolution of TTX resistance in two additional Nav paralogs, Nav1.6 and 1.7, which are known to be expressed in the peripheral nervous system and should thus be exposed to ingested TTX. Each paralog contains at least one TTX-resistant substitution identical to a substitution previously identified in Nav1.4. These sites are fixed across populations, suggesting that the resistant peripheral nerves antedate resistant muscle. In contrast, three sodium channels expressed solely in the central nervous system (Nav1.1-1.3) showed no evidence of TTX resistance, consistent with protection from toxins by the blood-brain barrier. We also report the exon-intron structure of six Nav paralogs, the first such analysis for snake genes. Our results demonstrate that the molecular basis of adaptation may be both repeatable across members of a gene family and predictable based on functional considerations.
authors.affiliation: Department of Biological Sciences, Virginia Tech, Blacksburg, VA Department of Biology, University of Virginia joelmcg@vt.edu.
authors.forename: Joel W
authors.initials: JW
authors.lastname: McGlothlin
authors.affiliation: Department of Biology, University of Virginia Bio-Rad Laboratories, Hercules, CA.
authors.forename: John P
authors.initials: JP
authors.lastname: Chuckalovcak
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University Division of Genetics and Developmental Biology, National Institutes of Health, Bethesda, MD.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Janes
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University.
authors.forename: Scott V
authors.initials: SV
authors.lastname: Edwards
authors.affiliation: Department of Biology, University of Nevada, Reno.
authors.forename: Chris R
authors.initials: CR
authors.lastname: Feldman
authors.affiliation: Department of Biology, Utah State University.
authors.forename: Edmund D
authors.initials: ED
authors.lastname: Brodie
authors.affiliation: Department of Biological Sciences and Environmental Change Initiative, University of Notre Dame.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Pfrender
authors.affiliation: Department of Biology, University of Virginia Mountain Lake Biological Station, University of Virginia.
authors.forename: Edmund D
authors.initials: ED
authors.lastname: Brodie
mesh_terms: D000222:Adaptation, Physiological; D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D022202:Chromosomes, Artificial, Bacterial; D017812:Colubridae / Q000235:genetics*; D004251:DNA Transposable Elements; D004351:Drug Resistance / Q000235:genetics*; D005091:Exons; D015723:Gene Library; D007438:Introns; D018895:Microsatellite Repeats; D008969:Molecular Sequence Data; D011235:Predatory Behavior; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012455:Salamandridae / Q000502:physiology; D016415:Sequence Alignment; D026941:Sodium Channel Blockers / Q000378:metabolism / Q000633:toxicity*; D013779:Tetrodotoxin / Q000096:biosynthesis / Q000633:toxicity*; D061566:Voltage-Gated Sodium Channels / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
chemical_list: D004251:DNA Transposable Elements; D020033:Protein Isoforms; D026941:Sodium Channel Blockers; D061566:Voltage-Gated Sodium Channels; D013779:Tetrodotoxin
keywords: adaptation; coevolution; gene families; molecular evolution; predator–prey interactions; toxins