Experimental evolution on heat tolerance and thermal performance curves under contrasting thermal selection in Drosophila subobscura

Gut microbiota modulation enhances the immune capacity of lizards under climate warming

BACKGROUND

Host-microbial interactions are expected to affect species' adaptability to climate change but have rarely been explored in ectothermic animals. Some studies have shown that short-term warming reduced gut microbial diversity that could hamper host functional performance.

RESULTS

However, our longitudinal experiments in semi-natural conditions demonstrated that warming decreased gut microbiota diversity at 2 months, but increased diversity at 13 and 27 months in a desert lizard (Eremias multiocellata). Simultaneously, long-term warming significantly increased the antibacterial activity of serum, immune responses (higher expression of intestinal immune-related genes), and the concentration of short-chain fatty acids (thereby intestinal barrier and immunity) in the lizard. Fecal microbiota transplant experiments further revealed that increased diversity of gut microbiota significantly enhanced antibacterial activity and the immune response of lizards. More specifically, the enhanced immunity is likely due to the higher relative abundance of Bacteroides in warming lizards, given that the bacteria of Bacteroides fragilis regulated IFN-β expression to increase the immune response of lizards under a warming climate.

CONCLUSIONS

Our study suggests that gut microbiota can help ectotherms cope with climate warming by enhancing host immune response, and highlights the importance of long-term studies on host-microbial interactions and their biological impacts.

Heat-induced female biased sex ratio during development is not mitigated after prolonged thermal selection

BACKGROUND

The negative impacts of climate change on biodiversity are consistently increasing. Developmental stages are particularly sensitive in many ectotherms. Moreover, sex-specific differences in how organisms cope with thermal stress can produce biased sex ratios upon emergence, with potentially major impacts on population persistence. This is an issue that needs investigation, particularly testing whether thermal selection can alleviate sex ratio distortions in the long-term is a critical but neglected issue. Here, we report an experiment analyzing the sex ratio patterns at different developmental temperatures in Drosophila subobscura populations subjected to long-term experimental evolution (~ 30 generations) under a warming environment.

RESULTS

We show that exposure to high developmental temperatures consistently promotes sex ratio imbalance upon emergence, with a higher number of female than male offspring. Furthermore, we found that thermal selection resulting from evolution in a warming environment did not alleviate such sex ratio distortions generated by heat stress.

CONCLUSIONS

We demonstrate that heat stress during development can lead to clear sex ratio deviations upon emergence likely because of differential survival between sexes. In face of these findings, it is likely that sex ratio deviations of this sort occur in natural populations when facing environmental perturbation. The inability of many insects to avoid thermal shifts during their (more) sessile developmental stages makes this finding particularly troublesome for population subsistence in face of climate warming events.

Slow and population specific evolutionary response to a warming environment

Adaptation to increasingly warmer environments may be critical to avoid extinction. Whether and how these adaptive responses can arise is under debate. Though several studies have tackled evolutionary responses under different thermal selective regimes, very few have specifically addressed the underlying patterns of thermal adaptation under scenarios of progressive warming conditions. Also, considering how much past history affects such evolutionary response is critical. Here, we report a long-term experimental evolution study addressing the adaptive response of Drosophila subobscura populations with distinct biogeographical history to two thermal regimes. Our results showed clear differences between the historically differentiated populations, with adaptation to the warming conditions only evident in the low latitude populations. Furthermore, this adaptation was only detected after more than 30 generations of thermal evolution. Our findings show some evolutionary potential of Drosophila populations to respond to a warming environment, but the response was slow and population specific, emphasizing limitations to the ability of ectotherms to adapt to rapid thermal shifts.

Polygenic adaptation to overnutrition reveals a role for cholinergic signaling in longevity

Overnutrition by high-sugar (HS) feeding reduces both the lifespan and healthspan across taxa. Pressuring organisms to adapt to overnutrition can highlight genes and pathways important for the healthspan in stressful environments. We used an experimental evolution approach to adapt four replicate, outbred population pairs of Drosophila melanogaster to a HS or control diet. Sexes were separated and aged on either diet until mid-life, then mated to produce the next generation, allowing enrichment for protective alleles over time. All HS-selected populations increased their lifespan and were therefore used as a platform to compare allele frequencies and gene expression. Pathways functioning in the nervous system were overrepresented in the genomic data and showed evidence for parallel evolution, although very few genes were the same across replicates. Acetylcholine-related genes, including the muscarinic receptor mAChR-A, showed significant changes in allele frequency in multiple selected populations and differential expression on a HS diet. Using genetic and pharmacological approaches, we show that cholinergic signaling affects Drosophila feeding in a sugar-specific fashion. Together, these results suggest that adaptation produces changes in allele frequencies that benefit animals under conditions of overnutrition and that it is repeatable at the pathway level.

Linking critical thermal maximum to mortality from thermal stress in a cold-water frog

Estimates of organismal thermal tolerance are frequently used to assess physiological risk from warming, yet the assumption that these estimates are predictive of mortality has been called into question. We tested this assumption in the cold-water-specialist frog, Ascaphus montanus. For seven populations, we used dynamic experimental assays to measure tadpole critical thermal maximum (CTmax) and measured mortality from chronic thermal stress for 3 days at different temperatures. We tested the relationship between previously estimated population CTmax and observed mortality, as well as the strength of CTmax as a predictor of mortality compared to local stream temperatures capturing varying timescales. Populations with higher CTmax experienced significantly less mortality in the warmest temperature treatment (25°C). We also found that population CTmax outperformed stream temperature metrics as the top predictor of observed mortality. These results demonstrate a clear link between CTmax and mortality from thermal stress, contributing evidence that CTmax is a relevant metric for physiological vulnerability assessments.

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.

Protection from overheating of simulated Sceloporus horridus lizards in a biosphere reserve of seasonally dry tropical forest in central Mexico

In seasonally dry ecosystems, loss of vegetation cover leads to warmer microclimates that can increase lizards' body temperatures to the point of threatening their performance. Preserving vegetation by establishing protected areas may mitigate these effects. We used remote sensing to test these ideas in the Sierra de Huautla Biosphere Reserve (REBIOSH) and surrounding areas. First, we determined whether vegetation cover was higher in the REBIOSH compared to adjacent unprotected areas to the north (NAA) and south (SAA). Then, we used a mechanistic niche model to test whether simulated Sceloporus horridus lizards in the REBIOSH experienced a cooler microclimate, higher thermal safety margin, longer foraging duration, and lower basal metabolic rate compared to adjacent unprotected areas. We compared these variables between 1999, when the reserve was declared, and 2020. We found that vegetation cover increased from 1999 to 2020 in all three areas; it was higher in the REBIOSH than in the more anthropized NAA, and was intermediate in the less anthropized SAA in both years. The microclimate temperature decreased from 1999 to 2020 and was lower in the REBIOSH and SAA than in the NAA. Thermal safety margin increased from 1999 to 2020; it was higher in the REBIOSH than in the NAA and intermediate in the SAA. Foraging duration increased from 1999 to 2020 and was similar among the three polygons. Basal metabolic rate decreased from 1999 to 2020 and was higher in the NAA than in the REBIOSH and SAA. Our results suggest that the REBIOSH provides cooler microclimates that increase the thermal safety margin and lower the metabolic rate of this generalist lizard compared to the NAA, and that the REBIOSH could contribute to increased vegetation cover in its surroundings. Besides, protecting original vegetation cover is an important part of climate change mitigation strategies more generally.

Rapid evolution of unimodal but not of linear thermal performance curves in Daphnia magna

Species may cope with warming through both rapid evolutionary and plastic responses. While thermal performance curves (TPCs), reflecting thermal plasticity, are considered powerful tools to understand the impact of warming on ectotherms, their rapid evolution has been rarely studied for multiple traits. We capitalized on a 2-year experimental evolution trial in outdoor mesocosms that were kept at ambient temperatures or heated 4°C above ambient, by testing in a follow-up common-garden experiment, for rapid evolution of the TPCs for multiple key traits of the water flea Daphnia magna. The heat-selected Daphnia showed evolutionary shifts of the unimodal TPCs for survival, fecundity at first clutch and intrinsic population growth rate toward higher optimum temperatures, and a less pronounced downward curvature indicating a better ability to keep fitness high across a range of high temperatures. We detected no evolution of the linear TPCs for somatic growth, mass and development rate, and for the traits related to energy gain (ingestion rate) and costs (metabolic rate). As a result, also the relative thermal slope of energy gain versus energy costs did not vary. These results suggest the overall (rather than per capita) top-down impact of D. magna may increase under rapid thermal evolution.

Experimental temperatures shape host microbiome diversity and composition

Global climate change has led to more extreme thermal events. Plants and animals harbour diverse microbial communities, which may be vital for their physiological performance and help them survive stressful climatic conditions. The extent to which microbiome communities change in response to warming or cooling may be important for predicting host performance under global change. Using a meta-analysis of 1377 microbiomes from 43 terrestrial and aquatic species, we found a decrease in the amplicon sequence variant-level microbiome phylogenetic diversity and alteration of microbiome composition under both experimental warming and cooling. Microbiome beta dispersion was not affected by temperature changes. We showed that the host habitat and experimental factors affected microbiome diversity and composition more than host biological traits. In particular, aquatic organisms-especially in marine habitats-experienced a greater depletion in microbiome diversity under cold conditions, compared to terrestrial hosts. Exposure involving a sudden long and static temperature shift was associated with microbiome diversity loss, but this reduction was attenuated by prior-experimental lab acclimation or when a ramped regime (i.e., warming) was used. Microbial differential abundance and co-occurrence network analyses revealed several potential indicator bacterial classes for hosts in heated environments and on different biome levels. Overall, our findings improve our understanding on the impact of global temperature changes on animal and plant microbiome structures across a diverse range of habitats. The next step is to link these changes to measures of host fitness, as well as microbial community functions, to determine whether microbiomes can buffer some species against a more thermally variable and extreme world.

Finding the right thermal limit: a framework to reconcile ecological, physiological and methodological aspects of CTmax in ectotherms

Upper thermal limits (CTmax) are frequently used to parameterize the fundamental niche of ectothermic animals and to infer biogeographical distribution limits under current and future climate scenarios. However, there is considerable debate associated with the methodological, ecological and physiological definitions of CTmax. The recent (re)introduction of the thermal death time (TDT) model has reconciled some of these issues and now offers a solid mathematical foundation to model CTmax by considering both intensity and duration of thermal stress. Nevertheless, the physiological origin and boundaries of this temperature–duration model remain unexplored. Supported by empirical data, we here outline a reconciling framework that integrates the TDT model, which operates at stressful temperatures, with the classic thermal performance curve (TPC) that typically describes biological functions at permissive temperatures. Further, we discuss how the TDT model is founded on a balance between disruptive and regenerative biological processes that ultimately defines a critical boundary temperature (Tc) separating the TDT and TPC models. Collectively, this framework allows inclusion of both repair and accumulation of heat stress, and therefore also offers a consistent conceptual approach to understand the impact of high temperature under fluctuating thermal conditions. Further, this reconciling framework allows improved experimental designs to understand the physiological underpinnings and ecological consequences of ectotherm heat tolerance.

Urban evolution of thermal physiology in a range-expanding, mycophagous fruit fly, Drosophila tripunctata

In Drosophila spp., their often high number of annual generations, large population sizes and large amounts of standing genetic variation should predispose them to undergo contemporary adaptation to climatic warming. However, a number of laboratory experimental evolution studies in this group of organisms suggest strong limits on the rate and magnitude of contemporary thermal adaptation. Here, we explore this discrepancy by examining the potential for rapid evolutionary divergence between wild populations of Drosophila tripunctata Loew, 1862 from rural and urban sites. We performed a multi-generation common garden study and found evidence for the evolution of higher heat tolerance (critical thermal maximum) in flies from urban populations. We also detected evolutionary divergence in cold resistance (chill coma recovery time), with diminished cold resistance in flies from urban populations, although the effect was weaker than the shift in heat tolerance. Our study provides evidence of contemporary urban thermal adaptation, although the magnitude of phenotypic change lagged the magnitude of environmental temperature change across the urbanization gradient, suggesting potential limits on the evolution of urban thermal physiology.

Symbiosis and host responses to heating

Virtually all organisms are colonized by microbes. Average temperatures are rising because of global climate change - accompanied by increases in extreme climatic events and heat shock - and symbioses with microbes may determine species persistence in the 21st century. Although parasite infection typically reduces host upper thermal limits, interactions with beneficial microbes can facilitate host adaptation to warming. The effects of warming on the ecology and evolution of the microbial symbionts remain understudied but are important for understanding how climate change might affect host health and disease. We present a framework for untangling the contributions of symbiosis to predictions of host persistence in the face of global change.

A fast pace-of-life is traded off against a high thermal performance

The integration of life-history, behavioural and physiological traits into a ‘pace-of-life syndrome’ is a powerful concept in understanding trait variation in nature. Yet, mechanisms maintaining variation in ‘pace-of-life’ are not well understood. We tested whether decreased thermal performance is an energetic cost of a faster pace-of-life. We characterized the pace-of-life of larvae of the damselfly Ischnura elegans from high-latitude and low-latitude regions when reared at 20°C or 24°C in a common-garden experiment, and estimated thermal performance curves for a set of behavioural, physiological and performance traits. Our results confirm a faster pace-of-life (i.e. faster growth and metabolic rate, more active and bold behaviour) in the low-latitude and in warm-reared larvae, and reveal increased maximum performance, R_max, but not thermal optimum T_opt, in low-latitude larvae. Besides a clear pace-of-life syndrome integration at the individual level, larvae also aligned along a ‘cold–hot’ axis. Importantly, a faster pace-of-life correlated negatively with a high thermal performance (i.e. higher T_opt for swimming speed, metabolic rate, activity and boldness), which was consistent across latitudes and rearing temperatures. This trade-off, potentially driven by the energetically costly maintenance of a fast pace-of-life, may be an alternative mechanism contributing to the maintenance of variation in pace-of-life within populations.

Potential distribution of invasive crop pests under climate change: incorporating mitigation responses of insects into prediction models

Climate change facilitates biological invasions globally. Predicting potential distribution shifts of invasive crop pests under climate change is essential for global food security in the context of ongoing world population increase. However, existing predictions often omit the capacity of crop pests to mitigate the impacts of climate change by using microclimates, as well as through thermoregulation, life history variation and evolutionary responses. Microclimates provide refugia buffering climate extremes. Thermoregulation and life history variation can reduce the effects of diurnal and seasonal temperature variability. Evolutionary responses allow insects to adapt to long-term climate change. Neglecting these ecological processes may lead to overestimations in the negative impacts of climate change on invasive pests whereas in turn cause underestimations in their range expansions. To improve model predictions, we need to incorporate the fine-scale microclimates experienced by invasive crop pests and the mitigation responses of insects to climate change into species distribution models.

Transient Receptor Potential-Vanilloid (TRPV1-TRPV4) Channels in the Atlantic Salmon, Salmo salar. A Focus on the Pineal Gland and Melatonin Production

Fish are ectotherm, which rely on the external temperature to regulate their internal body temperature, although some may perform partial endothermy. Together with photoperiod, temperature oscillations, contribute to synchronizing the daily and seasonal variations of fish metabolism, physiology and behavior. Recent studies are shedding light on the mechanisms of temperature sensing and behavioral thermoregulation in fish. In particular, the role of some members of the transient receptor potential channels (TRP) is being gradually unraveled. The present study in the migratory Atlantic salmon, Salmo salar, aims at identifying the tissue distribution and abundance in mRNA corresponding to the TRP of the vanilloid subfamilies, TRPV1 and TRPV4, and at characterizing their putative role in the control of the temperature-dependent modulation of melatonin production-the time-keeping hormone-by the pineal gland. In Salmo salar, TRPV1 and TRPV4 mRNA tissue distribution appeared ubiquitous; mRNA abundance varied as a function of the month investigated. In situ hybridization and immunohistochemistry indicated specific labeling located in the photoreceptor cells of the pineal gland and the retina. Additionally, TRPV analogs modulated the production of melatonin by isolated pineal glands in culture. The TRPV1 agonist induced an inhibitory response at high concentrations, while evoking a bell-shaped response (stimulatory at low, and inhibitory at high, concentrations) when added with an antagonist. The TRPV4 agonist was stimulatory at the highest concentration used. Altogether, the present results agree with the known widespread distribution and role of TRPV1 and TRPV4 channels, and with published data on trout (Oncorhynchus mykiss), leading to suggest these channels mediate the effects of temperature on S. salar pineal melatonin production. We discuss their involvement in controlling the timing of daily and seasonal events in this migratory species, in the context of an increasing warming of water temperatures.

Plasticity of thermal performance curves in a narrow range endemic water beetle

Thermal history can plastically alter the response of ectotherms to temperature, and thermal performance curves (TPCs) are powerful tools for exploring how organismal-level performance varies with temperature. Plasticity in TPCs may be favoured in thermally variable habitats, where it can result in fitness benefits. However, thermal physiology remains insufficiently studied for freshwater insects despite freshwater biodiversity being at great risk under global change. Here, we assess how acclimation at either summer or winter average temperatures changes TPCs for locomotion activity and metabolism in Enochrus jesusarribasi (Hydrophilidae), a water beetle endemic to shallow saline streams in SE Spain. This beetle is a bimodal gas exchanger and so we also assessed how aerial and aquatic gas exchange varied across temperatures for both acclimation treatments. Responses of locomotory TPCs to thermal acclimation were relatively weak, but high temperature acclimated beetles tended to exhibit higher maximum locomotor activity and reduced TPC breadth than those acclimated at lower temperature. High temperature acclimation increased the thermal sensitivity of metabolic rates, contrary to the response generally found in aquatic organisms. Higher metabolic rates upon high temperature acclimation were achieved by increasing aerial, rather than aquatic oxygen uptake. Such plastic respiratory behaviour likely contributed to enhanced locomotor performance at temperatures around the optimum and thermal plasticity could thus be an important component in the response of aquatic insects to climate change. However, high temperature acclimation appeared to be detrimental for locomotion in subsequent exposure at upper sublethal temperatures, suggesting that this narrow range endemic may be vulnerable to future climate warming. This study demonstrates that TPCs are context-specific, differing with performance metric as well as thermal history. Such context dependency must be considered when using TPCs to predict organismal responses to climate change.

No evidence for short-term evolutionary response to a warming environment in Drosophila

Adaptive evolution is key in mediating responses to global warming and may sometimes be the only solution for species to survive. Such evolution will expectedly lead to changes in the populations' thermal reaction norm and improve their ability to cope with stressful conditions. Conversely, evolutionary constraints might limit the adaptive response. Here, we test these expectations by performing a real-time evolution experiment in historically differentiated Drosophila subobscura populations. We address the phenotypic change after nine generations of evolution in a daily fluctuating environment with average constant temperature, or in a warming environment with increasing average and amplitude temperature across generations. Our results showed that (1) evolution under a global warming scenario does not lead to a noticeable change in the thermal response; (2) historical background appears to be affecting responses under the warming environment, particularly at higher temperatures; and (3) thermal reaction norms are trait dependent: although lifelong exposure to low temperature decreases fecundity and productivity but not viability, high temperature causes negative transgenerational effects on productivity and viability, even with high fecundity. These findings in such an emblematic organism for thermal adaptation studies raise concerns about the short-term efficiency of adaptive responses to the current rising temperatures.