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Abstract Phenotypic expression is often constrained by functional conflicts between traits, and the resulting trade-offs impose limits on phenotypic and taxonomic diversity. However, the underlying mechanisms that maintain trade-offs or allow organisms to resolve them via phenotypic plasticity are often challenging to detect. The trade-off between gas exchange and water loss across respiratory surfaces represents a fundamental trade-off that constrains phenotypic diversity in terrestrial life. Here, we investigate plastic mechanisms that mitigate this trade-off in lungless salamanders that breathe exclusively across their skin. Our field and laboratory experiments identified plastic responses to environmental variation in water loss and oxygen uptake, and gene expression analyses identified putative pathways that regulate this trade-off. Although the trade-off was generally strong, its strength covaried with environmental conditions. At the molecular level, antagonistic pleiotropy in multiple biological pathways (e.g., vasoconstriction and upregulation of aerobic respiration) putatively produce the trade-off, while other pathways mitigate the trade-off by affecting a single trait (e.g., oxygen binding affinity, melanin synthesis). However, organisms are likely to encounter novel trade-offs in the process of bypassing another. Our study provides evidence that alternative pathways allow organisms to mitigate pleiotropic conflicts, which ultimately may allow greater phenotypic diversity and persistence in novel environments. 

ABSTRACT Identifying populations at highest risk from climate change is a critical component of conservation efforts. However, vulnerability assessments are usually applied at the species level, even though intraspecific variation in exposure, sensitivity and adaptive capacity play a crucial role in determining vulnerability. Genomic data can inform intraspecific vulnerability by identifying signatures of local adaptation that reflect population‐level variation in sensitivity and adaptive capacity. Here, we address the question of local adaptation to temperature and the genetic basis of thermal tolerance in two stream frogs (Ascaphus trueiandA. montanus). Building on previous physiological and temperature data, we used whole‐genome resequencing of tadpoles from four sites spanning temperature gradients in each species to test for signatures of local adaptation. To support these analyses, we developed the first annotated reference genome forA. truei. We then expanded the geographic scope of our analysis using targeted capture at an additional 11 sites per species. We found evidence of local adaptation to temperature based on physiological and genomic data inA. montanusand genomic data inA. truei, suggesting similar levels of sensitivity (i.e., susceptibility) among populations regardless of stream temperature. However, invariant thermal tolerances across temperatures inA. trueisuggest that populations occupying warmer streams may be most sensitive. We identified high levels of evolutionary potential in both species based on genomic and physiological data. While further integration of these data is needed to comprehensively evaluate spatial variation in vulnerability, this work illustrates the value of genomics in identifying spatial patterns of climate change vulnerability. 

Abstract 1. Critical thermal limits represent an important component of an organism's capacity to cope with future temperature changes. Understanding the drivers of variation in these traits may uncover patterns in physiological vulnerability to climate change. Local temperature extremes have emerged as a major driver of thermal limits, although their effects can be mediated by the exploitation of fine‐scale spatial variation in temperature through behavioural thermoregulation. 2. Here, we investigated thermal limits along elevation gradients within and between two cold‐water frog species (Ascaphusspp.), one with a coastal distribution (A. truei) and the other with a continental range (A. montanus). We quantified thermal limits for over 700 tadpoles, representing multiple populations from each species. We combined local temporal and fine‐scale spatial temperature data to quantify local thermal landscapes (i.e., thermalscapes), including the opportunity for behavioural thermoregulation. 3. Lower thermal limits for either species could not be reached experimentally without the water freezing, suggesting that cold tolerance is <0.3°C. By contrast, upper thermal limits varied among populations, but this variation only reflected local temperature extremes inA. montanus, perhaps as a consequence of the greater variation in stream temperatures across its range. Lastly, we found minimal fine‐scale spatial variability in temperature, suggesting limited opportunity for behavioural thermoregulation and thus increased vulnerability to warming for all populations. 4. By quantifying local thermalscapes, we uncovered different trends in the relative vulnerability of populations across elevation for each species. InA. truei, physiological vulnerability decreased with elevation, whereas inA. montanus, all populations were equally physiologically vulnerable. These results highlight how similar environments can differentially shape physiological tolerance and patterns of vulnerability of species, and in turn impact their vulnerability to future warming.