Thermal Reaction Norms Research Articles

Re-Equilibrating Sex Ratios: Adjustment of Reaction Norms in Species With Temperature-Dependent Sex Determination.

Fisher's general principle for sex allocation holds that population sex ratios are typically balanced because parents producing the rare sex are benefited and the rare sex alternates over time. In species that have temperature-dependent sex determination (TSD), thermal reaction norms need to be adjusted at the population level to avoid extremely biased sex ratios and extinction. Extant species with TSD experienced drastic climatic changes in the geological past and must necessarily have mechanisms of adaptation. I propose here a conceptual framework to explain how TSD curves could be adjusted by means of natural selection, based on Fisher's equilibrium sex-ratio principle. Through a process that alternatively favors mothers that tend to produce the rare sex under new temperatures, sex ratios eventually return toward a theoretical equilibrium. Prerequisites for this model are variability among mothers in the tendency to produce a particular sex at a given temperature (i.e., variability in the thermal reaction norm), inheritance of this trend, and higher fitness of the rare sex. This straightforward mechanism could facilitate thermal adaptation in species with TSD over multiple generations.

Developmental Thermal Reaction Norms of Leatherback Marine Turtles at Nesting Beaches.

Accurate scientific information is critical for undertaking appropriate conservation and management practices for imperiled species. One source of concern is that research findings might vary for non-biological reasons, including experimental design and analytical methods. To illustrate, we provide detailed modern analysis of reproductive data for leatherback turtles (Dermochelys coriacea). This species exhibits significant fluctuations in nesting densities across different regions, possibly driven by local rather than global factors. Key factors influencing these changes include hatching success and sex determination, both sensitive to incubation temperatures (e.g., lower temperatures yield more males, higher temperatures yield females). This study updates the understanding of temperature-dependent sex determination (TSD) in this species using Bayesian statistics. Growth rate data from the West Pacific and Northwest Atlantic populations show a similar, monotone increase with temperature, affirming the reliability of the models used. The analysis of TSD patterns indicates that observed differences are more likely due to study methodologies and clutch-specific factors rather than regional differences. These findings challenge previous assumptions, showing that leatherback TSD does not conform to a simple on/off pattern but is influenced by multiple, interacting environmental factors. Population dynamics models must account for these complexities, recognizing that both sex ratios and hatching success are critical to understand the rapid changes observed in some leatherback populations.

A quantitative model of temperature-dependent diapause progression

Winter diapause in insects is commonly terminated through cold exposure, which, like vernalization in plants, prevents development before spring arrives. Currently, quantitative understanding of the temperature dependence of diapause termination is limited, likely because diapause phenotypes are generally cryptic to human eyes. We introduce a methodology to tackle this challenge. By consecutively moving butterfly pupae of the species Pieris napi from several different cold conditions to 20 °C, we show that diapause termination proceeds as a temperature-dependent rate process, with maximal rates at relatively cold temperatures and low rates at warm and extremely cold temperatures. Further, we show that the resulting thermal reaction norm can predict P. napi diapause termination timing under variable temperatures. Last, we show that once diapause is terminated in P. napi, subsequent development follows a typical thermal performance curve, with a maximal development rate at around 31 °C and a minimum at around 2 °C. The sequence of these thermally distinct processes (diapause termination and postdiapause development) facilitates synchronous spring eclosion in nature; cold microclimates where diapause progresses quickly do not promote fast postdiapause development, allowing individuals in warmer winter microclimates to catch up, and vice versa. The unveiling of diapause termination as one temperature-dependent rate process among others promotes a parsimonious, quantitative, and predictive model, wherein winter diapause functions both as an adaptation against premature development during fall and winter and for synchrony in spring.

Thermal plasticity in protective wing pigmentation is modulated by genotype and food availability in an insect model of seasonal polyphenism

Abstract Phenotypic variation in natural populations results from complex interactions between organisms and their changing environments. The environment shapes both phenotypic frequencies (during adaptation) and organismal phenotypes (through phenotypic plasticity). Developmental plasticity, in particular, refers to the phenomenon whereby an organism's phenotype depends on the environmental conditions during development. It can match phenotype to ecological conditions and help organisms to cope with environmental heterogeneity, including differences between alternating seasons. Experimental studies of developmental plasticity often focus on the impact of individual environmental cues and do not take explicit account of genetic variation. In contrast, natural environments are complex, comprising multiple variables with combined effects that are poorly understood and may vary among genotypes. We investigated the effects of multifactorial environments on the development of the seasonally plastic eyespots of Bicyclus anynana butterflies. Eyespot size depends on developmental temperature and is involved in alternative seasonal strategies for predator avoidance. In nature, both temperature and food availability undergo seasonal fluctuations. However, our understanding of how thermal plasticity in eyespot size varies in response to food availability and across genotypes remains limited. To address this, we investigated the combined effects of temperature (T; two levels: 20°C and 27°C) and food availability (N; two levels: control and limited) during development. We examined their impact on wing and eyespot size in adult males and females from multiple genotypes (G; 28 families). We found evidence of thermal and nutritional plasticity and temperature‐by‐nutrition interactions (significant T × N) on the size of eyespots in both sexes. Food limitation resulted in relatively smaller eyespots and tempered the effects of temperature. Additionally, we found differences among families for thermal plasticity (significant G × T effects), but not for nutritional plasticity (non‐significant G × N effects) nor for the combined effects of temperature and food limitation (non‐significant G × T × N effects). Our results reveal the context dependence of thermal plasticity, with the slope of thermal reaction norms varying across genotypes and across nutritional environments. We discuss these results in the light of the ecological significance of pigmentation and the value of considering thermal plasticity in studies of the biological impact of climate change. Read the free Plain Language Summary for this article on the Journal blog.

Modelling thermal reaction norms for development and viability in Drosophila suzukii under constant, fluctuating and field conditions

Phenological models for insect pests often rely on knowledge of thermal reaction norms. These may differ in shape depending on developmental thermal conditions (e.g. constant vs. fluctuating) and other factors such as life-stages. Here, we conducted an extensive comparative study of the thermal reaction norms for development and viability in the invasive fly, Drosophila suzukii, under constant and fluctuating thermal regimes. Flies, were submitted to 15 different constant temperatures (CT) ranging from 8 to 35 °C. We compared responses under CT with patterns observed under 15 different fluctuating temperature (FT) regimes. We tested several equations for thermal performance curves and compared various models to obtain thermal limits and degree-day estimations. To validate the model's predictions, the phenology was monitored in two artificial field-like conditions and two natural conditions in outdoor cages during spring and winter. Thermal reaction norm for viability from egg to pupa was broader than that from egg to adult. FT conditions yielded a broader thermal breadth for viability than CT, with a performance extended towards the colder side, consistent with our field observations in winter. Models resulting from both CT and FT conditions made accurate predictions of degree-day as long as the temperature remained within the linear part of the developmental rate curve. Under cold artificial and natural winter conditions, a model based on FT data made more accurate predictions. Model based on CT failed to predict adult's emergence in winter. We also document the first record of development and adult emergence throughout winter in D. suzukii. Population dynamics models in D. suzukii are all based on summer phenotype and CT. Accounting for variations between seasonal phenotypes, stages, and thermal conditions (CT vs. FT) could improve the predictive power of the models.

Among- and Within-Individual Variance in Metabolic Thermal Reaction Norms.

AbstractIn ectotherms, temperature has a strong effect on metabolic rate (MR), yet the extent to which the thermal sensitivity of MR varies among versus within individuals is largely unknown. This is of interest because significant among-individual variation is a prerequisite for the evolution of metabolic thermal sensitivity. Here, we estimated the repeatability (R) of the thermal sensitivity of MR in individual virgin, adult male Drosophila melanogaster () by taking repeated overnight measures of their MRs at two temperatures (~24°C and ~27°C). At the population level, thermal sensitivity decreased with locomotor activity, and older individuals showed a higher thermal sensitivity of MR than younger individuals. Taking these effects (and body mass) into account, we detected significant repeatability in both the centered intercept () and the slope () of the metabolic thermal reaction norms, which respectively represent average MR and thermal sensitivity of MR. Furthermore, individuals with a higher overall MR also displayed greater increases in MR as temperature increased from ~24°C to ~27°C (). Average MR and thermal sensitivity of MR were also positively correlated within individuals (). Our study represents a point of departure for future larger studies, in which more complex protocols (e.g., wider temperature range, breeding design) can be applied to quantify the causal components of variation in thermal sensitivity that are needed to make accurate predictions of adaptive responses to global warming.

Terrestrial amphibians respond to rapidly changing temperatures with individual plasticity of exploratory behaviour

Terrestrial ectotherms react to acute changes in environmental temperatures by adjusting their behaviour. Evaluating the adaptive potential of these behavioural adjustments requires information on their repeatability and plasticity. We examined behavioural response (exploration) to acute temperature change in two amphibian taxa, alpine (Ichthyosaura alpestris) and smooth (Lissotriton vulgaris) newts. These responses were investigated at both population and individual levels under multiple thermal contexts (dimensions), represented by the direction and range of changing temperature and rearing thermal regimes. Population-level analyses showed species-specific, non-additive effects of direction and range of temperature change on acute thermal reaction norms for exploration, but explained only a low amount (7–23%) of total variation in exploration. In contrast, within- and among-individual variation in acute thermal reaction norm parameters explained 42–50% of total variation in the examined trait. Although immediate thermal responses varied among individuals (repeatability = 0.07 to 0.53), they were largely shaped by environmental contexts during repeated trials. We conclude that these amphibians respond to acute temperature change through individual plasticity of behavioural traits. A repeated-measures approach under multiple thermal contexts will be needed to identify the selective and plastic potential of behavioural responses used by juvenile newts and perhaps other ectotherm taxa to cope with rapidly changing environmental temperatures.

Olive Ridley Sea Turtle Incubation in Natural Conditions Is Possible on Guatemalan Beaches

The Guatemalan strategy for sea turtle conservation was defined by the National Council of Protected Areas (CONAP) in 1989. Hatcheries lie at the core of this strategy: egg collectors are allowed to deliver 20% of a nest to a hatchery in exchange for selling or eating the remaining eggs. Consequently, nearly 100% of nests are collected, with no nests being left on the beaches. Hatchery design promotes shading using roofs made from vegetation. The logic behind this recommendation is that the natural incubation of eggs is supposedly impossible due to the overly high temperatures on the beach. However, changing the incubation temperature of sea turtle eggs can profoundly alter the sex ratio in sea turtles with temperature-dependent sex determination. It can also modify the physiology or behavior of juvenile turtles. Here, we test whether incubation in natural conditions is possible on Guatemalan beaches, and for the first time, we determine the thermal reaction norm of embryo growth to ensure hatching success in sea turtles. We show that incubation in natural conditions is possible since three out of the four monitored nests produced hatchlings. We urge the Guatemala National Council of Protected Areas to reevaluate its strategy for sea turtle conservation in Guatemala in light of these results.

Population resistance and recovery after an extreme heat event are explained by thermal effects on life‐history traits

Extreme heat events lower the fitness of organisms by inducing physiological stress and increasing metabolic costs. Yet, little is known about the role of life‐history traits in elucidating population responses to extreme heat events. Here, we used a trait‐based approach to understand population resistance and recovery using four closely related species of soil‐dwelling Collembola. We measured thermal reaction norms of life‐history traits (survival and reproductive traits) and used this information to identify ecological mechanisms linked to population responses after an extreme heat event (i.e. one week at 26–30°C, representing + 10°C above ambient conditions). Furthermore, we investigated potential shifts in the body size distribution of recovering populations to better understand if extreme heat events can restructure body size spectra within populations. While resistance remained unaltered across species in our study, the recovery response of the most heat‐sensitive species (Protaphorura pseudovanderdrifti, predominantly a boreal species) was strongly affected by the extreme heat event (−54% population change compared to ambient conditions). Given that the fecundity (linked to recovery) of P. pseudovanderdrifti was more sensitive to heat than their survival (linked to resistance), we detected a decoupling between population resistance and recovery to an extreme heat event in this species. In addition, the detrimental effects of heat on fecundity were largely responsible for a drop in the proportion of small‐sized (juvenile) individuals in the recovering populations of P. pseudovanderdrifti. Thermally insensitive resistance and recovery in the other three species (P. armata, P. fimata, P. tricampata; predominantly temperate species) can be explained by their high survival and fecundity at warmer temperatures. We highlight that life‐history trait responses to warming can help explain population resistance and recovery after extreme heat events.

Multi-trait analysis reveals large interspecific differences for phytoplankton in response to thermal change

Understanding the responses of multiple traits in phytoplankton, and identifying interspecific variabilities to thermal changes is crucial for predicting the impacts of ocean warming on phytoplankton distributions and community structures in future scenarios. Here, we applied a trait-based approach by examining the patterns in multi-traits variations (eight traits) and interspecific variabilities in five phytoplankton species (two diatoms, three dinoflagellates) in response to a wide range of ecologically relevant temperatures (14–30 °C). Our results show large inter-traits and interspecific variabilities of thermal reaction norms in all of the tested traits. We also found that the interspecific variability exceeded the variations induced by thermal changes. Constrained variations and trade-offs between traits both revealed substantial interspecific differences and shifted as the temperature changed. Our study helps to understand the species-specific response patterns of multiple traits to ocean warming and to investigate the implications of these responses in the context of global change.

Population-specific effects of temperature and photoperiod on development and body mass in Cassida vibex (Coleoptera: Chrysomelidae).

An interplay of genetic divergence and phenotypic plasticity in shaping geographic variation is increasingly receiving attention in the entomological literature. Two major environmental variables that govern life histories are temperature and photoperiod. Studies of thermal and photoperiodic reaction norms help us understand how insect diversity evolved and how insects respond to environmental change. We studied survival, development, and body mass in three geographic populations of the beetle Cassida vibex reared in the laboratory under several combinations of constant temperature (16, 19, 22, 25, and 28°C) and photoperiod (short-day and long-day). The three collection sites are situated along a climatic gradient and separated by hundreds of kilometers. Each population subtly but significantly differs in the absolute values of survival rate, developmental rate, and body mass as well as in the thermal and photoperiodic plasticity of these traits, but the geographic differences do not form a latitudinal cline. The southernmost population from a relatively warm climate survives worse at low temperatures than the other two, but the overall survival is lowest in the latitudinally intermediate population. Short-day conditions tend to accelerate postembryonic development and increase the slope of the developmental rate-temperature relationship, especially so in the intermediate population, followed by the southernmost population and then by the northernmost population. The latter, which inhabits a harsh climate, has the fastest and most temperature-sensitive development, regardless of photoperiod, and attains the largest body mass among the three populations. The intermediately located and photoperiodically plastic population, which lives in a cool but mild climate, in contrast, has the smallest body size. Hence, although the importance of short-day conditions as a seasonal cue increases poleward, the photoperiodic responses do not always become more pronounced in colder, high-latitude environments. Our results emphasize that insect life-history traits can exhibit quite sophisticated patterns of variation along climatic gradients.

Evaluation of Sub-Antarctic and Antarctic sea urchins’ thermal reaction norm through righting behavior and comparison with in situ measurements

Sea urchin's survival may depend on their capacity to recover proper orientation rapidly and effectively after inversion, enabling escape from predator and preventing desiccation. This righting behavior has been used as a repeatable and reliable indicator to assess echinoderms performance across environmental conditions, including thermal sensitivity and thermal stress. The current study aims at evaluating and comparing the thermal reaction norm for righting behavior (time for righting (TFR) and capacity to self-right) of three common sea urchins from high latitude, the Patagonian sea urchins Loxechinus albus and Pseudechinus magellanicus, and the Antarctic sea urchin Sterechinus neumayeri. In addition, to infer the ecological implications of our experiments, we compared laboratory-based and in situ TFR of these three species. We observed that populations of the Patagonian sea urchins L. albus and P. magellanicus presented similar trend of righting behavior, overly accelerating with increasing temperature (from 0 to 22°C). Little variations and high inter-individual variability were observed below 6°C in the Antarctic sea urchin TFR, and righting success strongly decreased between 7 and 11°C. For the three species, TFR was lower in in situ experiments compared to the laboratory. Overall, our results suggest that the populations of Patagonian sea urchin exhibit a wide thermal tolerance and, based on S. neumayeri's TFR, aligning with the narrow thermal tolerance of Antarctic benthos. Finally, the differences between laboratory and in situ experiments highlights the importance of considering the complexity of marine environments for future predictions.

Repeated elevational clines of early life‐history traits and their proximate mechanisms in brown trout

Abstract Climate warming imposes a severe threat to freshwater ecosystems, which are dominated by ectotherms such as fish and aquatic insects. To better predict the effects of climate warming on thermally sensitive ecosystems, information on how temperature affects individual traits within populations is fundamental. Patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients provide valuable information. Immediate temperature effects on individual traits can be inferred from the shape of the thermal reaction norm. The way that temperature and associated environmental conditions (to which populations have been exposed over generations) affect individual traits through transgenerational plasticity in, and/or natural selection on, these traits can also be inferred from patterns of trait variation along a geographic thermal gradient. Many studies have documented patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients. However, most previous studies cannot exclude the possibility that the observed geographic patterns are solely explained by random processes, such as isolation by distance, due to the lack of replication of geographic gradients. Here, we show consistent patterns in intraspecific trait variation along geographic thermal gradients using Salmo trutta (brown trout), which is an ecologically and economically important fish in alpine streams. We kept trout embryos collected from 52 families from 14 populations along wide and replicated elevational gradients from three Alpine drainages (Danube, Po, and Rhine) in two temperature treatments (3.2 and 6.2°C). Timing and body size at emergence from the nest, which are key early life‐history traits of trout affecting their early growth and survival, were measured. Besides faster embryonic development at warmer temperatures, we found that offspring from low‐elevation parents took very slightly longer to reach emergence from fertilisation and were larger than offspring from high‐elevation parents. Importantly, this was evident for all three drainages. Further analyses found that the longer duration until‐ and larger body length at‐ emergence of low‐elevation trout were mainly due to the larger eggs laid by low‐elevation females, which had larger body size than high‐elevation females. Trout female body size, which is positively correlated with egg size, is susceptible to temperatures and associated environmental conditions. Consequently, climate warming may not only immediately accelerate development rate but also shift timing and size at emergence through egg size‐mediated maternal effects within a relatively short time scale.

Past history shapes evolution of reproductive success in a global warming scenario

Adaptive evolution is critical for animal populations to thrive in the fast-changing natural environments. Ectotherms are particularly vulnerable to global warming and, although their limited coping ability has been suggested, few real-time evolution experiments have directly accessed their evolutionary potential. Here, we report a long-term experimental evolution study addressing the evolution of Drosophila thermal reaction norms, after ∼30 generations under different dynamic thermal regimes: fluctuating (daily variation between 15 and 21 °C) or warming (daily fluctuation with increases in both thermal mean and variance across generations). We analyzed the evolutionary dynamics of Drosophila subobscura populations as a function of the thermally variable environments in which they evolved and their distinct background. Our results showed clear differences between the historically differentiated populations: high latitude D. subobscura populations responded to selection, improving their reproductive success at higher temperatures whereas their low latitude counterparts did not. This suggests population variation in the amount of genetic variation available for thermal adaptation, an aspect that needs to be considered to allow for better predictions of future climate change responses. Our results highlight the complex nature of thermal responses in face of environmental heterogeneity and emphasize the importance of considering inter-population variation in thermal evolution studies.

Rapid shifts in thermal reaction norms and tolerance of brooded coral larvae following parental heat acclimation.

Thermal priming of reef corals can enhance their heat tolerance; however, the legacy effects of heat stress during parental brooding on larval resilience remain understudied. This study investigated whether preconditioning adult coral Pocillopora damicornis to high temperatures (29°C and 32°C) could better prepare their larvae for heat stress. Results showed that heat-acclimated adults brooded larvae with reduced symbiont density and shifted thermal performance curves. Reciprocal transplant experiments demonstrated higher bleaching resistance and better photosynthetic and autotrophic performance in heat-exposed larvae from acclimated adults compared to unacclimated adults. RNA-seq revealed strong cellular stress responses in larvae from heat-acclimated adults that could have been effective in rescuing host cells from stress, as evidenced by the widespread upregulation of genes involved in cell cycle and mitosis. For symbionts, a molecular coordination between light harvesting, photoprotection and carbon fixation was detected in larvae from heat-acclimated adults, which may help optimize photosynthetic activity and yield under high temperature. Furthermore, heat acclimation led to opposing regulations of symbiont catabolic and anabolic pathways and favoured nutrient translocation to the host and thus a functional symbiosis. Notwithstanding, the improved heat tolerance was paralleled by reduced light-enhanced dark respiration, indicating metabolic depression for energy saving. Our findings suggest that adult heat acclimation can rapidly shift thermal tolerance of brooded coral larvae and provide integrated physiological and molecular evidence for this adaptive plasticity, which could increase climate resilience. However, the metabolic depression may be maladaptive for long-term organismal performance, highlighting the importance of curbing carbon emissions to better protect corals.

Limited sex differences in plastic responses suggest evolutionary conservatism of thermal reaction norms: A meta-analysis in insects.

Temperature has a profound effect on the growth and development of ectothermic animals. However, the extent to which ecologically driven selection pressures can adjust thermal plastic responses in growth schedules is not well understood. Comparing temperature-induced plastic responses between sexes provides a promising but underexploited approach to evaluating the evolvability of thermal reaction norms: males and females share largely the same genes and immature environments but typically experience different ecological selection pressures. We proceed from the idea that substantial sex differences in plastic responses could be interpreted as resulting from sex-specific life-history optimization, whereas similarity among the sexes should rather be seen as evidence of an essential role of physiological constraints. In this study, we performed a meta-analysis of sex-specific thermal responses in insect development times, using data on 161 species with comprehensive phylogenetic and ecological coverage. As a reference for judging the magnitude of sex specificity in thermal plasticity, we compared the magnitude of sex differences in plastic responses to temperature with those in response to diet. We show that sex-specific responses of development times to temperature variation are broadly similar. We also found no strong evidence for sex specificity in thermal responses to depend on the magnitude or direction of sex differences in development time. Sex differences in temperature-induced plastic responses were systematically less pronounced than sex differences in responses induced by variations in larval diet. Our results point to the existence of substantial constraints on the evolvability of thermal reaction norms in insects as the most likely explanation. If confirmed, the low evolvability of thermal response is an essential aspect to consider in predicting evolutionary responses to climate warming.

Univariate and multivariate plasticity in response to incubation temperature in an Australian lizard.

Environments, particularly developmental environments, can generate a considerable amount of phenotypic variation through phenotypic plasticity. Plasticity in response to incubation temperature is well characterised in egg-laying reptiles. However, traits do not always vary independently of one another, and studies encompassing a broad range of traits spanning multiple categories are relatively rare but crucial to better understand whole-organism responses to environmental change, particularly if covariation among traits may constrain plasticity. In this study, we investigated multivariate plasticity in response to incubation across three temperatures in the delicate skink, Lampropholis delicata, and whether this was affected by covariation among traits. At approximately 1month of age, a suite of growth, locomotor performance, thermal physiology and behavioural traits were measured. Plasticity in the multivariate phenotype of delicate skinks was distinct for different incubation temperatures. Cool temperatures drove shifts in growth, locomotor performance and thermal physiology, while hot temperatures primarily caused changes in locomotor performance and behaviour. These differences are likely due to variation in thermal reaction norms, as there was little evidence that covariation among traits or phenotypic integration influenced plasticity, and there was no effect of incubation temperature on the direction or strength of covariation. While there were broad themes in terms of which trait categories were affected by different incubation treatments, traits appeared to be affected independently by developmental temperature. Comparing reaction norms of a greater range of traits and temperatures will enable better insight into these patterns among trait categories, as well as the impacts of environmental change.

Developmental Plasticity in Butterfly Eyespot Mutants: Variation in Thermal Reaction Norms Across Genotypes and Pigmentation Traits.

Developmental plasticity refers to the property by which a genotype corresponds to distinct phenotypes depending on the environmental conditions experienced during development. This dependence of phenotype expression on environment is graphically represented by reaction norms, which can differ between traits and between genotypes. Even though genetic variation for reaction norms provides the basis for the evolution of plasticity, we know little about the genes that contribute to that variation. This includes understanding to what extent those are the same genes that contribute to inter-individual variation in a fixed environment. Here, we quantified thermal plasticity in butterfly lines that differ in pigmentation phenotype to test the hypothesis that alleles affecting pigmentation also affect plasticity therein. We characterized thermal reaction norms for eyespot color rings of distinct Bicyclus anynana genetic backgrounds, corresponding to allelic variants affecting eyespot size and color composition. Our results reveal genetic variation for the slope and curvature of reaction norms, with differences between eyespots and between eyespot color rings, as well as between sexes. Our report of prevalent temperature-dependent and compartment-specific allelic effects underscores the complexity of genotype-by-environment interactions and their consequence for the evolution of developmental plasticity.

Microclimatic variation affects developmental phenology, synchrony and voltinism in an insect population

Abstract Temperature influences the rate of most biological processes. Nonlinearities in the thermal reaction norms of such processes complicate intuitive predictions of how ectothermic organisms respond to naturally fluctuating temperatures, and by extension, to climate warming. Additionally, organisms developing close to the ground experience a highly variable microclimate landscape that often is poorly captured by coarse standard climate data. Using a butterfly population in central Sweden as a model, we quantified the consequences of small‐scale temperature variation on phenology, emergence synchrony and number of annual reproductive cycles (voltinism). By combining empirical microclimate and thermal performance data, we project development of individual green‐veined white butterflies (Pieris napi) across 110 sites in an exceptionally high‐resolved natural microclimate landscape. We demonstrate that differences among microclimates just meters apart can have large impacts on the rate of development and emergence synchrony of neighbouring butterflies. However, when considering the full development from egg to adult, these temporal differences were reduced in some scenarios, due to negative correlations in development times among life stages. The negative correlations were caused by temperatures at some sites beginning to exceed the optimum for development as the season progressed. Indeed, which sites were optimal for fast development could change across the lifetimes of individual butterflies, that is, ‘fast’ sites could become ‘slow’ sites. Thus, from a thermal point of view, there seem to be no consistently optimal microsites. Importantly, the fast sites were not always the warmest sites. We showed that such unintuitive effects could play an important role in the regulation of phenological synchrony and voltinism in insects, as most sites consistently favoured two generations. The results were generally robust across years and three different egg‐laying dates. Using high‐resolved empirical climate data on organism‐relevant temporal and spatial scales and considering nonlinear responses to temperature, we demonstrated the large and unintuitive population‐level consequences of locally and temporarily high temperatures. We suggest to—whenever possible—incorporate species‐ and life stage‐specific nonlinear responses to temperature when studying the effects of natural microclimate variation and climate change on organisms. Read the free Plain Language Summary for this article on the Journal blog.

Evidence for evolutionary adaptation of mixotrophic nanoflagellates to warmer temperatures.

Mixotrophs, organisms that combine photosynthesis and heterotrophy to gain energy, play an important role in global biogeochemical cycles. Metabolic theory predicts that mixotrophs will become more heterotrophic with rising temperatures, potentially creating a positive feedback loop that accelerates carbon dioxide accumulation in the atmosphere. Studies testing this theory have focused on phenotypically plastic (short-term, non-evolutionary) thermal responses of mixotrophs. However, as small organisms with short generation times and large population sizes, mixotrophs may rapidly evolve in response to climate change. Here, we present data from a 3-year experiment quantifying the evolutionary response of two mixotrophic nanoflagellates to temperature. We found evidence for adaptive evolution (increased growth rates in evolved relative to acclimated lineages) in the obligately phototrophic strain, but not in the facultative phototroph. All lineages showed trends of increased carbon use efficiency, flattening of thermal reaction norms, and a return to homeostatic gene expression. Generally, mixotrophs evolved reduced photosynthesis and higher grazing with increased temperatures, suggesting that evolution may act to exacerbate mixotrophs' effects on global carbon cycling.