Toxic Newts Research Articles

Both novelty and conspicuousness influence selection by mammalian predators on the colour pattern ofPlethodon cinereus(Urodela: Plethodontidae)

Predators influence the evolution of colour pattern in prey species, yet how these selective forces might differ among predators is rarely considered. In particular, prey colour patterns that indicate unpalatability to some predator species may not carry the same signal for other predators. We test several hypotheses of selection on patterning between mammal predators and the polymorphic salamander Plethodon cinereus, which, under an avian visual system appears as a mimic of the toxic newt Notophthalmus viridescens. We fit each hypothesis against field observations of mammalian attacks on salamander clay replicas. We then develop a novel analytical procedure that enables the combination of multiple non-exclusive models in a likelihood framework. We find that mammals do not follow any single hypothesis proposed, including the hypothesis of mimicry. Instead, mammals in this system use visual cues while foraging to avoid unfamiliar, novel prey and attack conspicuous prey. We propose that mammals may help to maintain colour pattern polymorphism within populations of P. cinereus by avoiding novel, unfamiliar colour morphs. Additionally, selective pressures from multiple predators and variation in predator communities among sites may contribute to the maintenance of colour polymorphism within and among localities in this salamander species.

Point-Mutations in Skeletal Muscle Voltage-Gated Sodium Channels Confer Resistance to Tetrodotoxin: But at a Cost?

The interaction between toxic newts (Taricha) and several resistant garter snakes (Thamnophis) provides a model system for the study of predator-prey coevolution (Brodie & Brodie 1999). Pacific newts secrete through their skin the potent neurotoxin, tetrodotoxin (TTX, Fuhrman et al., 1964), a compound that specifically binds to the outer pore of voltage-gated sodium channels (NaV proteins, Fozzard et al., 1994). TTX-ligation prevents sodium ion movement across the membrane and therefore abolishes excitability in skeletal muscle and peripheral nerves (Hille, 2001). Three species of garter snake that prey on sympatric newts have adapted to these toxic newts by circumventing the block through several independently evolved mutations within the pore loop of the skeletal muscle channel variant, NaV1.4 (Feldman et al., 2012). These mutations reduce the affinity of TTX to the pore and provide physiological resistance (Feldman et al., 2009). However, these same mutations are also expected to alter the biophysical properties of the channel, in particular the permeation and selectivity (Hille, 2001). We show that animals with greater TTX resistance displayed diminished skeletal muscle performance (phasic and tetanic contractions). We are currently testing the hypothesis that changes in skeletal muscle sodium channel current density, activation, inactivation, and recovery could potentially explain the tissue-level findings.

Molecular structures of two tetrodotoxin analogs containing a monooxa-hydrocarbon cage: A computational study

Using quantum chemical calculations we investigate the molecular structures of two tetrodotoxin (TTX) analogs recently isolated from the Japanese toxic newt Cynops ensicauda popei. These novel analogs are characterized by a monooxa-hydrocarbon cage with a direct C5–C10 bond that replaces one of the ether bridges in the canonical dioxa-adamantane cage of TTX. The computed change in the 13C NMR chemical shifts is in good agreement with the change in the corresponding experimental values that results from the above chemical modification. This confirms the chemical structure assigned to the TTX analogs. A topological analysis of the theoretical electronic charge density indicates that the removal of the oxygen bridge in TTX increases the magnitude of the charge density at the cage critical point. A database search indicates that the monooxa-hydrocarbon cage is also present in other natural products such as cinnzeylanine and platensimycin whose molecular structures have been characterized by single-crystal X-ray diffraction analyses.

Functional and physiological resistance of crayfish to amphibian toxins: tetrodotoxin resistance in the white river crayfish (Procambarus acutus)

Freshwater crayfish are reported to consume early life-history stages of a number of toxic amphibians. Although previous research indicates toxic amphibians are palatable to crayfish, the potential toxicity associated with consumption of toxic prey has been poorly described. We sought to characterise the supposed tetrodotoxin (TTX) resistance of freshwater crayfish, which have been observed to eat the eggs and larvae of toxic Taricha Gray, 1850 newts. White river crayfish (Procambarus acutus (Girard, 1852)) consumed 7.7 ± 4.0 Rough-skinned Newt (Taricha granulosa (Skilton, 1849)) eggs (mean ± SD) when offered 10 eggs in controlled feeding trials. Eggs were determined to contain a concentration of 1239 ± 571 ng (mean ± SD) of TTX. A dose-response assay was then performed to compare ingested doses with physiological TTX resistance. Crayfish were highly susceptible to TTX when administered as an intramuscular injection; TTX doses of 0.1 mass-adjusted mouse units were lethal to 100% of P. acutus crayfish. We established that while crayfish were capable consumers of highly toxic newt eggs, these decapods did not demonstrate physiological resistance to TTX. These findings suggest that crayfish have some functional resistance that renders them capable of consuming TTX-bearing prey despite a lack of physiological resistance to TTX.

Survival and growth of the caddisfly Limnephilus flavastellus after predation on toxic eggs of the Rough-skinned Newt (Taricha granulosa)

The Rough-skinned Newt ( Taricha granulosa (Skilton, 1849)) possesses a powerful neurotoxin, tetrodotoxin, in the skin that is secondarily deposited in the ova. Although assumed to serve an antipredator function in the eggs, empirical evidence of the toxin’s role in preventing egg predation is lacking. In this study, we characterized the aquatic macroinvertebrate community at a location sympatric with extremely toxic newts and estimated the abundance of caddisflies. We tested aquatic macroinvertebrates sympatric with toxic newts for their capacity to consume the toxic eggs, and examined the propensity of egg predation and its effect on growth of the only known predator of newt eggs, caddisfly larvae. Limnephilid caddisfly larvae were the only invertebrate observed to consume substantial quantities of toxic newt eggs. Survival and growth of the caddisfly Limnephilus flavastellus Banks, 1918 continued when larvae consumed toxic eggs and did not differ from L. flavastellus that also had access to an alternative food source (detritus). Limnephilus flavastellus that had access to eggs + detritus consumed a similar number of eggs compared with those provided with eggs only. These results, combined with the abundance of caddisflies, suggest that caddisflies are important predators of eggs of T. granulosa.

Toxic Prey and Predator Avoidance: Responses of Toxic Newts to Chemical Stimuli from a Predator and Injured Conspecifics

Abstract Predator avoidance behavior minimizes predation risk because the prey never enters the perceptual field of the predator. For toxic species, avoiding potential predators indiscriminately is unlikely to be the best predator avoidance strategy because many predators will be incapable of consuming the toxin. We investigated the role of chemical-based avoidance behavior in predator-prey interactions between a toxic prey species, the Rough-skinned Newt (Taricha granulosa), and its major predator, the Common Garter Snake (Thamnophis sirtalis). Terrestrial newts avoided predator kairomones only when the predator had recently consumed a conspecific, but did not avoid damage-released alarm cues. These results indicate newts may be able to chemically differentiate between toxin-resistant and toxin-sensitive snakes in the same population and avoid only those snakes actively preying upon their cohort.

Predators Make (Temporary) Escape from Coevolutionary Arms Race

Predators Make (Temporary) Escape from Coevolutionary Arms Race

Evolutionary diversification of TTX-resistant sodium channels in a predator–prey interaction

Understanding the molecular genetic basis of adaptations provides incomparable insight into the genetic mechanisms by which evolutionary diversification takes place. Whether the evolution of common traits in different lineages proceeds by similar or unique mutations, and the degree to which phenotypic evolution is controlled by changes in gene regulation as opposed to gene function, are fundamental questions in evolutionary biology that require such an understanding of genetic mechanisms. Here we identify novel changes in the molecular structure of a sodium channel expressed in snake skeletal muscle, tsNa(V)1.4, that are responsible for differences in tetrodotoxin (TTX) resistance among garter snake populations coevolving with toxic newts. By the functional expression of tsNa(V)1.4, we show how differences in the amino-acid sequence of the channel affect TTX binding and impart different levels of resistance in four snake populations. These results indicate that the evolution of a physiological trait has occurred through a series of unique functional changes in a gene that is otherwise highly conserved among vertebrates.

Parallel Arms Races between Garter Snakes and Newts Involving Tetrodotoxin as the Phenotypic Interface of Coevolution

Parallel "arms races" involving the same or similar phenotypic interfaces allow inference about selective forces driving coevolution, as well as the importance of phylogenetic and phenotypic constraints in coevolution. Here, we report the existence of apparent parallel arms races between species pairs of garter snakes and their toxic newt prey that indicate independent evolutionary origins of a key phenotype in the interface. In at least one area of sympatry, the aquatic garter snake, Thamnophis couchii, has evolved elevated resistance to the neurotoxin tetrodotoxin (TTX), present in the newt Taricha torosa. Previous studies have shown that a distantly related garter snake, Thamnophis sirtalis, has coevolved with another newt species that possesses TTX, Taricha granulosa. Patterns of within population variation and phenotypic tradeoffs between TTX resistance and sprint speed suggest that the mechanism of resistance is similar in both species of snake, yet phylogenetic evidence indicates the independent origins of elevated resistance to TTX.

The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts.

The "geographic mosaic" approach to understanding coevolution is predicated on the existence of variable selection across the landscape of an interaction between species. A range of ecological factors, from differences in resource availability to differences in community composition, can generate such a mosaic of selection among populations, and thereby differences in the strength of coevolution. The result is a mixture of hotspots, where reciprocal selection is strong, and coldspots, where reciprocal selection is weak or absent, throughout the ranges of species. Population subdivision further provides the opportunity for nonadaptive forces, including gene flow, drift, and metapopulation dynamics, to influence the coevolutionary interaction between species. Some predicted results of this geographic mosaic of coevolution include maladapted or mismatched phenotypes, maintenance of high levels of polymorphism, and prevention of stable equilibrium trait combinations. To evaluate the potential for the geographic mosaic to influence predator-prey coevolution, we investigated the geographic pattern of genetically determined TTX resistance in the garter snake Thamnophis sirtalis over much of the range of its ecological interaction with toxic newts of genus Taricha. We assayed TTX resistance in over 2900 garter snakes representing 333 families from 40 populations throughout western North America. Our results provide dramatic evidence that geographic structure is an important component in coevolutionary interactions between predators and prey. Resistance levels vary substantially (over three orders of magnitude) among populations and over short distances. The spatial array of variation is consistent with two areas of intense evolutionary response by predators ("hotspots") surrounded by clines of decreasing resistance. Some general predictions of the geographic mosaic process are supported, including clinal variation in phenotypes, polymorphism in some populations, and divergent outcomes of the interaction between predator and prey. Conversely, our data provide little support for one of the major predictions, mismatched values of interacting traits. Two lines of evidence suggest selection is paramount in determining population variation in resistance. First, phylogenetic information indicates that two hotspots of TTX resistance have evolved independently. Second, in the one region that TTX levels in prey have been quantified, resistance and toxicity levels match almost perfectly over a wide phenotypic and geographic range. However, these results do not preclude the role the nonadaptive forces in generating the overall geographic mosaic of TTX resistance. Much work remains to fill in the geographic pattern of variation among prey populations and, just as importantly, to explore the variation in the ecology of the interaction that occurs within populations.

Great Horned Owl Death from Predation of a Toxic California Newt

We report the first records of the ingestion of a toxic California newt (Taricha torosa) and a particular species of Jerusalem cricket (Stenopelmatus fuscus) by a Great Horned Owl (Bubo virginianus). This is one of only a few records of avian predation on this genus of toxic newt. Ingestion of the extremely poisonous tetrodotoxin present in the newt's skin was likely the cause of the owl's death.

EVOLUTIONARY RESPONSE OF PREDATORS TO DANGEROUS PREY: PREADAPTATION AND THE EVOLUTION OF TETRODOTOXIN RESISTANCE IN GARTER SNAKES.

Coevolutionary interactions typically involve only a few specialized taxa. The factors that cause some taxa and not others to respond evolutionarily to selection by another species are poorly understood. Preadaptation may render some species predisposed for evolutionary response to new pressures, whereas a lack of genetic variation may limit the evolutionary potential of other taxa. We evaluate these factors in the predator-prey interaction between toxic newts (Taricha granulosa) and their resistant garter snake predators (Thamnophis sirtalis). Using a bioassay of resistance to tetrodotoxin (TTX), the primary toxin in the prey, we examined phenotypic evolution in the genus Thamnophis. Reconstruction of ancestral character states suggests that the entire genus Thamnophis, and possibly natricine snakes in general, has slightly elevated TTX resistance compared to other lineages of snakes. While this suggests that T. sirtalis is indeed predisposed to evolving TTX resistance, it also indicates that the potential exists in sympatric congeners not expressing elevated levels of TTX resistance. We also detected significant family level variation for TTX resistance in a species of Thamnophis that does not exhibit elaborated levels of the trait. This finding suggests that evolutionary response in other taxa is not limited by genetic variability. In this predator-prey system, species and population differences in resistance appear to be largely determined by variation in the selective environment rather than preadaptation or constraint.

Repeated Injections of TTX Do Not Affect TTX Resistance or Growth in the Garter Snake Thamnophis sirtalis

The garter snake Thamnophis sirtalis is a known predator of the toxic newt Taricha granulosa. Resistance to the primary toxin of the newt, tetrodotoxin (TTX), varies among individuals and has a genetic basis. We investigated the consequences of longterm exposure to this toxin in an experiment designed to determine (1) whether increased tolerance to tetrodotxin occurs following repeated injections of tetrodotoxin in T. sitralis and (2) whether repeated exposure to tetrodotoxin has an effect on the growth of juvenile snakes. We measured resistance to tetrodotoxin for each individual based on reduction in crawl speed. Pairs of neonatal snakes were chosen based on similar resistance and size and randomly split into two groups, control and experimental. Each group received intraperitoneal injections every two weeks of either saline (control) or tetrodotoxin (experimental) for six months. Mass data were collected five times during the year. Length data were collected twice during the year. After one year, resistance was again measured and compared with neonate resistance. Analyses showed that repeated injections of tetrodotoxin had no effect on resistance or growth.

Evolution, Predator Avoidance, and Unsuccessful Predation

Commentaires sur les criteres de Vermeij (Ibid., 1982, 120, 701-720) pour l'identification des adaptations liees a la viabilite et, en particulier, aux defenses contre les predateurs