Phenotypic consequences of adaptive toxin production and resistance in coevolutionary partners
The effect that ecological interactions between species have on their evolution is an impactful but understudied area of research. The influence species have on each other's evolution has taken on new relevance as anthropogenic change alters where, when, and how species interact. Partnered species that directly influence selection for traits in one another represent one such interaction where selection may be intense. In this case, evolutionary constraints and consequences are measurable in phenotypic traits that mediate the interaction. Antagonistic coevolution, which can arise between predator and prey, may influence traits directly responsible for survival and may favor the predator who only risks loss of a meal while their prey risk loss of life (e.g. life-dinner principle). When prey are dangerous, selection may be more balanced with both species exerting selective pressure on the other. This reciprocal selection can lead to rapid adaptation as predator and prey evolve to avoid deadly outcomes. However, specialized adaptations require a balance between cost and benefit. Evolutionary changes depend on existing genetic architecture and must maintain the function of essential traits. Thus, tradeoffs may arise, and they may limit the extent to which coevolutionary arms-race dynamics between predator and prey develop, determining which party will best the other. In the western U.S., toxic Pacific newts (Taricha) are defended by a highly potent neurotoxin. This neurotoxin makes them too dangerous for nearly all vertebrates to consume. Taricha are defended by tetrodotoxin (TTX) which blocks voltage-gated sodium channels (Nav) and inhibits nerve activation and muscle function, typically leading to death by respiratory failure. Amazingly, multiple species of garter snakes (Thamnophis) have adapted to overcome this defense, harboring mutations that prevent TTX from binding to its target. These mutations, however, also decrease protein and muscle function, which suggests garter snakes may suffer evolutionary tradeoffs. Due to the severity of effects that sodium channel mutations cause in humans, we expect that altering a trait that is highly conserved across vertebrates would have similar costs in snakes. However, many specialized adaptations evolve repeatedly. For example, TTX resistance has evolved independently at least three or four times in Thamnophis, which suggests selection may favor specialization in specific environments. We first measured a key physiological process, standard metabolic rate, to investigate whether specialized adaptations in toxin-resistant garter snakes affect baseline energy expenditure. In snakes, skeletal muscles power ventilation which influences metabolism, so inefficiencies of sodium channels in these muscles might impact whole-animal energy expenditure. Because sodium channels are membrane-bound proteins, suboptimal temperatures may increase membrane rigidity, further altering sodium channel conformation and exacerbating inefficiencies of channel kinetics and performance. We measured standard metabolic rate in two species, Th. atratus and Th. sirtalis. Because these two species independently evolved tetrodotoxin resistance through unique mutations, we can treat them as natural replicates with distinct underlying genetics and potential physiological costs. Despite our expectations, neither resistance phenotype nor sodium channel genotype affected metabolism, and resistant snakes did not perform worse under suboptimal body temperature. Instead, Th. atratus and Th. sirtalis had nearly identical rates of mass-adjusted energy expenditure at both temperatures, despite differing eco-morphologies, life histories, and distant phylogenetic positions. We then examined whether biophysical tradeoffs in a gene of major importance impact whole-animal performance by measuring speed, endurance, and antipredator behavior in three Thamnophis species with an array of Nav genotypes and corresponding resistance phenotypes. We predicted that resistant snakes with weaker underlying skeletal muscles would be slower, with lower endurance, and they would rely on less energetically costly but riskier antipredator defensive behavior rather than attempting to flee. Contrary to our predictions, we found little evidence that protein and muscle costs impact sprint or endurance abilities. However, some TTX-resistant animals differed in time spent in evasive versus defensive behavior. Taken together, our results suggest that TTX-resistant snakes might exhibit compensatory mechanisms to offset the costs of mutations. Finally, we sought to understand how Pacific newts balance the cost of toxin production and maintenance, as well as foraging for carotenoid pigments to produce both TTX and warning coloration. When threatened, Taricha assume a specific defensive posture known as the 'unken reflex' in which they display bright orange bellies against a dark dorsal surface. Because newts are defended by TTX, this behavior is assumed to be an aposematic signal to warn predators that an attack would be unprofitable or even fatal. However, there have been no analyses of pacific newt coloration, so it is unknown whether coloration in Taricha is qualitatively honest (signaling toxic defense) or quantitatively honest (signaling toxin level). We measured ventral color and brightness as well as the contrast between ventrum and dorsum for two species (T. granulosa, T. sierrae) within and across populations. We predicted that contrasting body regions exposed during unken reflex would be visible to predators (qualitatively honesty) but would not correlate with toxin level (quantitatively dishonest). Using two color-capture methods (spectrometry and hyper-spectral imaging), we found support for a model of qualitative honesty but did not find quantitative honesty in the form of correlation between any colorimetric and toxin level at either the intra- or interpopulation level for either species. These three projects address if and how tradeoffs impose constraints on the pace and outcomes of adaptive evolution. Despite clear negative protein performance impacts stemming from a beneficial adaptation in toxin producing newts and their resistant garter snake predators, neither showed evidence of whole-animal tradeoffs. The geographic mosaic of coevolution between these species is comprised of populations where snakes are sufficiently resistant that they could survive eating any newt they encounter as well as populations in which newts could incapacitate and escape any garter snake. It may be that the existence of these extreme levels of toxin production and toxin resistance provides enough evidence that newts and snakes are able to compensate for underlying physiological costs before they impose tradeoffs at the whole-animal level, allowing the arms-race to continue.
Adaptation, Aposematism, Coevolution, Constraint, Toxin resistance, Tradeoff
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