Reversible Phenotypic Plasticity Research Articles

Great tits differ in glucocorticoid plasticity in response to spring temperature.

Fluctuations in environmental temperature affect energy metabolism and stimulate the expression of reversible phenotypic plasticity in vertebrate behavioural and physiological traits. Changes in circulating concentrations of glucocorticoid hormones often underpin environmentally induced phenotypic plasticity. Ongoing climate change is predicted to increase fluctuations in environmental temperature globally, making it imperative to determine the standing phenotypic variation in glucocorticoid responses of free-living populations to evaluate their potential for coping via plastic or evolutionary changes. Using a reaction norm approach, we repeatedly sampled wild great tit (<i>Parus major</i>) individuals for circulating glucocorticoid concentrations during reproduction across five years to quantify individual variation in glucocorticoid plasticity along an environmental temperature gradient. As expected, baseline and stress-induced glucocorticoid concentrations increased with lower environmental temperatures at the population and within-individual level. Moreover, we provide unique evidence that individuals differ significantly in their plastic responses to the temperature gradient for both glucocorticoid traits, with some displaying greater plasticity than others. Average concentrations and degree of plasticity covaried for baseline glucocorticoids, indicating that these two reaction norm components are linked. Hence, individual variation in glucocorticoid plasticity in response to a key environmental factor exists in a wild vertebrate population, representing a crucial step to assess their potential to endure temperature fluctuations.

Environmental change and the rate of phenotypic plasticity.

With rapid and less predictable environmental change emerging as the ‘new norm’, understanding how individuals tolerate environmental stress via plastic, often reversible changes to the phenotype (i.e., reversible phenotypic plasticity, RPP), remains a key issue in ecology. Here, we examine the potential for better understanding how organisms overcome environmental challenges within their own lifetimes by scrutinizing a somewhat overlooked aspect of RPP, namely the rate at which it can occur. Although recent advances in the field provide indication of the aspects of environmental change where RPP rates may be of particular ecological relevance, we observe that current theoretical models do not consider the evolutionary potential of the rate of RPP. Whilst recent theory underscores the importance of environmental predictability in determining the slope of the evolved reaction norm for a given trait (i.e., how much plasticity can occur), a hitherto neglected possibility is that the rate of plasticity might be a more dynamic component of this relationship than previously assumed. If the rate of plasticity itself can evolve, as empirical evidence foreshadows, rates of plasticity may have the potential to alter the level predictability in the environment as perceived by the organism and thus influence the slope of the evolved reaction norm. However, optimality in the rate of phenotypic plasticity, its evolutionary dynamics in different environments and influence of constraints imposed by associated costs remain unexplored and may represent fruitful avenues of exploration in future theoretical and empirical treatments of the topic. We conclude by reviewing published studies of RPP rates, providing suggestions for improving the measurement of RPP rates, both in terms of experimental design and in the statistical quantification of this component of plasticity.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments.

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations. This tolerance is achieved through a suite of physiological and behavioural responses that allow animals to maintain homeostasis, including the ability to dynamically modulate their physiology through reversible phenotypic plasticity. However, maintaining the plasticity to adjust to some stresses in a dynamic environment may trade off with the capacity to deal with other stressors. This paper will explore studies on select fishes and invertebrates exposed to fluctuations in dissolved oxygen, salinity and pH. We assess the physiological mechanisms these species employ to achieve homeostasis, with a focus on the plasticity of their responses, and consider the resulting physiological trade-offs in function. Finally, we discuss additional factors that may influence organismal responses to fluctuating environments, such as the presence of multiple stressors, including parasites. We echo recent calls from experimental biologists to consider physiological responses to life in naturally fluctuating environments, not only because they are interesting in their own right but also because they can reveal mechanisms that may be crucial for living with increasing environmental instability as a consequence of climate change.

Low Transcriptomic Plasticity of Antarctic Giant Isopod Glyptonotus antarcticus Juveniles Exposed to Acute Thermal Stress

The Western Antarctic Peninsula (WAP) is among the areas of the planet showing some of the most significant increases in air and water temperature. It is projected that increasing temperature will modulate coastal ecosystems at species ecological performance and molecular composition. The main way that the organisms can cope with large thermal variation is by having a reversible phenotypic plasticity, which provides the organisms with a compensatory physiological response when facing challenging conditions. The giant Antarctic isopod Glyptonotus antarcticus is one of most common species in Antarctic waters. This species has a larval development inside of the maternal marsupium, where juveniles have a short period to acclimate to environmental conditions after birth. In this sense, we hypothesize that juveniles exposed to unusual temperature increases even for short periods, would not respond adequately showing a narrow phenotypic plasticity. We experimentally assessed if early juveniles of G. antarcticus have the molecular plasticity when exposed to increased temperature at 5°C during 1, 6, 12 and 24 h. Sequenced libraries were compared between control (0°C) and each experimental treatment to detect differentially expressed transcripts. The main molecular pathways affected by thermal stress were antioxidant, proteases, endopeptidases and ubiquination transcripts which were up-regulated and mitochondrial respiratory chain, cuticle, cytoskeleton and a molt transcript which were down-regulated. Regarding the HSP transcript, only 3 were up-regulated at least in two points of the stress kinetic, without classical Hsp70 and Hsp90 transcripts. This study shows that juveniles of G. antarcticus do not show molecular phenotypic plasticity to cope with acute short-term heat stress, even for one or few hours of exposure with an absence of an eco-physiological capacity to respond. This may have consequences at the ecological population level, showing a reduced individual ability to survive decreasing population recruitment.

Reversible and irreversible root phenotypic plasticity under fluctuating soil physical conditions

Roots grow in a highly heterogeneous physical environment due to the spatial complexity of soil structure. Thereby, the root growth zone repeatedly experiences soil physical stress such as hypoxia or increased penetration resistance. To mimic the highly variable physical environment surrounding the root growth zone, we subjected pea and wheat seedlings to periodic soil physical stress. One day of soil hypoxia or increased penetration resistance reduced root elongation rate of both species by at least 20 %. Upon stress release, root elongation rate of pea could recover within one day, while no such recovery occurred in wheat. Similarly, the diameter of the root elongation zone in pea increased by 15 % and 20 % due to hypoxia and increased penetration resistance, respectively, but decreased again once the stresses were released. In contrast, the diameter of the elongation zone of wheat roots started to decrease with the onset of soil physical stress and this trend continued upon stress release. Hence, root responses to short-term soil physical stress were reversible in pea and irreversible in wheat, indicating reversible and irreversible root phenotypic plasticity, respectively. This suggests that strategies to cope with periodic soil physical stress may vary among species. The differentiation between reversible and irreversible phenotypic plasticity is crucial to advance our understanding on soil exploration, resource acquisition, whole plant growth, and ultimately crop yield formation on structured soil.

Body size, but not age‐at‐maturation or context, affects the expression of predator‐induced behavioural plasticity in female green swordtails (Xiphophorus hellerii)

AbstractBehavioural plasticity is a form of reversible phenotypic plasticity in which a genotype can express different behavioural phenotypes under different environmental conditions. Though an interest in among‐individual differences in behavioural plasticity has flourished in recent decades, few studies have considered the effects of intrinsic factors, such as life‐history or morphological traits, in tandem with extrinsic factors, such as presence of conspecifics in different social contexts, on predator‐induced behavioural plasticity. Here, we present a study conducted with female green swordtail fishes, Xiphophorus hellerii, designed to assess the effects of age‐at‐maturation and body size on the expression of predator‐induced behavioural plasticity in two social contexts: (a) female‐only (two females) and (b) female‐and‐male (two females and a male). We further examined the extent to which individual expression of behavioural plasticity is consistent across these two social contexts. We found that in the presence of a predator, focal females were more timid in response to the stimulus and more tolerant of the non‐focal female, and small females expressed this change from bold/less tolerant to timid/more tolerant to a greater degree than large females, regardless of age‐at‐maturation. However, individuals were not consistent in the degree or direction of plasticity expressed in the behaviours of interest between the female‐only and the female‐and‐male context. Here, we show that within‐ and among‐individual differences in behavioural expression are common but inconsistent. How intrinsic and extrinsic factors independently or together drive expression of plasticity in antipredator and agonistic behaviours is varied and warrants further study.

Social flexibility and environmental unpredictability in African striped mice

The resilience of an individual to environmental change depends on its ability to respond adaptively. Phenotypic flexibility, i.e., reversible phenotypic plasticity, is such an adaptive response, which has been predicted to evolve in unpredictable environments. We present data on the environmental predictability for 17 generations of socially flexible African striped mice Rhabdomys pumilio, which can switch from group living to solitary living and back to group living. Population density during the breeding season is the main predictor of social organization in striped mice, which become solitary breeding when population density is low and plural breeding when population density is high. Using time series analysis, we could not reject randomness for the variation in population density and found a 6-year cycle for food availability. However, food availability when individual females grew up did not predict the environmental conditions during which they bred in the next year, their only breeding season. Group size was predictable and most females bred plurally in communal groups. However, single breeding is the preferred tactic to avoid infanticide but for single breeding females, it was not predictable from the environment in which they grew up whether they would become single breeders in the next breeding season. Our study indicates unpredictability in the factors most important for determining the optimal breeding tactics for 322 female striped mice. In sum, striped mice exhibit social flexibility in an unpredictable environment, making it an adaptive trait.

Transient Stability of Epigenetic Population Differentiation in a Clonal Invader.

Epigenetic variation may play an important role in how plants cope with novel environments. While significant epigenetic differences among plants from contrasting habitats have often been observed in the field, the stability of these differences remains little understood. Here, we combined field monitoring with a multi-generation common garden approach to study the dynamics of DNA methylation variation in invasive Chinese populations of the clonal alligator weed (Alternanthera philoxeroides). Using AFLP and MSAP markers, we found little variation in DNA sequence but substantial epigenetic population differentiation. In the field, these differences remained stable across multiple years, whereas in a common environment they were maintained at first but then progressively eroded. However, some epigenetic differentiation remained even after 10 asexual generations. Our data indicate that epigenetic variation in alligator weed most likely results from a combination of environmental induction and spontaneous epimutation, and that much of it is neither rapidly reversible (phenotypic plasticity) nor long-term stable, but instead displays an intermediate level of stability. Such transient epigenetic stability could be a beneficial mechanism in novel and heterogeneous environments, particularly in a genetically impoverished invader.

The coevolution of lifespan and reversible plasticity

Reversible phenotypic plasticity, the ability to change one’s phenotype repeatedly throughout life, can be selected for in environments that do not stay constant throughout an individual’s lifetime. It might also mitigate senescence, as the mismatch between the environment and a non-plastic individual’s traits is likely to increase as time passes. To understand why reversible plasticity may covary with lifespan, studies tend to assume unidirectional causality: plasticity evolves under suitable rates of environmental variation with respect to life history. Here we show that if lifespan also evolves in response to plasticity, then long life is not merely a context that sets the stage for lifelong plasticity. Instead, the causality is bidirectional because plasticity itself can select for longevity. Highly autocorrelated environmental fluctuations predict low investment in reversible plasticity and a phenotype that is poorly matched to the environment at older ages. Such environments select for high reproductive effort and short lifespans.

Role of cellular reprogramming and epigenetic dysregulation in acquired chemoresistance in breast cancer.

Acquired resistance to chemotherapy is a major limitation in clinical treatment for breast cancer. Accumulating evidence from in vitro, in vivo and clinical studies suggest that acquired chemoresistance is progressive, multifactorial and involve genetic and epigenetic aberrations. Among various mechanisms that contribute to chemoresistance, cellular reprogramming has extensively been implicated in breast cancer resistance lately. Cellular reprogramming events such as acquisition of epithelial to mesenchymal transition (EMT) and cancer stemness (CSCs) not only provide cancer cells with reversible phenotypic plasticity and survival advantage against cytotoxicity but also leads to aggressiveness, metastasis, clinical resistance, tumor recurrence and poor survival. The transient and reversible nature of cellular reprogramming processes and their controlled interaction with epigenetic regulatory complexes strongly support the involvement of dynamic epigenetic regulatory network in governing the cellular reprogramming and associated acquired chemoresistance. Further, epigenetic modulations are also gaining interest as promising interventions addressing the cancer cell reprogramming machinery to overcome acquired chemoresistance. This review discusses the previous reports and our recent findings that lead to current understanding of epigenetic dysregulation dictating the cellular reprogramming processes such as acquisition of EMT and CSCs phenotype and how they co-ordinate to establish acquired drug resistance in breast cancer.

Thermal Acclimatization in Overwintering Tadpoles of the Green Frog, Lithobates clamitans (Latreille, 1801)

Seasonal acclimatization permits organisms to maintain function in the face of environmental change. Tadpoles of the green frog (Lithobates clamitans) overwinter as tadpoles in much of their range. Because they are active in winter, we hypothesized that green frog tadpoles would display acclimatization of metabolic and locomotor function. We collected tadpoles in Sewanee, Tennessee (35.2°N) in winter and summer. Tadpoles collected during each season were tested at both winter (8°C) and summer (26°C) temperatures. Winter tadpoles were able to maintain swimming performance at both temperatures, whereas swimming performance decreased at cold temperatures in summer tadpoles. There was no evidence for seasonal acclimatization of whole-animal metabolic rate. Although whole-animal metabolic acclimatization was not observed, the activities of cytochrome c oxidase, citrate synthase, and lactate dehydrogenase measured in skeletal muscle homogenates showed higher activity in winter-acclimatized tadpoles indicating compensation for temperature. Further, the composition of muscle membranes of winter tadpoles had less saturated and more monounsaturated fatty acids and a higher ω-3 balance, unsaturation index, and peroxidation index than summer tadpoles. These data indicate that reversible phenotypic plasticity of thermal physiology occurs in larval green frog tadpoles. They appear to compensate for colder temperatures to maintain burst-swimming velocity and the ability to escape predators without the cost of maintaining a constant, higher standard metabolic rate in the winter.

The overshoot and phenotypic equilibrium in characterizing cancer dynamics of reversible phenotypic plasticity

The paradigm of phenotypic plasticity indicates reversible relations of different cancer cell phenotypes, which extends the cellular hierarchy proposed by the classical cancer stem cell (CSC) theory. Since it is still questionable if the phenotypic plasticity is a crucial improvement to the hierarchical model or just a minor extension to it, it is worthwhile to explore the dynamic behavior characterizing the reversible phenotypic plasticity. In this study we compare the hierarchical model and the reversible model in predicting the cell-state dynamics observed in biological experiments. Our results show that the hierarchical model shows significant disadvantages over the reversible model in describing both long-term stability (phenotypic equilibrium) and short-term transient dynamics (overshoot) in cancer cell populations. In a very specific case in which the total growth of population due to each cell type is identical, the hierarchical model predicts neither phenotypic equilibrium nor overshoot, whereas the reversible model succeeds in predicting both of them. Even though the performance of the hierarchical model can be improved by relaxing the specific assumption, its prediction to the phenotypic equilibrium strongly depends on a precondition that may be unrealistic in biological experiments. Moreover, it still does not show as rich dynamics as the reversible model in capturing the overshoots of both CSCs and non-CSCs. By comparison, it is more likely for the reversible model to correctly predict the stability of the phenotypic mixture and various types of overshoot behavior.

Multiple Myeloma Subtyping That Associates Normal B-Cell Subset Phenotypes, do Correlate to Disease Stage and Prognosis - Indication of Reversible Phenotypic Plasticity as a Hallmark

e48 p1⁄40.0005). To further validate this signature, we generated 500 randomly computed re-sampling of the data sets. The three-miRNA signature was consistently significant with a p-value < 0.05 in more than 78% and < 0.10 in 86% of the 500 randomizations. In a multivariable Cox regression model, the circulating exosomal miRNA signature was an independent prognostic marker after adjusting for cytogenetics and ISS. In a receiver operating characteristic (ROC) analysis, a combination of this signature together with International Staging System (ISS) and cytogenetics had a better prognostic value than ISS and cytogenetics alone in the training set (2 years area under the ROC curve 0.64 [95% CI 0.560.72] vs. 0.60 [95% CI 0.52-0.69]) and the validation set (0.67 [0.59-0.75] vs. 0.58 [0.50-0.66]). Interpretation: This study demonstrates unprecedented evidence of the prognostic significance of exosomal miRNAs in patients with MM. We identified a threemiRNA signature in circulating exosomes that adds prognostic value to ISS and cytogenetic status and helps improve prognostic identification of newly diagnosed MM patients.

Pretransplant induction with VTD (Bortezomib/Thalidomide/Dexamethasone) significantly improves PFS: long-term results of the randomized phase 3 PETHEMA/GEM study

Kingdom; Department of Hematopathology, Aalborg University Hospital, Denmark; Clinical Cancer Research Center, Aalborg University Hospital, Denmark and The Department of Clinical Medicine, Aalborg University, Denmark Background: Today’s diagnostic tests for multiple myeloma (MM) reflect the criteria of the updatedWHO classification based on biomarkers and clinicopathologic heterogeneity. To that end, we propose a new biological subtyping of myeloma plasma cells (PC) by B-cell subset associated gene signatures (BAGS), from the normal Bcell hierarchy in the bone marrow (BM). Here we document the prognostic and biological value of subtyping, as shown for DLBCL (JCO 2015 Apr 20;33:1379). Patients and Methods: We combined FACS and GEP to generate BAGS classifiers for the PreB-I, PreB-II, immature, naive, memory (M) and PC subsets in normal BM. Construction was based onmedian-centered probe sets from the BM data using regularized multinomial regression with six discrete outcomes representing BAGS, by a total of 55 genes varying from 1524 per subtype. Each patient underwent BAGS assignment according to the highest predicted probability score above 0.45 or was otherwise unclassified. The impact of BAGS was analyzed using six clinical cohorts, gathered across geographical regions, time eras, and sampling methods. The analysis estimated subtype frequencies and included a prognostic meta-analysis of 926 patients treated with high dose melphalan as first line therapy in 3 prospective trials: UAMS, HOVON65/GMMG-HD4, MRC Myeloma IX data with the Affymetrix U133 plus 2.0 microarray data available from myeloma PC samples. To compensate for cohort-wise technical batch effects, each cohort was median centered and adjusted probe set-wise to have same variance as the BM data. Results: The resultant tumor assignments exhibited similar BAGS subtype frequencies across 1302 individual MM cases with identical frequencies in the six cohorts. The BAGS subtypes were significantly associated with overall and progression free survival (OS, P1⁄41.1e-16 and PFS, P1⁄47.3e-9) in the metaanalysis of 926 pts. The major impact was observed within the PreB-II and M subtypes conferred with significant inferior prognosis compared to the Im, N and PC subtypes as illustrated in Figure 2. Cox proportional hazard meta-analysis showed that the BAGS subtypes added significant and independent prognostic information to the TC classification system and ISS staging. In parallel we found significant correlation between the PreBII subtypes and the proliferation index, risk profiling (P<0.001). Conclusion: We have documented myeloma PC differences with prognostic impact in support of reversible phenotypic plasticity in MM. 2. Multiple Myeloma Therapy in Newly Diagnosed Patients including Transplantation

Reversible phenotypic plasticity with continuous adaptation.

We introduce a novel model for continuous reversible phenotypic plasticity. The model includes a one-dimensional environmental gradient, and we describe performance of an organism as a function of the environmental state by a Gaussian tolerance curve. Organisms are assumed to adapt their tolerance curve after a change of the environmental state. We present a general framework for calculating the genotype fitness if such adaptations happen in a continuous manner and apply the model to a periodically changing environment. Significant differences of our model with previous models for plasticity are the continuity of adaptation, the presence of intermediate phenotypes, that the duration of transformations depends on their extent, fewer restrictions on the distribution of the environment, and a higher robustness with respect to assumptions about environmental fluctuations. Further, we show that continuous reversible plasticity is beneficial mainly when environmental changes occur slow enough so that fully developed phenotypes can be exhibited. Finally we discuss how the model framework can be generalized to a wide variety of biological scenarios from areas that include population dynamics, evolution of environmental tolerance and physiology.

Fully reversible phenotypic plasticity of digestive physiology in young house sparrows: lack of long-term effect of early diet composition

Feeding conditions during the nestling period may significantly affect whole-life fitness in altricial birds but little is known about the physiological mechanisms responsible for these effects. Permanent changes (irreversible developmental plasticity) in digestive physiology caused by the neonatal diet may form such a mechanism. We previously showed that the lack of starch in the diet of house sparrow (Passer domesticus) nestlings between 3 and 12 days post-hatching significantly decreased the activity of intestinal maltase, an enzyme essential for starch digestion. To check whether diet-induced variation in maltase activity in young house sparrows is reversible, we raised them under laboratory conditions from 3 until 30 days of age on diets with either 0% starch or 25% starch, with some individuals experiencing a switch in their assigned diet at 12 days of age. We found evidence for the presence of an internal, presumably genetic, program for changes in the activity of maltase and sucrase, which was, however, significantly affected by diet composition (i.e. environmental factor). Digestive enzyme activity in 30 day old birds was not influenced by diet composition prior to day 12 but instead depended only on diet that was fed between days 12 and 30. We conclude that plasticity in the activity of intestinal disaccharidases in house sparrow nestlings represents completely reversible phenotypic flexibility that can help young sparrows to cope with unpredictable variation in food composition during ontogeny without long-term effects on their digestive system. However, comparison with other species suggests that the magnitude of digestive flexibility in young passerines may be evolutionarily matched to species-specific variation in feeding conditions.

Beneficial acclimation: sex specific thermal acclimation of metabolic capacity in the striped marsh frog (Limnodynastes peronii)

Reproductive success in thermally varying environments will depend on maintaining metabolic capacity of tissues that are important in mating behaviours. Here we test the hypothesis that cold acclimation will occur in those tissues that are important for reproduction, and that acclimation will be sex specific, reflecting behavioural differences between the sexes. We used the frog Limnodynastes peronii as a model because anurans engage in energetically demanding reproductive behaviour, and many species, including L. peronii, are reproductively active across seasons. Additionally, reproductive behaviours such as calling and amplexus are sex specific. We acclimated animals to naturally occurring autumn (15 degrees C, N=10) and summer (25 degrees C, N=10) temperatures. Whole-animal resting oxygen consumption decreased with lowered temperature, but there was no difference in oxygen consumption between acclimation treatments or sexes. However, the respiratory control ratio (RCR) of mitochondria from the liver and external oblique calling muscle increased with cold acclimation. The increase in RCR with thermal acclimation was due to upregulation of state 3 respiration, and not to a decrease in state 4 respiration. Males had higher activity of citrate synthase, beta-hydroxyacyl CoA dehydrogenase and cytochrome c oxidase than females in the calling (external oblique) muscle, and males also showed thermal acclimation of these enzymes while females did not. Additionally, males had greater activity of metabolic enzymes in the principal muscle (extensor carpi radialis) used during amplexus. However, there were no differences in metabolic capacity between sexes in the gastrocnemius muscle and in liver, and both sexes showed significant acclimation of lactate dehydrogenase and cytochrome c oxidase in the former and latter, respectively. In L. peronii, thermal acclimation of metabolic capacities is linked to reproductive success, and reversible phenotypic plasticity therefore confers a selective advantage by extending the temporal and spatial extent of the animals' fundamental niche.

32.1. Thermoregulation and physiological performance in reptiles: What is the role of reversible phenotypic plasticity?

32.1. Thermoregulation and physiological performance in reptiles: What is the role of reversible phenotypic plasticity?

Selective advantage of irreversible and reversible phenotypic plasticity

A recent model on phenotypic plasticity is extended so that reversible and irreversible plasticity can be compared. It is assumed that the plastic organism receives an environmental cue that induces a change of the phenotype. Complete and incomplete information are treated as two extreme cases of the reliability of those cues. Relative fitness is calculated depending on the difference between inducing and non-inducing states of the environment and its coefficient of variation, on the time pattern and on the relative length of the inducing environment. In addition, fitness of reversible plasticity depends on the time delays for changing the phenotype from non-induced to induced and back. Irreversible plasticity can successfully compete with reversible plasticity only if at least one of these time delays becomes large, irrespective of the reliability of environmental cues. For either complete or incomplete information, there are parameter regions in which irreversibility is advantageous.

A review of thermoregulation and physiological performance in reptiles: what is the role of phenotypic flexibility?

Biological functions are dependent on the temperature of the organism. Animals may respond to fluctuation in the thermal environment by regulating their body temperature and by modifying physiological and biochemical rates. Phenotypic flexibility (reversible phenotypic plasticity, acclimation, or acclimatisation in rate functions occurs in all major taxonomic groups and may be considered as an ancestral condition. Within the Reptilia, representatives from all major groups show phenotypic flexibility in response to long-term or chronic changes in the thermal environment. Acclimation or acclimatisation in reptiles are most commonly assessed by measuring whole animal responses such as oxygen consumption, but whole animal responses are comprised of variation in individual traits such as enzyme activities, hormone expression, and cardiovascular functions. The challenge now lies in connecting the changes in the components to the functioning of the whole animal and its fitness. Experimental designs in research on reptilian thermal physiology should incorporate the capacity for reversible phenotypic plasticity as a null-hypothesis, because the significance of differential body temperature-performance relationships (thermal reaction norms) between individuals, populations, or species cannot be assessed without testing that null-hypothesis.