Thermal Reaction Norms Research Articles

The seasonal development cycle of Aphantopus hyperantus (L.) (Lepidoptera, Nymphalidae: Satyrinae) in Leningrad Province

The simultaneous influence of the photoperiodic and temperature conditions on pre-diapause and postdiapause larval development of the ringlet Aphantopus hyperantus was studied. At the short day (12 h of light a day) all larvae reached the III instar and entered diapause at all the temperatures tested (18–24°C). At the long day (22 h) the number of diapausing larvae increased with decreasing temperature, and the larvae diapaused at the III and IV instars. The I and II instar larvae (before winter) developed and grew faster under short-day, and the hibernated larvae, under long-day conditions. At the short day the growth rate of the I and II instar larvae did not depend on the temperature, and at the long day it decreased as the temperature increased. After hibernation the growth rate of the IV instar larvae increased with the temperature under long-day conditions, and did not depend on the temperature under short-day ones. Thus, acceleration or deceleration of development, depending on the photoperiodic and temperature conditions, regulated the timing of diapause in the overwintering stage (the III instar larva) and maintained the univoltine seasonal cycle. The males of A. hyperantus developed and grew faster than females while their weight at the larval and pupal stages was smaller. The thermal reaction norms for development (the lower temperature threshold and the coefficient of thermal sensitivity of development) were calculated for the eggs, IV and V instar larvae, and pupae. The coefficient of thermal sensitivity of development for the IV and V instar larvae taken together was lower, and the threshold was higher than the corresponding values for eggs and pupae. The thermal reaction norms for development of the latter stages were the same. The larvae of A. hyperantus retained their sensitivity to the photoperiod after hibernation. Resumption of development after cold reactivation occurred only under long-day conditions, whereas under short-day ones the overwintered larvae entered a repeated diapause. The possibility of photoperiodic reactivation and the absence of spontaneous resumption of development in these larvae were demonstrated.

Evolutionary divergence of reaction norms in ecological context: a commentary.

Evolutionary divergence of reaction norms in ecological context: a commentary.

Thermal plasticity in Drosophila melanogaster populations from eastern Australia: quantitative traits to transcripts.

The flexibility afforded to genotypes in different environments by phenotypic plasticity is of interest to biologists studying thermal adaptation because of the thermal lability of many traits. Differences in thermal performance and reaction norms can provide insight into the evolution of thermal adaptation to explore broader questions such as species distributions and persistence under climate change. One approach is to study the effects of temperature on fitness, morphological and more recently gene expression traits in populations from different climatic origins. The diverse climatic conditions experienced by Drosophilamelanogaster along the eastern Australian temperate-tropical gradient are ideal given the high degree of continuous trait differentiation, but reaction norm variation has not been well studied in this system. Here, we reared a tropical and temperate population from the ends of the gradient over six developmental temperatures and examined reaction norm variation for five quantitative traits including thermal performance for fecundity, and reaction norms for thermotolerance, body size, viability and 23 transcript-level traits. Despite genetic variation for some quantitative traits, we found no differentiation between the populations for fecundity thermal optima and breadth, and the reaction norms for the other traits were largely parallel, supporting previous work suggesting that thermal evolution occurs by changes in trait means rather than by reaction norm shifts. We examined reaction norm variation in our expanded thermal regime for a gene set shown to previously exhibit GxE for expression plasticity in east Australian flies, as well as key heat-shock genes. Although there were differences in curvature between the populations suggesting a higher degree of thermal plasticity in expression patterns than for the quantitative traits, we found little evidence to support a role for genetic variation in maintaining expression plasticity.

Evolutionary and environmental determinants of freshwater fish thermal tolerance and plasticity

Understanding the extent to which phylogenetic constraints and adaptive evolutionary forces help define the physiological sensitivity of species is critical for anticipating climate-related impacts in aquatic environments. Yet, whether upper thermal tolerance and plasticity are shaped by common evolutionary and environmental mechanisms remains to be tested. Based on a systematic literature review, we investigated this question in 82 freshwater fish species (27 families) representing 829 experiments for which data existed on upper thermal limits and it was possible to estimate plasticity using upper thermal tolerance reaction norms. Our findings indicated that there are strong phylogenetic signals in both thermal tolerances and acclimation capacity, although it is weaker in the latter. We found that upper thermal tolerances are correlated with the temperatures experienced by species across their range, likely because of spatially autocorrelated processes in which closely related species share similar selection pressures and limited dispersal from ancestral environments. No association with species thermal habitat was found for acclimation capacity. Instead, species with the lowest physiological plasticity also displayed the highest thermal tolerances, reflecting to some extent an evolutionary trade-off between these two traits. Although our study demonstrates that macroecological climatic niche features measured from species distributions are likely to provide a good approximation of freshwater fish sensitivity to climate change, disentangling the mechanisms underlying both acute and chronic heat tolerances may help to refine predictions regarding climate change-related range shifts and extinctions.

Thermal reaction norms can surmount evolutionary constraints: comparative evidence across leaf beetle species.

One of the leitmotifs of the ecophysiological research on ectotherms is the variation and evolution of thermal reaction norms for biological rates. This long‐standing issue is crucial both for our understanding of life‐history diversification and for predicting the phenology of economically important species. A number of properties of the organism's thermal phenotype have been identified as potential constraints on the evolution of the rate–temperature relationship. This comparative study addresses several such constraints by testing whether the actual interspecific variation of thermal reaction norms across nearly hundred leaf beetle species agrees with the expected patterns. The results show that developmental rate and its temperature‐dependent parameters are similar in closely related species and that the variation pattern depends on the taxonomic scale, the thermal reaction norms being mostly parallel for the representatives of distant subclades but intersecting more often farther down the phylogenetic tree. The parallel shift disagrees with the putative ubiquity of a positive slope–threshold relationship, whereby thermal reaction norms should normally intersect, and even more contradicts with the common‐intersection hypothesis. The ability to develop in cooler conditions is not traded off at higher temperatures, which is an exception to the “warmer is better” principle. A comparison of high‐ and low‐quality data indicates that some of these discrepancies with earlier findings may stem from a likely presence of noise in previous analyses, which may have affected the variation patterns observed. Overall, the failure to support the universality of the predicted patterns suggests that the evolution of thermal reaction norms in leaf beetles has largely overcome the hypothesized constraints.

Reptile embryos and climate change: Modelling limits of viability to inform translocation decisions

The embryos of many egg-laying species develop under the environmental conditions selected by one or both parents, and these conditions may cease to be optimal under a changed climate. Assisted colonization is an emerging option to relocate species that are threatened by unfavourable changes in the local climate, but the incubation requirements of embryos are rarely considered in conservation translocations despite suggestions that relocation of early life stages could be more effective than relocating older animals. Here we review examples and outcomes of relocations of reptile eggs, and the decision tools currently available for guiding translocation decisions from an embryonic perspective. We then demonstrate a mechanistic approach, using the modelling framework NicheMapR, for assessing the optimal translocation range for the Western Swamp Turtle (Pseudemydura umbrina), a Critically Endangered reptile with an extremely restricted natural distribution. We determine thermal reaction norms and critical thermal limits for embryonic development based on laboratory and field data, and model soil temperatures at typical nest depths, simulate embryonic development at these depths, and map the probability of survival of P. umbrina embryos within the major bioregions of south-western Australia. The same model forced by future climates for 2050–70 demonstrates a southerly shift in the regions where embryos would be viable. However, if P. umbrina is to be translocated in the near future to regions 300–400km south of its natural range, females will need to nest in relatively unshaded sites to achieve temperatures that are high enough to promote hatchling success.

Wing morphology of the active flyerCalliphora vicina(Diptera: Calliphoridae) during its invasion of a sub-Antarctic archipelago where insect flightlessness is the rule

The cosmopolitan blowfly Calliphora vicina became established in the sub-Antarctic Kerguelen Islands in the late 1970s, following a warming period that allowed its full development. Although temperature and wind may limit flight activity, the fly invaded the archipelago, reaching sites remote from the introduction point. Most native competitors have converged to flightlessness as a response to stringent environmental conditions and therefore the flight strategy of C. vicina might be either a handicap or a competitive advantage under ongoing climate change. Using geometric morphometrics, we investigated whether the wing had changed over time in C. vicina within the archipelago (1998 vs. 2009) and compared its morphology with that of a continental population from a temperate area (1983 vs. 2009). Wing shape plasticity to temperature was also experimentally investigated. We found no clues of relaxed selection on flight morphology in the range invaded. However, rapid changes of wing shape occurred over time in females from the Kerguelen Islands compared with both males and females of the continental population, despite a shorter time-lag between samples in the former. The thermal reaction norms for wing shape found for C. vicina from Kerguelen were also different from those of the continental population, but it remains unknown whether this resulted from or preceded the introduction. These combined findings are consistent with a fingerprint of local adaptation in the invasive population. However, the adaptive significance of the changes, in terms of their aerodynamic consequences and the future evolution of C. vicina in the Kerguelen Islands, requires further investigation. From an evolutionary standpoint, sustaining flight capability under the novel sub-Antarctic conditions might be critical to the invasive success of C. vicina as most competitors are flightless.

Swift thermal reaction norm evolution in a key marine phytoplankton species.

Temperature has a profound effect on the species composition and physiology of marine phytoplankton, a polyphyletic group of microbes responsible for half of global primary production. Here, we ask whether and how thermal reaction norms in a key calcifying species, the coccolithophore Emiliania huxleyi, change as a result of 2.5 years of experimental evolution to a temperature ≈2°C below its upper thermal limit. Replicate experimental populations derived from a single genotype isolated from Norwegian coastal waters were grown at two temperatures for 2.5 years before assessing thermal responses at 6 temperatures ranging from 15 to 26°C, with pCO 2 (400/1100/2200 μatm) as a fully factorial additional factor. The two selection temperatures (15°/26.3°C) led to a marked divergence of thermal reaction norms. Optimal growth temperatures were 0.7°C higher in experimental populations selected at 26.3°C than those selected at 15.0°C. An additional negative effect of high pCO 2 on maximal growth rate (8% decrease relative to lowest level) was observed. Finally, the maximum persistence temperature (T max) differed by 1–3°C between experimental treatments, as a result of an interaction between pCO 2 and the temperature selection. Taken together, we demonstrate that several attributes of thermal reaction norms in phytoplankton may change faster than the predicted progression of ocean warming.

Thermal plasticity in farmed, wild and hybrid Atlantic salmon during early development: has domestication caused divergence in low temperature tolerance?

BackgroundIn the past three decades, millions of domesticated Atlantic salmon Salmo salar L. have escaped from farms into the wild. Their offspring display reduced survival in the natural environment, which demonstrates that gene-flow is likely to have a negative effect on wild populations. However, inter-population differences in introgression of farmed salmon have been observed, and the underlying ecological mechanisms remain enigmatic. We hypothesised that domestication-driven divergence in tolerance to low temperatures during early development may contribute to lower survival of farmed salmon offspring in the wild, which in turn, may influence patterns of introgression among populations exposed to different temperature regimes. We reared the offspring of 35 families of wild, farmed and hybrid origin at three temperatures (3.9, 5.6 and 12 °C) from the onset of exogenous feeding and throughout their first summer. Thermal reaction norms for growth and survival were investigated along the gradient.ResultsThe main results of this study, which is based upon the analysis of juvenile salmon from five wild strains, two farmed strains and two hybrid strains, can be summarised as; (i) salmon of all origins were able to successfully initiate feeding at all temperatures and similar survival reaction norms were detected in all strains across the temperature gradient; (ii) deviating growth reaction norms were detected between strains, although this result was most likely due to an overall lack of growth in the lower temperature treatments.ConclusionsThis study revealed no evidence of domesticated-driven divergence in low temperature tolerance in Atlantic salmon during early development. Although the potential interaction between low temperature and other river-specific factors cannot be excluded, our results indicate that the reduced survival of farmed offspring in the wild is not explained by farmed salmon displaying impaired abilities to initiate feeding at low temperatures. We therefore suggest that the observed inter-population patterns of introgression are not low-temperature driven and that other ecological or biological factors may explain why detection of farmed salmon in wild rivers is not synonymous with introgression. In general, our results support the literature indicating that phenotypic plasticity instead of thermal adaption has been selected for in Atlantic salmon.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0607-2) contains supplementary material, which is available to authorized users.

Climate and habitat interact to shape the thermal reaction norms of breeding phenology across lizard populations.

Substantial plastic variation in phenology in response to environmental heterogeneity through time in the same population has been uncovered in many species. However, our understanding of differences in reaction norms of phenology among populations from a given species remains limited. As the plasticity of phenological traits is often influenced by local thermal conditions, we expect local temperature to generate variation in the reaction norms between populations. Here, we explored temporal variation in parturition date across 11 populations of the common lizard (Zootoca vivipara) from four mountain chains as a function of air temperatures during mid-gestation. We characterized among-population variation to assess how local weather conditions (mean and variance of ambient temperatures during mid-gestation) and habitat openness (an index of anthropogenic disturbance) influence the thermal reaction norms of the parturition date. Our results provide evidence of interactive effects of anthropogenic disturbance and thermal conditions, with earlier parturition dates in warmer years on average especially in closed habitats. Variation in the reaction norms for parturition date was correlated with mean local thermal conditions at a broad geographical scale. However, populations exposed to variable thermal conditions had flatter thermal reaction norms. Assessing whether environmental heterogeneity drives differentiation among reaction norms is crucial to estimate the capacity of different populations to contend with projected climatic and anthropogenic challenges.

Keeping your options open: Maintenance of thermal plasticity during adaptation to a stable environment.

Phenotypic plasticity may allow species to cope with environmental variation. The study of thermal plasticity and its evolution helps understanding how populations respond to variation in temperature. In the context of climate change, it is essential to realize the impact of historical differences in the ability of populations to exhibit a plastic response to thermal variation and how it evolves during colonization of new environments. We have analyzed the real-time evolution of thermal reaction norms of adult and juvenile traits in Drosophila subobscura populations from three locations of Europe in the laboratory. These populations were kept at a constant temperature of 18ºC, and were periodically assayed at three experimental temperatures (13ºC, 18ºC, and 23ºC). We found initial differentiation between populations in thermal plasticity as well as evolutionary convergence in the shape of reaction norms for some adult traits, but not for any of the juvenile traits. Contrary to theoretical expectations, an overall better performance of high latitude populations across temperatures in early generations was observed. Our study shows that the evolution of thermal plasticity is trait specific, and that a new stable environment did not limit the ability of populations to cope with environmental challenges.

Fathers modify thermal reaction norms for hatching success in Atlantic cod, Gadus morhua

Climate-driven warming is altering marine ecosystems at an unprecedented rate and evolutionary adaptation may represent the last resort for many ectothermic organisms to avoid local extinction. The first step to elucidate the potential for adaptation to unfavorable thermal conditions is to assess the degree of genotype-based variation in thermal reaction norms of vital fitness traits. Marine broadcast spawning fishes experience extremely high rates of mortality during early life stages. Paternally derived (genetic) variation underlying offspring fitness in adverse environmental conditions may therefore hold important implications for resilience. This study examined how males differ in their ability to sire viable offspring and whether the paternal contribution modified thermal reaction norms for hatching success in two replicated trials with cod Gadus morhua from the Northwest Atlantic (trial 1) and Baltic Sea (trial 2). Each trial included five temperature treatments (2.0, 4.0, 6.0, 8.0, 10.0°C in trial 1, and 6.5, 8.0, 9.5, 11.0, 12.5°C in trial 2) encompassing optimum conditions as well as the amount of warming projected in various future pathways for the year 2100. In both trials, mean hatching success significantly decreased towards thermal extremes. However, half-sibling families varied in their response to different incubation temperatures as indicated by significant paternity×temperature interactions and crossing of reaction norms. The influence of paternity itself was highly significant and explained 56% and 44% of the observed variation in hatching success in trials 1 and 2, respectively. Early embryogenesis represented the most crucial developmental period in terms of thermal tolerance and paternally mediated variation in hatching success. High variation in daily embryo survival among half-sibling families and temperature treatments was observed during blastula and gastrulation stages (until 100% epiboly), while almost no mortality occurred during subsequent development and throughout the hatching period. The observed magnitude of genetic variation underlying thermal reaction norms for embryo viability represents a relevant resource for adaptive responses (favorable selection) of cod populations exposed to environmental variability and/or directional changes, such as ongoing ocean warming.

Thermal reaction norms for development and their photoperiodic modification in the Bryansk population of the european peacock butterfly <em>Inachis io</em> (Lepidoptera, Nymphalidae)

The thermal reaction norms for development (TRN, i.e. the regression coefficient or thermal sensitivity coefficient, the lower developmental threshold and the sum of degree-days) were investigated at 4 temperatures (16, 18, 20 and 22 °C) and 3 photoperiods (18, 16 and 12 hours of light per day) in the Bryansk population of European Peacock butterfly Inachis io. Photoperiodic modification of TRN for caterpillars was found: while the length of the day decreased there was a gradual increase of a lower threshold (10.7, 11.3 and 12.4 °C at 18, 16 and 12 h, respectively) and the slope of the regression line of development rate on temperature to abscissa axis became steeper. This means that there is an increase of the thermal sensitivity of development. Under short-day conditions pupae were lighter than under long days in all temperature regimes. The pupal weight increased with increasing temperature under long- and under short-day conditions. The development of caterpillars is characterized by a lower thermal sensitivity than the development of pupae.

Variation in plant thermal reaction norms along a latitudinal gradient – more than adaptation to season length

Little is known about the extent to which observed phenological responses to changes in climate are the result of phenotypic plasticity or genetic changes. We also know little about how plasticity, in terms of thermal reaction norms, vary spatially. We investigated if the thermal reaction norms for flower development of five crucifer species (Brassicaceae) differed among three regions along a south–north latitudinal gradient in replicated experiments. The mean response (elevation) of thermal reaction norms of flowering differed among regions in all study species, while sensitivity of flower development to temperature (slope) differed in only one of the species. Differences in mean responses corresponded to cogradient patterns in some species, but countergradient patterns in other. This suggests that differences among regions were not solely the result of adaptation to differences in the length of the growing season, but that other factors, such as herbivory, play an important role. Differences in development rate within species were mainly explained by variation in early phases of bud formation in some species but by variation in later phases of bud formation in other species. The differences in latitudinal patterns of thermal reaction norms among species observed in this study are important, both to identify agents of selection and to predict short‐ and long‐term responses to a warming climate.

Thermal plasticity of growth and development varies adaptively among alternative developmental pathways.

Polyphenism, the expression of discrete alternative phenotypes, is often a consequence of a developmental switch. Physiological changes induced by a developmental switch potentially affect reaction norms, but the evolution and existence of alternative reaction norms remains poorly understood. Here, we demonstrate that, in the butterfly Pieris napi (Lepidoptera: Pieridae), thermal reaction norms of several life history traits vary adaptively among switch-induced alternative developmental pathways of diapause and direct development. The switch was affected both by photoperiod and temperature, ambient temperature during late development having the potential to override earlier photoperiodic cues. Directly developing larvae had higher development and growth rates than diapausing ones across the studied thermal gradient. Reaction norm shapes also differed between the alternative developmental pathways, indicating pathway-specific selection on thermal sensitivity. Relative mass increments decreased linearly with increasing temperature and were higher under direct development than diapause. Contrary to predictions, population phenology did not explain trait variation or thermal sensitivity, but our experimental design probably lacks power for finding subtle phenology effects. We demonstrate adaptive differentiation in thermal reaction norms among alternative phenotypes, and suggest that the consequences of an environmentally dependent developmental switch primarily drive the evolution of alternative thermal reaction norms in P. napi.

Temperature-dependent development in Chrysomela vigintipunctata (Coleoptera: Chrysomelidae), a stenothermal early-season breeder

Chrysomela vigintipunctata (Scopoli) is a univoltine leaf beetle commonly encountered on willows across the Palearctic forest zone. The preimaginal development in this species takes place during a short time period, from May to June, because larvae are unable to consume mature leaves of the host plant. Therefore, the diet quality imposes a time constraint, and it was expected that the temperature dependence of development in C. vigintipunctata should be adaptively adjusted to the shortness and cool conditions of the favorable season. It was experimentally determined that this leaf beetle was stenothermal at the larval stage, required 275.5 degree-days above the threshold of 9.0°C for total development from oviposition to adult emergence, and attained greater body mass at lower temperatures. However, in all of these aspects, the thermal ecology of C. vigintipunctata was similar to that of two related multivoltine species, C. populi and C. scripta. The interspecific similarity of thermal reaction norms for development rate and body size suggests that these reaction norms in C. vigintipunctata were unlikely to have been shaped by selection favoring faster development or growth early in the season. The results are discussed in terms of the “ecological fitting” concept, which states that a species may be successful in exploiting novel environments while retaining ecophysiological traits evolved elsewhere.

Local divergence of thermal reaction norms among amphibian populations is affected by pond temperature variation.

Although temperature variation is known to cause large-scale adaptive divergence, its potential role as a selective factor over microgeographic scales is less well-understood. Here, we investigated how variation in breeding pond temperature affects divergence in multiple physiological (thermal performance curve and critical thermal maximum [CTmax]) and life-history (thermal developmental reaction norms) traits in a network of Rana arvalis populations. The results supported adaptive responses to face two main constraints limiting the evolution of thermal adaptation. First, we found support for the faster-slower model, indicating an adaptive response to compensate for the thermodynamic constraint of low temperatures in colder environments. Second, we found evidence for the generalist-specialist trade-off with populations from colder and less thermally variable environments exhibiting a specialist phenotype performing at higher rates but over a narrower range of temperatures. By contrast, the local optimal temperature for locomotor performance and CTmax did not match either mean or maximum pond temperatures. These results highlight the complexity of the adaptive multiple-trait thermal responses in natural populations, and the role of local thermal variation as a selective force driving diversity in life-history and physiological traits in the presence of gene flow.

Thermotolerant Yeast Strains Adapted by Laboratory Evolution Show Trade-Off at Ancestral Temperatures and Preadaptation to Other Stresses.

ABSTRACTA major challenge for the production of ethanol from biomass-derived feedstocks is to develop yeasts that can sustain growth under the variety of inhibitory conditions present in the production process, e.g., high osmolality, high ethanol titers, and/or elevated temperatures (≥40°C). Using adaptive laboratory evolution, we previously isolated seven Saccharomyces cerevisiae strains with improved growth at 40°C. Here, we show that genetic adaptations to high temperature caused a growth trade-off at ancestral temperatures, reduced cellular functions, and improved tolerance of other stresses. Thermotolerant yeast strains showed horizontal displacement of their thermal reaction norms to higher temperatures. Hence, their optimal and maximum growth temperatures increased by about 3°C, whereas they showed a growth trade-off at temperatures below 34°C. Computational analysis of the physical properties of proteins showed that the lethal temperature for yeast is around 49°C, as a large fraction of the yeast proteins denature above this temperature. Our analysis also indicated that the number of functions involved in controlling the growth rate decreased in the thermotolerant strains compared with the number in the ancestral strain. The latter is an advantageous attribute for acquiring thermotolerance and correlates with the reduction of yeast functions associated with loss of respiration capacity. This trait caused glycerol overproduction that was associated with the growth trade-off at ancestral temperatures. In combination with altered sterol composition of cellular membranes, glycerol overproduction was also associated with yeast osmotolerance and improved tolerance of high concentrations of glucose and ethanol. Our study shows that thermal adaptation of yeast is suitable for improving yeast resistance to inhibitory conditions found in industrial ethanol production processes.

Effects of temperature and nitrogen availability on the growth of invasive and native cyanobacteria

Rising temperatures are expected to favour the growth of bloom-forming cyanobacteria in temperate lakes, but may also change the composition of cyanobacterial communities. To predict future community and bloom dynamics, it is therefore important to understand how bloom-forming species respond to temperature. Cylindrospermopsis raciborskii (Woloszynska) Seenayya & Subba Raju is an invasive, toxin-producing, nitrogen-fixer that may benefit from warming. To understand how changing temperatures will influence its ability to compete against native North American bloom-formers, we characterized the thermal reaction norms and temperature traits of three C. raciborskii strains, four strains of Microcystis aeruginosa (Kutzing) Kutzing and one strain of Anabaena flos-aquae (Lyng.) Breb. C. raciborskii strains had higher optimum temperatures and survived higher temperatures than toxic M. aeruginosa strains, but had no apparent advantage over the non-toxic M. aeruginosa strain or A. flos-aquae. M. aeruginosa strains and A. flos-aquae tolerated lower temperatures than C. raciborskii, suggesting that fitness differences at low temperature may be important in limiting the latter’s spread. Furthermore, we found that nutrient availability strongly influenced thermal reaction norm shape: nitrogen deprivation lowered growth rates and decreased both low- and high-temperature tolerance, but did not affect the optimum temperature in C. raciborskii.

Macronutrient Balance Modulates the Temperature-Size Rule in an Ectotherm.

Most ectotherms mature at a larger body size in colder conditions, a phenomenon known as the temperature-size rule. While a number of hypotheses have been proposed to explain this rule, little work has been done to understand it from a nutritional perspective. We have used the final-instar caterpillars of Spodoptera litura to investigate how dietary protein∶carbohydrate (P∶C) balance influences the relationship between temperature and body size. The strength and direction of the thermal reaction norm for body size were significantly altered by dietary P∶C balance. The slope of the reaction norm was nearly flat for caterpillars raised on a balanced food ([Formula: see text]) but was significantly negative for those on nutritionally imbalanced foods (1∶5 or 5∶1), especially when carbohydrates were in considerable excess. These nutrient-dependent effects of temperature on body size were caused mainly by corresponding changes in body lipid storage. When allowed to choose between imbalanced diets, caterpillars increased their preference for carbohydrates to meet high energy demands at higher temperatures. The slope of the thermal reaction norm for body size was substantially reduced by such a temperature-driven shift in nutrient preference, indicating that the impact of high temperature on body size was buffered by altered food selection. This study highlights the importance of macronutrient balance as a key factor modulating the relationship between temperature and body size in ectotherms and provides a novel approach for understanding the temperature-size rule.