Molecular Mechanisms of Winter Survival

Bioenergetic responses mediate interactive effects of pharmaceuticals and warming on freshwater arthropod populations and ecosystem functioning

Freshwater ecosystems are increasingly impacted by pharmaceutical contaminants (PhACs) and climate change-induced warming. Yet, their joint effects on freshwater taxa remain unclear. This is partly due to poorly understood mechanisms linking the effects on (sub)individual scales to higher levels of ecological organisation. We investigated the responses of two aquatic arthropods, Asellus aquaticus and Cloeon dipterum, to environmentally realistic levels of a 15-PhAC mixture (total concentration: 2.9 µg/L) and warming (+4 °C above ambient) in outdoor pond mesocosms (1000 L) across winter and summer. We measured physiological traits (bioenergetic responses based on quantification of energy consumption and energy stored in proteins, sugars and lipids, and oxidative damage based on malondialdehyde [MDA] levels), population density and ecosystem functions (leaf litter decomposition and insect emergence). In winter, PhACs reduced energy availability and increased MDA levels. In contrast, PhACs increased energy availability and decreased MDA levels in summer. The stressors reduced Asellus abundance, leading to reduced leaf litter decomposition, while Cloeon emergence in summer declined due to a PhAC-induced decline in larval abundance. Warming alone consistently decreased arthropod abundances and emergence, except for Asellus abundance in winter. The stressor effects through changes in bioenergetics were stronger than their direct effects on population abundances and ecosystem functions. Our findings highlight the vulnerability of aquatic arthropods to PhAC pollution and warming, emphasising the need for effective management strategies to mitigate the effects of emerging contaminants and climate change on freshwater biota.

Freeze-tolerant crickets fortify their actin cytoskeleton in fat body tissue

Animals that overwinter in temperate climates must prevent or repair damage to their cells to survive winter, but we know little about how they protect cellular structure at the cytoskeletal level. Both chilling (no ice formation) and freezing (ice formation) are hypothesized to cause substantial challenges to cell structure and the cytoskeleton. The spring field cricket Gryllus veletis becomes freeze tolerant following a 6 week acclimation to autumn-like conditions, during which they differentially express multiple cytoskeleton-related genes. We tested the hypothesis that G. veletis alter their cytoskeleton during acclimation to support maintenance of cytoskeletal structure during freezing and thawing. We used immunocytochemistry and confocal microscopy to characterize changes in microfilaments (F-actin, a polymer of G-actin) and microtubules (a polymer of α- and β-tubulin) in three tissues. While we saw no effect of acclimation on microtubules, crickets increased the abundance of microfilaments in fat body tissue and Malpighian tubules. When we chilled or froze these freeze-tolerant crickets, there was no apparent damage to the actin or tubulin cytoskeleton in fat body tissue, but there was decreased cytoskeleton abundance in Malpighian tubules. When we froze freeze-intolerant (unacclimated) crickets, microfilament abundance decreased in fat body tissue, while microfilaments were unaffected by chilling to the same subzero temperature. Our study shows that freeze-tolerant crickets are able to prevent or rapidly repair ice-induced damage to the actin cytoskeleton in fat body tissue, likely as a result of preparatory changes in advance of freezing, i.e. during acclimation. We suggest future directions examining the mechanisms that underlie these structural changes.

Tissue- and temperature-dependent expression, enzyme activity, and RNAi knockdown of Catalase in a freeze-tolerant insect

Organisms that overwinter in temperate climates may experience freezing and freezing-induced oxidative stress during winter. While many insect species can survive freezing, reverse genetics techniques such as RNA interference (RNAi) have not been used to understand the physiological mechanisms underlying freeze tolerance. The spring field cricket Gryllus veletis can survive freezing following a 6-week fall-like acclimation. We used RNAi to knock down expression of an antioxidant enzyme in G. veletis to test the hypothesis that minimizing oxidative stress is important for freeze tolerance. In fat body tissue, Catalase mRNA abundance and enzyme activity increased during the fall-like acclimation that induces freeze tolerance. Other tissues such as midgut and Malpighian tubules had more stable or lower Catalase expression and activity during this acclimation. In summer-acclimated (freeze-intolerant) crickets, RNA interference (RNAi) effectively knocked down production of the Catalase mRNA and protein in fat body and midgut, but not Malpighian tubules. In fall-acclimated (freeze-tolerant) crickets, RNAi efficacy was temperature-dependent, functioning well at warm (c. 22 °C) but not cool (15 °C or lower) temperatures. This highlights a challenge of using RNAi in organisms acclimated to low temperatures, as they may need to be warmed up for RNAi to work, potentially affecting their stress physiology. Knockdown of Catalase via RNAi in fall-acclimated crickets also had no effect on the ability of the crickets to survive a mild freeze treatment, suggesting that Catalase may not be necessary for freeze tolerance. Our study is the first to demonstrate that RNAi is possible in a freeze-tolerant insect, but further research is needed to examine whether other genes and antioxidants are needed for G. veletis freeze tolerance.

Lipid Properties and Metabolism in Response to Cold

Temperature directly shapes insect physiology and has a preponderant effect on life history traits. Winter conditions in temperate and polar regions are especially challenging for insects. Extremely low temperatures can indeed compromise insect survival by promoting freezing of body fluids, but mild cold temperatures above 0 °C (i.e., chilling) can also lead to complex and severe physiological dysregulations. Among physiological damages due to freezing and chilling, insect lipids are one of the primary targets. As low temperatures tend to rigidify phospholipid bilayers, membrane functions are compromised in the cold. Lipid rigidification due to cold also decreases the accessibility of fat stores for metabolic enzymes, and therefore their availability for basal metabolism. These deleterious effects, combined with low food availability in winter, result in substantial nutritional challenges for overwintering insects. Consequently, lipid modifications such as homeoviscous adaptation of cell membranes, fluidity maintenance of fat reserves, cuticular lipid accumulation, and production of antifreeze glycolipids are essential components of the physiological response to cold stress. The aim of the present chapter is to present the physiological challenges caused by low temperatures, the lipid modifications linked with cold tolerance in insects, and the molecular regulation of lipid metabolism during cold exposure.

Cold hardiness of Corythucha marmorata (Hemiptera: Tingidae) on the functional crop Helianthus tuberosus

The invasive phytophagous lace bug, Corythucha marmorata, threatens the functional food crop Helianthus tuberosus, but its overwintering ecology on this plant is poorly understood. This study evaluated the cold hardiness of C. marmorata at various life stages, focusing on the differences between female and male adults. C. marmorata overwinter as adults on H. tuberosus, based on a four-year winter field investigation. The supercooling and equilibrium freezing points of C. marmorata decline with development. Female adults showed the greatest supercooling capacity. The lower lethal temperature (female - 15 °C, male - 16 °C) is above the supercooling point (- 26 °C). The low temperature exposure mortality of C. marmorata female and male adults exhibited different regularities. We conclude that C. marmorata belongs to chill susceptible insects. October to February is the most recommended period for C. marmorata control by harvesting H. tuberosus. Weed removal, such as Erigeron bonariensis, Erigeron canadensis, and Ambrosia trifida, is an early control measure. These results enhance our understanding of C. marmorata's cold tolerance and inform targeted pest management strategies for H. tuberosus crops.

The role of insulin receptor InR in photoperiod-regulated reproductive diapause of Chrysoperla nipponensis

Insects usually diapause, a process regulated by hormonal signals as an adaptive mechanism developed through long-term evolution to survive unfavorable environmental conditions. Chrysoperla nipponensis is classified as a photoperiod-sensitive insect. Treatments with short-day (SD) and long-day (LD) conditions have distinct effects on ovarian development and lipid accumulation in adults, with SD condition inducing diapause. Injecting bovine insulin promoted ovarian development and egg formation in diapause females, while injecting insulin receptor induced diapause-like traits in reproductive females. This study investigate the biological function of insulin signaling in the reproductive diapause of females of C. nipponensis. Under SD treatment the mRNA expression level of InR1 and InR2, as well as the protein expression level of InR1 were significantly reduced. This reduction led to stagnant ovarian development, increased adipose tissue mass, and a significant rise in triglyceride (TG) content. Silencing InR1 under LD conditions resulted in halted ovarian development and enhanced lipid accumulation, with the expression levels of Akt, Kr-h1, and Vg significantly decreased mirroring those observed under SD conditions. Interestingly, silencing InR2 under LD condition did not affect ovarian development. Furthermore, transcriptome analysis identified six genes (Akt, PkN, Skp2, CycB3, BTrC, and AurkA) associated with reproductive regulation and eight genes (FadΔ11, EchA, EcI, Ugts (2A3, 1-9), AR, Gpdh and Cbr) linked to lipid metabolism, all of which are involved in InR1 mediated regulation of C. nipponensis reproduction.

Lipid Metabolism in Diapause

Organisms living in temperate and polar environments encounter seasonal fluctuations that entail changes in temperature, resource availability, and biotic interactions. Thus, adaptations for synchronizing the life cycle with essential resources and persisting through unfavorable conditions are critical. Diapause, a programmed period of developmental arrest and metabolic depression, is widely used by insects to survive winter and synchronize the life cycle. In some cases, insects spend over half the year (or in some cases, multiple years) in a nonfeeding diapause state. Thus, diapause is energetically challenging, and insects accumulate surplus energy stores and/or suppress metabolism to make it through the winter. As the most energy-dense, and often most abundant, energy reserve in insects, lipids play a central role in diapause energetics. In this chapter, we provide an overview of lipid metabolism in the context of diapause. First, as this is the only chapter in this book that covers diapause, we present some of the general features of diapause. We then discuss the role of lipids as an essential energy store during diapause, focusing on patterns of lipid accumulation before diapause and patterns of utilization during diapause. In the next section, we outline some other roles of lipids during diapause in addition to their role as an energy store. Finally, we end the chapter by discussing the molecular regulation of lipid metabolism in diapause, which has received increased attention in recent years.

Cold tolerance and prediction of northern distribution of Histia rhodope (Lepidoptera: Zygaenidae) in China

Histia rhodope (Cramer) (Lepidoptera: Zygaenidae) is one of the most destructive defoliating pests of the landscape tree Bischofia polycarpa (Levl.) S in China and other Southeast Asian regions, posing a critical threat to urban landscapes and their ecological benefits. This pest has shown a trend of northward range shift in recent years in China, making it urgent to understand its potential distribution. This study investigated the cold tolerance of overwintering H. rhodope larvae from October 2022 to March 2023 and estimated their overwintering potential in China. The results showed that the supercooling points (SCP) differed significantly across months. The SCP tended to decrease as the ambient temperature dropped until January, after which it gradually increased until the end of winter. The highest monthly mean SCP was -7.5 ± 2.22°C (October 2022), while the lowest monthly mean SCP was -15.09 ± 2.61°C (January 2023). The mortality rate increased with longer exposure times and lower exposure temperatures but decreased as winter progressed. Moreover, 50% and 90% lethal temperature (Ltemp50 and Ltemp90) exhibited a similar trend, decreasing to a minimum in January 2023, which indicates increased cold tolerance during the colder months. Using Ltemp90 in January as the isotherm for its northern limit indicated that H. rhodope may be limited by low temperatures along the 40°N latitude. These results provide a basis for predicting the dispersal potential and possible geographic range of this pest in China.

HSP70 is upregulated after heat but not freezing stress in the freeze-tolerant cricket Gryllus veletis

Heat shock proteins (HSPs) are well known to prevent and repair protein damage caused by various abiotic stressors, but their role in low temperature and freezing stress is not well-characterized in insects compared to other thermal challenges such as heat stress. Ice formation in and around cells is hypothesized to cause protein damage, yet many species of insects can survive freezing, suggesting HSPs may be an important mechanism in freeze tolerance. Here, we studied HSP70 in a freeze-tolerant cricket Gryllus veletis to better understand the role of HSPs in this phenomenon. We measured expression of one heat-inducible HSP70 isoform at the mRNA level (using RT-qPCR), as well as the relative abundance of total HSP70 protein (using semi-quantitative Western blotting), in five tissues from crickets exposed to a survivable heat treatment (2 h at 40 °C), a 6-week fall-like acclimation that induces freeze tolerance, and a survivable freezing treatment (1.5 h at -8 °C). While HSP70 expression was upregulated by heat at the mRNA or protein level in all tissues studied (fat body, Malphigian tubules, midgut, femur muscle, nervous system ganglia), no tissue exhibited HSP70 upregulation within 2-24 h following a survivable freezing stress. During fall-like acclimation to mild low temperatures, we only saw moderate upregulation of HSP70 at the protein level in muscle, and at the RNA level in fat body and nervous tissue. Although HSP70 is important for responding to a wide range of stressors, our work suggests that this chaperone may be less critical in the preparation for, and response to, moderate freezing stress.

Soilscapes of Mortality Risk Suggest a Goldilocks Effect for Overwintering Ectotherms

AbstractChanging climates are driving population declines in diverse animals worldwide. Winter conditions may play an important role in these declines but are often overlooked. Animals must not only survive winter but also preserve body condition, a key determinant of growing season success. We hypothesized that ectotherms overwintering in soil face a trade-off between risks of cold damage (including freezing) near the surface and elevated energy use at deeper depths. To test this hypothesis, we developed landscapes of mortality risk across depth for overwintering bumble bee queens. These critical pollinators are in decline in part because of climate change, but little is known about how climate affects overwintering mortality. We developed a mechanistic modeling approach combining measurements of freezing points and the temperature dependence of metabolic rates with soil temperatures from across the United States to estimate mortality risk across depth under historic conditions and under several climate change scenarios. Under current conditions, overwintering queens face a Goldilocks effect: temperatures can be too cold at shallow depths because of substantial freezing risk but too hot at deep depths where they risk prematurely exhausting lipid stores. Models suggest that increases in mean temperatures and in seasonal and daily temperature variation will increase risk of overwinter mortality. Better predictions of effects of changing climate on dormant ectotherms require more measurements of physiological responses to temperature during dormancy across diverse taxa.

Advances in understanding Lepidoptera cold tolerance

Ambient thermal conditions mediate insect growth, development, reproduction, survival, and distribution. With increasingly frequent and severe cold spells, it is critical to determine low-temperature performance and cold tolerances of ecologically and economically essential insect groups to predict their responses to global environmental change. This review covers the cold tolerance strategies of 49 species of Lepidoptera (moths and butterflies), focusing on species that are known as crop pests and crop storage facilities. We synthesize cold tolerance strategies of well-studied species within this order, finding that diapause is a distinctive mechanism that has independently evolved in different genera and families of Lepidoptera. However, the occurrence of diapause in each life stage is specific to the species, and in most studied lepidopteran species, the feeding stage (as larva) is the predominant overwintering stage. We also found that the onset of diapause and the improvement of cold tolerance are interdependent phenomena that typically occur together. Moreover, adopting a cold tolerance strategy is not an inherent, fixed trait and is greatly influenced by a species' geographic distribution and rearing conditions. This review further finds that freeze avoidance rather than freeze tolerance or chill susceptibility is the primary cold tolerance strategy among lepidopteran species. The cold hardiness of lepidopteran insects primarily depends on the accumulation of cryoprotectants and the depression of the supercooling point. We highlight variations in cold tolerance strategies and mechanisms among a subset of Lepidoptera, however, further work is needed to elucidate these strategies for the vast numbers of neglected species and populations to understand broad-scale responses to global change.

Genome-wide analysis reveals transcriptional and translational changes during diapause of the Asian corn borer (Ostrinia furnacalis)

BACKGROUND

Diapause, a pivotal phase in the insect life cycle, enables survival during harsh environmental conditions. Unraveling the gene expression profiles of the diapause process helps uncover the molecular mechanisms that underlying diapause, which is crucial for understanding physiological adaptations. In this study, we utilize RNA-seq and Ribo-seq data to examine differentially expressed genes (DEGs) and translational efficiency during diapause of Asian corn borer (Ostrinia furnacalis, ACB).

RESULTS

Our results unveil genes classified as "forwarded", "exclusive", "intensified", or "buffered" during diapause, shedding light on their transcription and translation regulation patterns. Furthermore, we explore the landscape of lncRNAs (long non-coding RNAs) during diapause and identify differentially expressed lncRNAs, suggesting their roles in diapause regulation. Comparative analysis of different types of diapause in insects uncovers shared and unique KEGG pathways. While shared pathways highlight energy balance, exclusive pathways in the ACB larvae indicate insect-specific adaptations related to nutrient utilization and stress response. Interestingly, our study also reveals dynamic changes in the HSP70 gene family and proteasome pathway during diapause. Manipulating HSP protein levels and proteasome pathway by HSP activator or inhibitor and proteasome inhibitor affects diapause, indicating their vital role in the process.

CONCLUSIONS

In summary, these findings enhance our knowledge of how insects navigate challenging conditions through intricate molecular mechanisms.

Glycerol Metabolism is Important for the Low-Temperature Adaptation of a Global Quarantine Pest Anoplophora glabripennis Larvae

Anoplophora glabripennis is a critical global quarantine pest. Recently, its distribution has been extended to colder and higher-latitude regions. The adaptation to low temperatures is vital for the successful colonization of insects in new environments. However, the metabolic pathways of A. glabripennis larvae under cold stress remain undefined. This study analyzed the larval hemolymph under different low-temperature treatments using LC-MS/MS. The results showed that differential metabolites associated with sugar and lipid metabolism are pivotal in the larval chill coma process. Under low-temperature treatments, the glycerol content increased significantly compared with the control group. Cold stress significantly induced the expression of AglaGK2 and AglaGPDHs. After undergoing RNAi treatment for 48 h, larvae exposed to -20 °C for 1 h showed reduced recovery when injected with ds-AglaGK2 and ds-AglaGPDH1 compared to the control group, indicating that glycerol biosynthesis plays a role in the low-temperature adaptation of A. glabripennis larvae. Our results provide a theoretical basis for clarifying the molecular mechanism of A. glabripennis larvae in response to environmental stresses.

Structural and functional analysis of aquaporins in Bombus terrestris

Bombus terrestris are efficient pollinators in forestry and agriculture, with higher cold tolerance than other bees. Yet, their cold tolerance mechanism remains unclear. Aquaporins (AQPs) function as cell membrane proteins facilitating rapid water flow, aiding in osmoregulation. Recent studies highlight the importance of insect AQPs in dehydration and cold stress. Comparative transcriptome analysis of B. terrestris under cold stress revealed up-regulation of four AQPs, indicating their potential role in cold tolerance. Seven AQPs-Eglp1, Eglp2, Eglp3, DRIP, PRIP, Bib, and AQP12L-have been identified in B. terrestris. These are widely expressed in various tissues, particularly in the alimentary canal and Malpighian tubules. Functional analysis of BterAQPs in the Xenopus laevis oocytes expressing system showed distinct water and glycerol selectivity, with BterDrip exhibiting the highest water permeability. Molecular modeling of BterDrip revealed six transmembrane domains, two NPA motifs, and an ar/R constriction region (Phe131, His256, Ser265, and Arg271), likely contributing to its water selectivity. Silencing BterDRIP accelerated mortality in B. terrestris under cold stress, highlighting the crucial role of BterDRIP in their cold tolerance and providing a molecular mechanism for their cold adaptation.

Ice in the intertidal: patterns and processes of freeze tolerance in intertidal invertebrates

Many intertidal invertebrates are freeze tolerant, meaning that they can survive ice formation within their body cavity. Freeze tolerance is a fascinating trait, and understanding its mechanisms is important for predicting the survival of intertidal animals during extreme cold weather events. In this Review, we bring together current research on the ecology, biochemistry and physiology of this group of freeze-tolerant organisms. We first introduce the ecology of the intertidal zone, then highlight the strong geographic and taxonomic biases within the current body of literature on this topic. Next, we detail current knowledge on the mechanisms of freeze tolerance used by intertidal invertebrates. Although the mechanisms of freeze tolerance in terrestrial arthropods have been well-explored, marine invertebrate freeze tolerance is less well understood and does not appear to work similarly because of the osmotic differences that come with living in seawater. Freeze tolerance mechanisms thought to be utilized by intertidal invertebrates include: (1) low molecular weight cryoprotectants, such as compatible osmolytes and anaerobic by-products; (2) high molecular weight cryoprotectants, such as ice-binding proteins; as well as (3) other molecular mechanisms involving heat shock proteins and aquaporins. Lastly, we describe untested hypotheses, methods and approaches that researchers can use to fill current knowledge gaps. Understanding the mechanisms and consequences of freeze tolerance in the intertidal zone has many important ecological implications, but also provides an opportunity to broaden our understanding of the mechanisms of freeze tolerance more generally.

Genome-wide profiles of H3K9me3, H3K27me3 modifications, and DNA methylation during diapause of Asian corn borer (Ostrinia furnacalis)

Diapause represents a crucial adaptive strategy used by insects to cope with changing environmental conditions. In North China, the Asian corn borer (Ostrinia furnacalis) enters a winter larval diapause stage. Although there is growing evidence implicating epigenetic mechanisms in diapause regulation, it remains unclear whether dynamic genome-wide profiles of epigenetic modifications exist during this process. By investigating multiple histone modifications, we have discovered the essential roles of H3K9me3 and H3K27me3 during diapause of the Asian corn borer. Building upon previous findings in vertebrates highlighting the connection between DNA methylation and repressive histone methylations, we have examined changes in the genome-wide profile of H3K9me3, H3K27me3, and DNA methylation at the nondiapause, prediapause, and diapause stages. Data analysis reveals significant alterations in these three modifications during diapause. Moreover, we observe a correlation between the H3K9me3 and H3K27me3 modification sites during diapause, whereas DNA modifications show little association with either H3K9me3 or H3K27me3. Integrative analysis of epigenome and expression data unveils the relationship between these epigenetic modifications and gene expression levels at corresponding diapause stages. Furthermore, by studying the function of histone modifications on genes known to be important in diapause, especially those involved in the juvenile pathway, we discover that the juvenile hormone pathway lies downstream from H3K9me3 and H3K27me3 histone modifications. Finally, the analysis of gene loci with modified modifications unreported in diapause uncovers novel pathways potentially crucial in diapause regulation. This study provides a valuable resource for future investigations aiming to elucidate the underlying mechanisms of diapause.

Cross-talk between low temperature and other environmental factors

Low temperatures are rarely experienced in isolation. The impacts of low temperatures on insects can be exacerbated or alleviated by the addition of other environmental factors, including, for example, desiccation, hypoxia, or infection. One way in which environmental factors can interact is through cross-talk where different factors enact common signaling pathways. In this review, I highlight the breadth of abiotic and biotic factors that can interact with low-temperature tolerance in both natural and artificial environments; and discuss some of the candidate pathways that are possibly responsible for cross-talk between several factors. Specifically, I discuss three interesting candidates: the neurohormone octopamine, circadian clock gene vrille, and microbes. Finally, I discuss applications of cross-talk studies, and provide recommendations for researchers.

A rapid return to normal: temporal gene expression patterns following cold exposure in the bumble bee Bombus impatiens

Bumble bees are common in cooler climates and many species likely experience periodic exposure to very cold temperatures, but little is known about the temporal dynamics of cold response mechanisms following chill exposure, especially how persistent effects of cold exposure may facilitate tolerance of future events. To investigate molecular processes involved in the temporal response by bumble bees to acute cold exposure, we compared mRNA transcript abundance in Bombus impatiens workers exposed to 0°C for 75 min (inducing chill coma) and control bees maintained at a constant ambient temperature (28°C). We sequenced the 3' end of mRNA transcripts (TagSeq) to quantify gene expression in thoracic tissue of bees at several time points (0, 10, 30, 120 and 720 min) following cold exposure. Significant differences from control bees were only detectable within 30 min after the treatment, with most occurring at the 10 min recovery time point. Genes associated with gluconeogenesis and glycolysis were most notably upregulated, while genes related to lipid and purine metabolism were downregulated. The observed patterns of expression indicate a rapid recovery after chill coma, suggesting an acute differential transcriptional response during recovery from chill coma and return to baseline expression levels within an hour, with no long-term gene expression markers of this cold exposure. Our work highlights the functions and pathways important for acute cold recovery, provides an estimated time frame for recovery from cold exposure in bumble bees, and suggests that cold hardening may be less important for these heterothermic insects.

Extracellular freezing induces a permeability transition in the inner membrane of muscle mitochondria of freeze-sensitive but not freeze-tolerant Chymomyza costata larvae

Background: Many insect species have evolved the ability to survive extracellular freezing. The search for the underlying principles of their natural freeze tolerance remains hampered by our poor understanding of the mechanistic nature of freezing damage itself. Objectives: Here, in search of potential primary cellular targets of freezing damage, we compared mitochondrial responses (changes in morphology and physical integrity, respiratory chain protein functionality, and mitochondrial inner membrane (IMM) permeability) in freeze-sensitive vs. freeze-tolerant phenotypes of the larvae of the drosophilid fly, Chymomyza costata. Methods: Larvae were exposed to freezing stress at -30°C for 1 h, which is invariably lethal for the freeze-sensitive phenotype but readily survived by the freeze-tolerant phenotype. Immediately after melting, the metabolic activity of muscle cells was assessed by the Alamar Blue assay, the morphology of muscle mitochondria was examined by transmission electron microscopy, and the functionality of the oxidative phosphorylation system was measured by Oxygraph-2K microrespirometry. Results: The muscle mitochondria of freeze-tolerant phenotype larvae remained morphologically and functionally intact after freezing stress. In contrast, most mitochondria of the freeze-sensitive phenotype were swollen, their matrix was diluted and enlarged in volume, and the structure of the IMM cristae was lost. Despite this morphological damage, the electron transfer chain proteins remained partially functional in lethally frozen larvae, still exhibiting strong responses to specific respiratory substrates and transferring electrons to oxygen. However, the coupling of electron transfer to ATP synthesis was severely impaired. Based on these results, we formulated a hypothesis linking the observed mitochondrial swelling to a sudden loss of barrier function of the IMM.

Comparative hemolymph proteomic analyses of the freezing and resistance-freezing Ostrinia furnacalis (Guenée)

The Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae), is one of the most harmful pests of maize in Asia. It poses a significant threat to maize production, causing economic losses due to its strong ecological adaptation. In this study, we compared and analyzed the hemolymph proteome between freezing and resistance-freezing O. furnacalis strains using two-dimensional gel electrophoresis to gain insights into the mechanisms of cold resistance. The results revealed that 300-400 hemolymph protein spots were common, with 24 spots showing differences between the two strains. Spectrometry analysis revealed 21 protein spots, including 17 upregulated spots and 4 downregulated ones. The expression of upregulation/downregulation proteins plays a crucial role in the metabolism, energy supply, and defense reaction of insects. Proteomics research not only provides a method for investigating protein expression patterns but also identifies numerous attractive candidates for further exploration.

Diapause survival requires a temperature-sensitive preparatory period

Diapause is a form of internally-controlled dormancy that allows insects to avoid stressful conditions and periods of low food availability. Eastern spruce budworm (Choristoneura fumiferana Clemens), like many cold-adapted insects, enter diapause well in advance of winter conditions, thus exposing them to elevated temperatures during fall that can deplete energy stores and impact post-diapause survival. We explored the impact of fall conditions on C. fumiferana by manipulating the length of the fall period and exposure temperatures during the diapause initiation phase of second instar larvae in a factorial design. We exposed second instar larvae to four fall temperatures (10, 15, 20, and 25°C) and five exposure times (1, 2, 4, 6, and 10 weeks) prior to standardized diapause conditions. We measured metabolites (glycogen, glycerol, and protein) prior to and during diapause for a subset of individuals. We also measured post-diapause survival by quantifying emergence following diapause conditions for a subset of individuals. We found that long, warm fall conditions depleted glycogen content and lowered post-diapause survival. We also found that short, cool conditions impacted post-diapause survival, although glycogen content remained high. Our results showed that fall conditions have substantial fitness consequences to overwintering insects. Optimal fall conditions struck a balance between exposure time and temperature. Our findings point to a potentially adaptive reason for early diapause onset: that an undescribed, but temperature-sensitive process is occurring in C. fumiferana larvae during the diapause initiation period that is essential for overwintering survival and successful post-diapause emergence.

The importance of fat accumulation and reserves for insect overwintering

Winter is a challenging season for ectothermic species such as insects. In addition to thermal stress imposed by cold temperatures, food scarcity during winter can lead to starvation and energy drain. In preparation for winter, most insects accumulate lipid (fat) reserves, which are the principal source of energetic fuel during overwintering. In this review, we highlight the most recent literature on lipid metabolism in response to cold. We first discuss how lipid metabolism is affected by biotic and abiotic environmental changes in preparation for winter. We then highlight how lipid dynamics are affected during winter, including physiological and (epi)genetic mechanisms. We end our review emphasizing the importance of remaining fat reserves in spring and how climate change can negatively impact lipid metabolism and fitness.

miR-252 targeting temperature receptor CcTRPM to mediate the transition from summer-form to winter-form of Cacopsylla chinensis

Temperature determines the geographical distribution of organisms and affects the outbreak and damage of pests. Insects seasonal polyphenism is a successful strategy adopted by some species to adapt the changeable external environment. Cacopsylla chinensis (Yang & Li) showed two seasonal morphotypes, summer-form and winter-form, with significant differences in morphological characteristics. Low temperature is the key environmental factor to induce its transition from summer-form to winter-form. However, the detailed molecular mechanism remains unknown. Here, we firstly confirmed that low temperature of 10 °C induced the transition from summer-form to winter-form by affecting the cuticle thickness and chitin content. Subsequently, we demonstrated that CcTRPM functions as a temperature receptor to regulate this transition. In addition, miR-252 was identified to mediate the expression of CcTRPM to involve in this morphological transition. Finally, we found CcTre1 and CcCHS1, two rate-limiting enzymes of insect chitin biosyntheis, act as the critical down-stream signal of CcTRPM in mediating this behavioral transition. Taken together, our results revealed that a signal transduction cascade mediates the seasonal polyphenism in C. chinensis. These findings not only lay a solid foundation for fully clarifying the ecological adaptation mechanism of C. chinensis outbreak, but also broaden our understanding about insect polymorphism.

Identification and characterization of cold-responsive aquaporins from the larvae of a crambid pest Agriphila aeneociliella (Eversmann) (Lepidoptera: Crambidae)

As small ectotherms, insects need to cope with the challenges of winter cold by regulating the water content through water transport. Aquaporins (AQPs) are key players to enhance the cold resistance by mediating essential homeostatic processes in many animals but remain poorly characterized in insects. Agriphila aeneociliella is a newly discovered winter wheat pest in China, and its early-stage larvae have strong tolerance to low temperature stress. Six AQP genes were identified, which belong to five AQP subfamilies (RPIP, Eglp, AQP12L, PRIP, DRIP). All of them contained six hydrophobic transmembrane helices (TMHs) and two relatively conservative Asparagine-Proline-Alanine motifs. The three-dimensional homology modeling showed that the six TMHs folded into an hourglass-like shape, and the imperceptible replace of four ar/R residues in contraction region had critical effects on changing the pore size of channels. Moreover, the transcript levels of AaAQP 1, 3, and 6 increased significantly with the treatment time below 0 °C. Combined with the results of pore radius variation, it is suggested that AaAQP1 and AaAQP3 may be considered to be the key anti-hypothermia proteins in A. aeneociliella by regulating rapid cell dehydration and allowing the influx of extracellular cold resistance molecules, thus avoiding death in winter.

Snow flies self-amputate freezing limbs to sustain behavior at sub-zero temperatures

Temperature profoundly impacts all living creatures. In spite of the thermodynamic constraints on biology, some animals have evolved to live and move in extremely cold environments. Here, we investigate behavioral mechanisms of cold tolerance in the snow fly (Chionea spp.), a flightless crane fly that is active throughout the winter in boreal and alpine environments of the northern hemisphere. Using thermal imaging, we show that adult snow flies maintain the ability to walk down to an average body temperature of -7°C. At this supercooling limit, ice crystallization occurs within the snow fly's hemolymph and rapidly spreads throughout the body, resulting in death. However, we discovered that snow flies frequently survive freezing by rapidly amputating legs before ice crystallization can spread to their vital organs. Self-amputation of freezing limbs is a last-ditch tactic to prolong survival in frigid conditions that few animals can endure. Understanding the extreme physiology and behavior of snow insects holds particular significance at this moment when their alpine habitats are rapidly changing due to anthropogenic climate change. VIDEO ABSTRACT.

Mortality caused by extracellular freezing is associated with fragmentation of nuclear DNA in larval haemocytes of two drosophilid flies

The great complexity of extracellular freezing stress, involving mechanical, osmotic, dehydration and chemical perturbations of the cellular milieu, hampers progress in understanding the nature of freezing injury and the mechanisms to cope with it in naturally freeze-tolerant insects. Here, we show that nuclear DNA fragmentation begins to occur in larval haemocytes of two fly species, Chymomyza costata and Drosophila melanogaster, before or at the same time as the sub-zero temperature is reached that causes irreparable freezing injury and mortality in freeze-sensitive larval phenotypes. However, when larvae of the freeze-tolerant phenotype (diapausing-cold acclimated-hyperprolinemic) of C. costata were subjected to severe freezing stress in liquid nitrogen, no DNA damage was observed. Artificially increasing the proline concentration in freeze-sensitive larvae of both species by feeding them a proline-enriched diet resulted in a decrease in the proportion of nuclei with fragmented DNA during freezing stress. Our results suggest that proline accumulated in diapausing C. costata larvae during cold acclimation may contribute to the protection of nuclear DNA against fragmentation associated with freezing stress.

Seasonal energetics: are insects constrained by energy during dormancy?

In seasonal environments, many animals, including insects, enter dormancy, where they are limited to a fixed energy budget. The inability to replenish energetic stores during these periods suggests insects should be constrained by pre-dormancy energy stores. Over the last century, the community of researchers working on survival during dormancy has operated under the strong assumption that energy limitation is a key fitness trait driving the evolution of seasonal strategies. That is, energy use has to be minimized during dormancy because insects otherwise run out of energy and die during dormancy, or are left with too little energy to complete development, reproductive maturation or other costly post-dormancy processes such as dispersal or nest building. But if energy is so strongly constrained during dormancy, how can some insects - even within the same species and population - be dormant in very warm environments or show prolonged dormancy for many successive years? In this Commentary, we discuss major assumptions regarding dormancy energetics and outline cases where insects appear to align with our assumptions and where they do not. We then highlight several research directions that could help link organismal energy use with landscape-level changes. Overall, the optimal energetic strategy during dormancy might not be to simply minimize metabolic rate, but instead to maintain a level that matches the demands of the specific life-history strategy. Given the influence of temperature on energy use rates of insects in winter, understanding dormancy energetic strategies is critical in order to determine the potential impacts of climate change on insects in seasonal environments.

Metabolomics as a tool to elucidate biochemical cold adaptation in insects

Metabolomics is an incredibly valuable tool in helping understand insect responses to cold. It not only characterizes how low temperature disrupts metabolic homeostasis, but also how it triggers fundamental adaptive responses, for example, homeoviscous adaptation and cryoprotectant accumulation. This review outlines the advantages and disadvantages of different metabolomic technologies (nuclear magnetic resonance- versus mass spectrometry-based) and screening approaches (targeted versus untargeted). We emphasize the importance of time-series and tissue-specific data, as well as the challenges of disentangling insect versus microbiome responses. In addition, we set out the need to move beyond simple correlations between metabolite abundance and tolerance phenotypes by undertaking functional assessments, for example, using dietary supplementation or injections. We highlight studies at the vanguard of employing these approaches, and where key knowledge gaps remain.

Winter Is (Not) Coming: Is Climate Change Helping Drosophila suzukii Overwintering?

Anthropogenic challenges, particularly climate change-associated factors, are strongly impacting the behavior, distribution, and survival of insects. Yet how these changes affect pests such as Drosophila suzukii, a cosmopolitan pest of soft-skinned small fruits, remains poorly understood. This polyphagous pest is chill-susceptible, with cold temperatures causing multiple stresses, including desiccation and starvation, also challenging the immune system. Since the invasion of Europe and the United States of America in 2009, it has been rapidly spreading to several European and American countries (both North and South American) and North African and Asian countries. However, globalization and global warming are allowing an altitudinal and latitudinal expansion of the species, and thus the colonization of colder regions. This review explores how D. suzukii adapts to survive during cold seasons. We focus on overwintering strategies of behavioral adaptations such as migration or sheltering, seasonal polyphenism, reproductive adaptations, as well as metabolic and transcriptomic changes in response to cold. Finally, we discuss how the continuation of climate change may promote the ability of this species to survive and spread, and what mitigation measures could be employed to overcome cold-adapted D. suzukii.