Hervé Colinet

Insects in Fluctuating Thermal Environments

All climate change scenarios predict an increase in both global temperature means and the magnitude of seasonal and diel temperature variation. The nonlinear relationship between temperature and biological processes means that fluctuating temperatures lead to physiological, life history, and ecological consequences for ectothermic insects that diverge from those predicted from constant temperatures. Fluctuating temperatures that remain within permissive temperature ranges generally improve performance. By contrast, those which extend to stressful temperatures may have either positive impacts, allowing repair of damage accrued during exposure to thermal extremes, or negative impacts from cumulative damage during successive exposures. We discuss the mechanisms underlying these differing effects. Fluctuating temperatures could be used to enhance or weaken insects in applied rearing programs, and any prediction of insect performance in the field-including models of climate change or population performance-must account for the effect of fluctuating temperatures.

Temporal expression of heat shock genes during cold stress and recovery from chill coma in adult Drosophila melanogaster.

A common physiological response of organisms to environmental stresses is the increase in expression of heat shock proteins (Hsps). In insects, this process has been widely examined for heat stress, but the response to cold stress has been far less studied. In the present study, we focused on 11 Drosophila melanogaster Hsp genes during the stress exposure and recovery phases. The temporal gene expression of adults was analyzed during 9 h of cold stress at 0 °C and during 8 h of recovery at 25 °C. Increased expression of some, but not all, Hsp genes was elicited in response to cold stress. The transcriptional activity of Hsp genes was not modulated during the cold stress, and peaks of expression occurred during the recovery phase. On the basis of their response, we consider that Hsp60, Hsp67Ba and Hsc70‐1 are not cold‐inducible, whereas Hsp22, Hsp23, Hsp26, Hsp27, Hsp40, Hsp68, Hsp70Aa and Hsp83 are induced by cold. This study suggests the importance of the recovery phase for repairing chilling injuries, and highlights the need to further investigate the contributions of specific Hsp genes to thermal stress responses. Parallels are drawn between the stress response networks resulting from heat and cold stress.

Host age and fitness‐related traits in a koinobiont aphid parasitoid

Abstract.  1. Trade‐offs play a key role in species evolution and should be found in host–parasitoid interactions where the host quality may differ between host age categories.

The impact of fluctuating thermal regimes on the survival of a cold-exposed parasitic wasp, Aphidius colemani

Abstract The impact of fluctuating thermal regimes on the cold tolerance of the parasitoid Aphidius colemani at the mummy stage is examined. The hypothesis is tested that, if a cold period is interrupted by a return to a higher temperature for a short time, a physiological recovery is possible and may lead to higher survival. Mummies are exposed to a constant temperature of 4 °C and, when periodic sudden transfers to 20 °C for 2 h are applied, survival of immature parasitoids is markedly improved, proportionally to the warming frequency. The time lag before emergence diminishes with the duration of cold exposure and with warming frequency. The sex ratio of emergent adults after cold exposure indicates that males may be more susceptible than females to cold‐injury. It is suggested that the transfer to 20 °C allows a re‐activation of the normal metabolism, leading to repair and recovery of any injuries caused by prolonged chilling. The study underlines the importance of cyclic temperature changes on insect survival.

Combined transcriptomic and metabolomic approach uncovers molecular mechanisms of cold tolerance in a temperate flesh fly

The ability to respond rapidly to changes in temperature is critical for insects and other ectotherms living in variable environments. In a physiological process termed rapid cold-hardening (RCH), exposure to nonlethal low temperature allows many insects to significantly increase their cold tolerance in a matter of minutes to hours. Additionally, there are rapid changes in gene expression and cell physiology during recovery from cold injury, and we hypothesize that RCH may modulate some of these processes during recovery. In this study, we used a combination of transcriptomics and metabolomics to examine the molecular mechanisms of RCH and cold shock recovery in the flesh fly, Sarcophaga bullata. Surprisingly, out of ∼15,000 expressed sequence tags (ESTs) measured, no transcripts were upregulated during RCH, and likewise RCH had a minimal effect on the transcript signature during recovery from cold shock. However, during recovery from cold shock, we observed differential expression of ∼1,400 ESTs, including a number of heat shock proteins, cytoskeletal components, and genes from several cell signaling pathways. In the metabolome, RCH had a slight yet significant effect on several metabolic pathways, while cold shock resulted in dramatic increases in gluconeogenesis, amino acid synthesis, and cryoprotective polyol synthesis. Several biochemical pathways showed congruence at both the transcript and metabolite levels, indicating that coordinated changes in gene expression and metabolism contribute to recovery from cold shock. Thus, while RCH had very minor effects on gene expression, recovery from cold shock elicits sweeping changes in gene expression and metabolism along numerous cell signaling and biochemical pathways.

Cold exposure and associated metabolic changes in adult tropical beetles exposed to fluctuating thermal regimes

Environmental stress deleteriously affects every aspect of an ectotherms biological function. Frequent exposure of terrestrial insects to temperature variation has thus led to the evolution of protective biochemical and physiological mechanisms. However, the physiological mechanisms underlying the positive impact of fluctuating thermal regimes (FTRs) on the fitness and survival of cold‐exposed insects have not been studied. We have thus investigated the metabolic changes in adults of the beetle Alphitobius diaperinus in order to determine whether FTRs trigger the initiation of a metabolic response involving synthesis of protective compounds, such as free amino acids (FAAs) and polyols. The metabolic profile was analyzed during constant fluctuating thermal regimes (the beetles had daily pulses at higher temperatures that enabled them to recover) and compared with constant cold exposure and untreated controls. The increase of several essential amino acids (Lys, Iso, Leu, Phe and Trp) in cold‐exposed beetles supports the conclusion that it results from the breakdown of proteins. Some FAAs have been shown to have cryoprotective properties in insects, but the relationship between FAAs, cold tolerance and survival has not yet been well defined. Instead of considering FAAs only as a part of the osmo‐ and cryoprotective arsenal, they should also be regarded as main factors involved in the multiple regulatory pathways activated during cold acclimation. Under FTRs, polyol accumulation probably contributes to the increased duration of survival in A. diaperinus.

Water Relations, Fat Reserves, Survival, and Longevity of a Cold-exposed Parasitic Wasp Aphidius colemani (Hymenoptera: Aphidiinae)

Abstract Parasitoids exposed to low temperatures may suffer from extreme physiological conditions inducing direct or indirect chill injuries. Under long cold exposure (which also includes starvation), individuals face a great challenge in maintaining both water balance and energy reserves. Key parameters associated with individual fitness and physiological parameters related to cold-hardiness were analyzed. One-day-old mummies of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiinae) were exposed to cold (2 and 4°C) for various periods (1–3 wk) under a high relative humidity (75 ± 5% RH) and darkness. High mortality and shortened adult longevity occurred with increasing duration of exposure at low temperatures. Individual parasitoid mass loss increased with cold storage duration and was associated to a marked decrease in dry mass caused by lipid reserve depletion. Water content (water mass/dry mass) slightly increased with cold exposure duration because of starvation. Similar patterns were observed for both temperatures tested. This study emphasizes (1) how energetic reserves may be critical to survive at low temperatures and (2) that the survival and longevity are related, at least in part, to the depletion of energy reserves during starvation.

Gene expression changes governing extreme dehydration tolerance in an Antarctic insect

Among terrestrial organisms, arthropods are especially susceptible to dehydration, given their small body size and high surface area to volume ratio. This challenge is particularly acute for polar arthropods that face near-constant desiccating conditions, as water is frozen and thus unavailable for much of the year. The molecular mechanisms that govern extreme dehydration tolerance in insects remain largely undefined. In this study, we used RNA sequencing to quantify transcriptional mechanisms of extreme dehydration tolerance in the Antarctic midge, Belgica antarctica, the world’s southernmost insect and only insect endemic to Antarctica. Larvae of B. antarctica are remarkably tolerant of dehydration, surviving losses up to 70% of their body water. Gene expression changes in response to dehydration indicated up-regulation of cellular recycling pathways including the ubiquitin-mediated proteasome and autophagy, with concurrent down-regulation of genes involved in general metabolism and ATP production. Metabolomics results revealed shifts in metabolite pools that correlated closely with changes in gene expression, indicating that coordinated changes in gene expression and metabolism are a critical component of the dehydration response. Finally, using comparative genomics, we compared our gene expression results with a transcriptomic dataset for the Arctic collembolan, Megaphorura arctica. Although B. antarctica and M. arctica are adapted to similar environments, our analysis indicated very little overlap in expression profiles between these two arthropods. Whereas several orthologous genes showed similar expression patterns, transcriptional changes were largely species specific, indicating these polar arthropods have developed distinct transcriptional mechanisms to cope with similar desiccating conditions.

Manipulation of parasitoid size using the temperature-size rule: fitness consequences.

The phenotypic effects of rearing temperature on several fitness components of the koinobiont parasitoid, Aphidius colemani, were examined. Temperatures experienced during development induced a plastic linear response in the dry and fat masses of the immature stage and a non-linear response in the growth rate as well as in the size of adults. We investigated if the phenotypic morphometrical plasticity exhibited by parasitoids reared at different temperatures can induce variations in fitness-related traits in females. We did not find any difference in immature (pupal) mortality in accordance to rearing temperature. However, when examining adult longevity, we found an inverse linear relation with developmental temperature, confirming the usual rule that larger and fatter wasps live longer than smaller ones. The pattern of female fecundity was non-linear; wasps that developed at high and low temperatures were less productive. We suggest that when development is short, the accumulated reserves are not adequate to support both fecundity and survival. By manipulating adult size through changes in the rearing temperature, we showed that the usual shape of the size/fitness function is not always linear as expected. Developmental temperature induced a plasticity in energy reserves which affected the functional constraints between survival and reproduction.

Metabolic effects of CO2 anaesthesia in Drosophila melanogaster

Immobilization of insects is necessary for various experimental purposes, and CO2 exposure remains the most popular anaesthetic method in entomological research. A number of negative side effects of CO2 anaesthesia have been reported, but CO2 probably brings about metabolic modifications that are poorly known. In this work, we used GC/MS-based metabolic fingerprinting to assess the effect of CO2 anaesthesia in Drosophila melanogaster adults. We analysed metabolic variation of flies submitted to acute CO2 exposure and assessed the temporal metabolic changes during short- and long-term recovery. We found that D. melanogaster metabotypes were significantly affected by the anaesthetic treatment. Metabolic changes caused by acute CO2 exposure were still manifested after 14 h of recovery. However, we found no evidence of metabolic alterations when a long recovery period was allowed (more than 24 h). This study points to some metabolic pathways altered during CO2 anaesthesia (e.g. energetic metabolism). Evidence of short-term metabolic changes indicates that CO2 anaesthesia should be used with utmost caution in physiological studies when a short recovery is allowed. In spite of this, CO2 treatment seems to be an acceptable anaesthetic method provided that a long recovery period is allowed (more than 24 h).