Natural selection often produces convergent changes in unrelated lineages, but the degree to which such adaptations occur via predictable genetic paths is unknown. If only a limited subset of ...possible mutations is fixed in independent lineages, then it is clear that constraint in the production or function of molecular variants is an important determinant of adaptation. We demonstrate remarkably constrained convergence during the evolution of resistance to the lethal poison, tetrodotoxin, in six snake species representing three distinct lineages from around the globe. Resistance-conferring amino acid substitutions in a voltage-gated sodium channel, Nav1.4, are clustered in only two regions of the protein, and a majority of the replacements are confined to the same three positions. The observed changes represent only a small fraction of the experimentally validated mutations known to increase Nav1.4 resistance to tetrodotoxin. These results suggest that constraints resulting from functional tradeoffs between ion channel function and toxin resistance led to predictable patterns of evolutionary convergence at the molecular level. Our data are consistent with theoretical predictions and recent microcosm work that suggest a predictable path is followed during an adaptive walk along a mutational landscape, and that natural selection may be frequently constrained to produce similar genetic outcomes even when operating on independent lineages.
Antagonistic coevolution between natural enemies can produce highly exaggerated traits, such as prey toxins and predator resistance. This reciprocal process of adaptation and counter‐adaptation may ...also open doors to other evolutionary novelties not directly involved in the phenotypic interface of coevolution. We tested the hypothesis that predator–prey coevolution coincided with the evolution of conspicuous coloration on resistant predators that retain prey toxins. In western North America, common garter snakes (Thamnophis sirtalis) have evolved extreme resistance to tetrodotoxin (TTX) in the coevolutionary arms race with their deadly prey, Pacific newts (Taricha spp.). TTX‐resistant snakes can retain large amounts of ingested TTX, which could serve as a deterrent against the snakes' own predators if TTX toxicity and resistance are coupled with a conspicuous warning signal. We evaluated whether arms race escalation covaries with bright red coloration in snake populations across the geographic mosaic of coevolution. Snake colour variation departs from the neutral expectations of population genetic structure and covaries with escalating clines of newt TTX and snake resistance at two coevolutionary hotspots. In the Pacific Northwest, bright red coloration fits an expected pattern of an aposematic warning to avian predators: TTX‐resistant snakes that consume highly toxic newts also have relatively large, reddish‐orange dorsal blotches. Snake coloration also seems to have evolved with the arms race in California, but overall patterns are less intuitively consistent with aposematism. These results suggest that interactions with additional trophic levels can generate novel traits as a cascading consequence of arms race coevolution across the geographic mosaic.
see also the Perspective by John N. Thompson
Where do the genetic variants underlying adaptive change come from? Are currently adaptive alíeles recruited by selection from standing genetic variation within populations, moved through ...introgression from other populations, or do they arise as novel mutations? Here, we examine the molecular basis of repeated adaptation to the toxin of deadly prey in 3 species of garter snakes (Thamnophis) to determine whether adaptation has evolved through novel mutations, sieving of existing variation, or transmission of beneficial alíeles across species. Functional amino acid substitutions in the skeletal muscle sodium channel $(Na_v 1.4)$ are largely responsible for the physiological resistance of garter snakes to tetrodotoxin found in their newt (Taricha) prey. Phylogenetic analyses reject the hypotheses that the unique resistance alíeles observed in multiple Thamnophis species were present before the split of these lineages, or that alíeles were shared among species through occasional hybridization events. Our results demonstrate that adaptive evolution has occurred independently multiple times in garter snakes via the de novo acquisition of beneficial mutations.
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.
Novel adaptations must originate and function within an already established genome 1. As a result, the ability of a species to adapt to new environmental challenges is predicted to be highly ...contingent on the evolutionary history of its lineage 2–6. Despite a growing appreciation of the importance of historical contingency in the adaptive evolution of single proteins 7–11, we know surprisingly little about its role in shaping complex adaptations that require evolutionary change in multiple genes. One such adaptation, extreme resistance to tetrodotoxin (TTX), has arisen in several species of snakes through coevolutionary arms races with toxic amphibian prey, which select for TTX-resistant voltage-gated sodium channels (Nav) 12–16. Here, we show that the relatively recent origins of extreme toxin resistance, which involve the skeletal muscle channel Nav1.4, were facilitated by ancient evolutionary changes in two other members of the same gene family. A substitution conferring TTX resistance to Nav1.7, a channel found in small peripheral neurons, arose in lizards ∼170 million years ago (mya) and was present in the common ancestor of all snakes. A second channel found in larger myelinated neurons, Nav1.6, subsequently evolved resistance in four different snake lineages beginning ∼38 mya. Extreme TTX resistance has evolved at least five times within the past 12 million years via changes in Nav1.4, but only within lineages that previously evolved resistant Nav1.6 and Nav1.7. Our results show that adaptive protein evolution may be contingent upon enabling substitutions elsewhere in the genome, in this case, in paralogs of the same gene family.
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•Toxin resistance in sensory neurons evolved in reptiles before the origin of snakes•Resistant motor neurons evolved four times in snakes that eat amphibians•Coevolutionary arms races with toxic prey were facilitated by resistant nerves
Some snakes are extremely resistant to toxins found in their prey, an adaptation requiring changes in several genes. Such changes must either arise simultaneously or accrue in a stepwise fashion over millions of years. To explore these alternatives, McGlothlin et al. reconstructed the history of toxin-resistance genes in snakes.
The repeated evolution of tetrodotoxin (TTX) resistance provides a model for testing hypotheses about the mechanisms of convergent evolution. This poison is broadly employed as a potent antipredator ...defence, blocking voltage‐gated sodium channels (Nav) in muscles and nerves, paralysing and sometimes killing predators. Resistance in taxa bearing this neurotoxin and a few predators appears to come from convergent replacements in specific Nav residues that interact with TTX. This stereotyped genetic response suggests molecular and phenotypic evolution may be constrained and predictable. Here, we investigate the extent of mechanistic convergence in garter snakes (Thamnophis) that prey on TTX‐bearing newts (Taricha) by examining the physiological and genetic basis of TTX resistance in the Sierra garter snake (Th. couchii). We characterize variation in this predatory adaptation across populations at several biological scales: whole‐animal TTX resistance; skeletal muscle resistance; functional genetic variation in three Nav encoding loci; and levels of gene expression for one of these loci. We found Th. couchii possess extensive geographical variation in resistance at the whole‐animal and skeletal muscle levels. As in other Thamnophis, resistance at both levels is highly correlated, suggesting convergence across the biological levels linking organism to organ. However, Th. couchii shows no functional variation in Nav loci among populations or difference in candidate gene expression. Local variation in TTX resistance in Th. couchii cannot be explained by the same relationship between genotype and phenotype seen in other taxa. Thus, historical contingencies may lead different species of Thamnophis down alternative routes to local adaptation.
The Geographic Mosaic Theory of Coevolution predicts that coevolutionary arms races will vary over time and space because of the diverse ecological settings and population histories of interacting ...species across the landscape. Thus, understanding coevolution may require investigating broad sets of populations sampled across the range of the interaction. In addition, comparing coevolutionary dynamics between similar systems may reveal the importance of specific factors that structure coevolution.
Here, we examine geographic patterns of prey traits and predator traits in the relatively unstudied interaction between the Sierra garter snake (Thamnophis couchii) and sympatric prey, the rough‐skinned newt (Taricha granulosa), Sierra newt (Ta. sierrae) and California newt (Ta. torosa). This system parallels, in space and phenotypes, a classic example of coevolution between predatory common garter snakes (Th. sirtalis) and their toxic newt prey exhibiting hotspots of newt tetrodotoxin (TTX) levels and matching snake TTX resistance.
We quantified prey and predator traits from hundreds of individuals across their distributions, and functional trait matching at sympatric sites.
We show strong regional patterns of trait covariation across the shared ranges of Th. couchii and newt prey. Traits differ significantly among localities, with lower newt TTX levels and snake TTX resistance at the northern latitudes, and higher TTX levels and snake resistance at southern latitudes. Newts and snakes in northern populations show the highest degree of functional trait matching despite possessing the least extreme traits. Conversely, newts and snakes in southern populations show the greatest mismatch despite possessing exaggerated traits, with some snakes so resistant to TTX they would be unaffected by any sympatric newt. Nevertheless, individual variation was substantial, and appears to offer the opportunity for continued reciprocal selection in most populations.
Overall, the three species of newts appear to be engaged in a TTX‐mediated arms race with Th. couchii. These patterns are congruent with those seen between newts and Th. sirtalis, including the same latitudinal gradient in trait covariation, and the potential ‘escape’ from the arms race by snake predators. Such concordance in broad scale patterns across two distinct systems suggests common phenomena might structure geographic mosaics in similar ways.
The authors examine a previously unexplored arms race that parallels, in both space and phenotypes, a classic coevolutionary system involving toxic newts and predatory garter snakes. They find tight covariation in prey and predator phenotypes across the landscape but also zones where snakes appear to outmatch their deadly prey. These patterns are congruent across predator–prey systems, suggesting common phenomena might structure coevolution in similar ways.
Landscape patterns of phenotypic coevolution are determined by variation in the outcome of predator–prey interactions. These outcomes may depend not only on the functional phenotypes that mediate ...species interactions, but also on aspects of the environment that enable encounters between coevolutionary partners.
Exploring the relationship between coevolutionary traits and the environment requires extensive sampling across the range of the interaction to determine the relationship between local ecological variation and coevolution.
In this study, we synthesized >30 years of data on predator–prey interactions between toxic newts (Taricha granulosa) and their snake predators (Thamnophis sirtalis) to explore the environmental predictors of arms race escalation.
We found that geographic variation in phenotypes at the interface of coevolution was best predicted by a combination of community and climatic variation. Coevolutionary phenotypes were greatest in environments with climate favourable for newt–snake overlap. We found prey toxicity was elevated in regions with more predator species, and predator resistance was higher in regions with more prey species.
Our results suggest specific environmental conditions reinforce the process of coevolution, signifying the phenotypic outcomes of coevolutionary arms races are sensitive to local ecological contexts that vary across the landscape.
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Read the free Plain Language Summary for this article on the Journal blog.
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 I1561V mutations in each lineage are not identical-by-decent, but rather they arose independently. Convergence in the evolution of channel resistance is likely the result of shared biases in the two lineages of T. sirtalis—only a few mutational routes can confer TTX resistance while maintaining the conserved function of voltage-gated sodium channels.
Reciprocal adaptation is the hallmark of arms race coevolution. Local coadaptation between natural enemies should generate a geographic mosaic pattern where both species have roughly matched ...abilities across their shared range. However, mosaic variation in ecologically relevant traits can also arise from processes unrelated to reciprocal selection, such as population structure or local environmental conditions. We tested whether these alternative processes can account for trait variation in the geographic mosaic of arms race coevolution between resistant garter snakes (Thamnophis sirtalis) and toxic newts (Taricha granulosa). We found that predator resistance and prey toxin levels are functionally matched in co‐occurring populations, suggesting that mosaic variation in the armaments of both species results from the local pressures of reciprocal selection. By the same token, phenotypic and genetic variation in snake resistance deviates from neutral expectations of population genetic differentiation, showing a clear signature of adaptation to local toxin levels in newts. Contrastingly, newt toxin levels are best predicted by genetic differentiation among newt populations, and to a lesser extent, by the local environment and snake resistance. Exaggerated armaments suggest that coevolution occurs in certain hotspots, but prey population structure seems to be of particular influence on local phenotypic variation in both species throughout the geographic mosaic. Our results imply that processes other than reciprocal selection, like historical biogeography and environmental pressures, represent an important source of variation in the geographic mosaic of coevolution. Such a pattern supports the role of “trait remixing” in the geographic mosaic theory, the process by which non‐adaptive forces dictate spatial variation in the interactions among species.
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