Proteins, Physiology, and Phenotypes: Assessment of Predator and Antipredator Adaptations in a Chemically Mediated System
Loading...
Authors
Robinson, Kelly Elizabeth
Issue Date
2024
Type
Dissertation
Language
Keywords
Adaptation , Antipredator Defenses , Chemical Defenses , Chemical Ecology , Color Defenses , Toxicokinetics
Alternative Title
Abstract
Organisms are continually confronted with new challenges and understanding how populations respond to these challenges and adapt to a changing world is a central goal in evolutionary biology. Adaptations can occur in various forms, from simple mutations that alter a protein, to specialized physiological structures, to complex behaviors. While adaptations usually arise to overcome a specific selective challenge ( i.e ., role), they can be co-opted for other purposes or have compounding impacts on other aspects of organismal life history and ecology. Thus, it is important to understand the mechanistic basis of specific adaptations, the various roles adaptations play across the life history of an organism and how selection is acting. Coevolutionary systems are ideal for studying adaptations because they often provide simple systems where two or more interacting species experience intense reciprocal selection. These relationships often lead to arms-races with escalating adaptations and counter adaptations. Here, I conducted research to explore the adaptations in a coevolving predator-prey system, that of toxic Pacific newts ( Taricha ) and resistant garter snakes ( Thamnophis ). Pacific newts are well defended prey that exude a highly lethal toxin �" tetrodotoxin (TTX). TTX is a potent neurotoxin that binds to voltage-gated sodium channels, which blocks the movement of sodium ions and thereby prevents the firing of action potentials in muscles and nerves, leading to paralysis or asphyxiation in predators. Despite this lethal defense, garter snakes from several populations in western North America are known to prey on sympatric newts with little to no ill effects. Through decades of research, it has been uncovered that the genetic and physiological basis of TTX resistance in Thamnophis stems from specific mutations in sodium channels that reduce the binding affinity of TTX to these channels. It has been repeatedly demonstrated that newts and snakes are engaged in arms race coevolution. This arms race is characterized by a pattern of escalating prey and predator traits across the landscape, whereby newt TTX levels and snake TTX resistance broadly correspond across their shared range. Where newts possess little TTX, sympatric snakes have correspondingly low resistance and in places where newts are especially potent, the snakes have high resistance. Nevertheless, there are several locations where Thamnophis are so extremely resistant to TTX that sympatric newts could not possibly harm these snakes. The fact that newts are over-matched in multiple locations might suggest some constraints on newts from producing or acquiring ever higher levels of TTX to combat snake predation. Another possibility is that newts may be investing in other avenues of defense ( i.e ., color) rather than TTX as TTX is costly to produce/acquire and/or maintain. We explored these two ideas with two different projects. We first examined the role that newt coloration plays in their defense against predators. Newts display a stark countershading coloration whereby they are dull and brown on their dorsum and vibrant orange on their ventrum. It has long been thought that this pattern provides an aposematic signal, in which newts display their vibrant bellies and throats to predators as a way to signal their toxicity and avoid predation. However, this signal has only been quantified in two of the four species. Furthermore, there is a need to understand how newts compare to their background substrates, as such, color must be understood not only within the individual but also across species and their background substrates. Newt color can play multiple roles in the life of the newt and may also serve as camouflage for newts to avoid detection by their predators. We quantified dorsal and ventral coloration of all four species of newts ( Taricha granulosa , T. rivularis , T. sierrae , T. torosa ) as well as quantified the color of common substrate types. We found support for the hypothesis that the newt dorsum provides camouflage to newts. Furthermore, the contrasts between the newt ventrum and the habitat may be an important contrast for predators as we identified that this is a stark contrast to predators. Lastly, we found a relationship between ventral color and TTX levels in newts, with newts showing an increasing contrast between ventrum and habitat was related to an increasing toxicity in the newts. Our results would suggest newts display an "honest" signal to predators. We then examined the complete chemical composition of newt secretions to gain a better understanding of the chemical compounds that newts utilize, besides TTX. Previous work on newt defenses has focused almost exclusively on TTX. We examined the skin secretion of all four species of Taricha with proteomic and metabolomic approaches. We found that newts display similar proteomic and metabolomic profiles but there was variation in the abundance of molecules between species. We identified 11 potential TTX analogs in our samples, and a few of these analogs only appeared in one or two of the newt species. Finally, we explored the relationship among TTX, TTX analog diversity, chemical diversity (excluding TTX and analogs) and TTX levels quantified through an assay. We found that TTX analog diversity had a strong positive relationship with TTX levels while chemical diversity had a strong negative relationship to TTX levels. However, our results show that there is no strong relationship between TTX analog diversity and chemical diversity, meaning that there may be some chemical constraints but there is not a direct tradeoff between chemical diversity and TTX analog diversity. This suggests that there may be some interactions between these compounds as those with high TTX levels have reduced chemical diversity whereby those with high TTX analog diversity have high TTX levels. These results highlight the complexity of chemical compound usage and composition in newts. Through this study we have gained insights into a more complete chemical profile of newt skin secretions as well as a deeper understanding of TTX diversity in newts. Finally, not all TTX-resistant snake species express ion channel mutations or contain allelic variation in ion channel genes that can explain phenotypic variation in TTX resistance. It seems likely that garter snakes employ additional measures of toxin resistance. Therefore, we sought to understand the toxicokinetics of TTX in snakes�"how TTX is processed, where it is distributed in the body, and how it is eliminated�"to understand how resistance may have evolved and how these snakes are eliminating TTX. We assessed how TTX was eliminated from TTX-resistant and TTX-sensitive populations of Thamnophis spp. as well as two species of snakes that do not consume TTX-bearing organisms. Although there was no clear pattern of elimination, we did find that the three species that have both resistant and non-resistant populations seem to eliminate TTX from their body at slower rates than the two outgroup species that are not resistant to TTX. However, within species, both resistant and non-resistant populations appear to eliminate TTX at similar rates. Finally, all snakes retain substantial amounts of TTX, and could be dangerous to their own predators' days to weeks following the ingestion of a single newt. Thus, aspects of toxin metabolism may have been key in driving predator-prey relationships, and important in determining other ecological interactions such as protection for garter snake from their own predators. These three projects address various aspects of the chemically mediated relationship between predator and prey, emphasizing the complexities of adaptations that surround coevolving species. These relationships can often be oversimplified, as research often focuses on the evolution of single traits ( i.e ., TTX and TTX resistance). We assessed two defensive traits in newts ( i.e. , color and chemical compounds) to better understand their defensive arsenal against predators. Furthermore, we explored other avenues of toxin resistance in snake predators and how these predators may actually gain protection from their own predators by consuming noxious prey. Predator-prey interactions lead to a wide array of adaptations associated with avoiding predation or overcoming prey defenses. It is important to describe the mechanistic basis of traits across multiple levels of biological organization in order to understand how those traits function in the organism and how those traits impact the ecology and evolutionary trajectories of species.