Larvae of Drosophila melanogaster exhibit transcriptional activation of immune response pathways and antimicrobial peptides during recovery from supercooling stress

Exploring cross-protective effects between cold and immune stress in Drosophila melanogaster

It is well established that environmental and biotic stressors like temperature and pathogens/parasites are essential for the life of small ectotherms. There are complex interactions between cold stress and pathogen infection in insects. Possible cross-protective mechanisms occur between both stressors, suggesting broad connectivity in insect stress responses. In this study, the functional significance of these interactions was tested, as well as the potential role of newly uncovered candidate genes, turandot. This was done using an array of factorial experiments exposing Drosophila melanogaster flies to a combination of different cold stress regimes (acute or chronic) and infections with the parasitic fungus Beauveria bassiana. Following these crossed treatments, phenotypic and molecular responses were assessed by measuring 1) induced cold tolerance, 2) immune resistance to parasitic fungus, and 3) activation of turandot genes. We found various responses in the phenotypic outcomes according to the various treatment combinations with higher susceptibility to infection following cold stress, but also significantly higher acute cold survival in flies that were infected. Regarding molecular responses, we found overexpression of turandot genes in response to most treatments, suggesting reactivity to both cold and infection. Moreover, maximum peak expressions were distinctly observed in the combined treatments (infection plus cold), indicating a marked synergistic effect of the stressors on turandot gene expression patterns. These results reflect the great complexity of cross-tolerance reactions between infection and abiotic stress, but could also shed light on the mechanisms underlying the activation of these responses.

Locust gut epithelia do not become more permeable to fluorescent dextran and bacteria in the cold

The insect gut, which plays a role in ion and water balance, has been shown to leak solutes in the cold. Cold stress can also activate insect immune systems, but it is unknown whether the leak of the gut microbiome is a possible immune trigger in the cold. We developed a novel feeding protocol to load the gut of locusts (Locusta migratoria) with fluorescent bacteria before exposing them to -2°C for up to 48 h. No bacteria were recovered from the hemolymph of cold-exposed locusts, regardless of exposure duration. To examine this further, we used an ex vivo gut sac preparation to re-test cold-induced fluorescent FITC-dextran leak across the gut and found no increased rate of leak. These results question not only the validity of FITC-dextran as a marker of paracellular barrier permeability in the gut, but also to what extent the insect gut becomes leaky in the cold.

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.

Cold-induced immune activation in chill-susceptible insects

Chilling injuries in chill-susceptible insects, like the model dipteran Drosophila melanogaster, have been well-documented as a consequence of stressful low temperature exposures. Cold stress also causes upregulation of genes in the insect immune pathways, some of which are also upregulated following other forms of sterile stress. The adaptive significance and underlying mechanisms surrounding cold-induced immune activation, however, are still unclear. Here, we review recent work on the roles of ROS, DAMPs, and AMPs in insect immune signalling or function. Using this emerging knowledge, we propose a conceptual model linking biochemical and molecular causes of immune activation to its consequences during and following cold stress.

Molecular Mechanisms of Winter Survival

Winter provides many challenges for insects, including direct injury to tissues and energy drain due to low food availability. As a result, the geographic distribution of many species is tightly coupled to their ability to survive winter. In this review, we summarize molecular processes associated with winter survival, with a particular focus on coping with cold injury and energetic challenges. Anticipatory processes such as cold acclimation and diapause cause wholesale transcriptional reorganization that increases cold resistance and promotes cryoprotectant production and energy storage. Molecular responses to low temperature are also dynamic and include signaling events during and after a cold stressor to prevent and repair cold injury. In addition, we highlight mechanisms that are subject to selection as insects evolve to variable winter conditions. Based on current knowledge, despite common threads, molecular mechanisms of winter survival vary considerably across species, and taxonomic biases must be addressed to fully appreciate the mechanistic basis of winter survival across the insect phylogeny.

Transcriptome analysis uncovers different avenues for manipulating cold performance in Chrysomya megacephala (Diptera, Calliphoridae)

Temperature strongly impacts the rates of physiological and biochemical processes, which in turn can determine the survival and population size of insects. At low temperatures performance is limited, however, cold tolerance and performance at low temperature can be improved after short- or long-term acclimation in many insect species. To understand mechanisms underlying acclimation, we sequenced and compared the transcriptome of the blowfly Chrysomya megacephala under rapid cold hardening (RCH) and long-term cold acclimation (LCA) conditions. The RCH response was dominated by genes related to immune response, spliceosome, and protein processing in endoplasmic reticulum with up-regulation during recovery from RCH. In contrast, LCA was associated with genes related to carbohydrate metabolism and cytoskeleton branching and stabilizing. Meanwhile, mRNA levels of genes related to glycerophospholipid metabolism, and some heat shock proteins (Hsps) were collectively up-regulated by both RCH and LCA. There were more genes and pathway adjustments associated with LCA than RCH. Overall, the transcriptome data provide basic information of molecular mechanisms underpinning the RCH and LCA response. The partly independent molecular responses to RCH and LCA suggest that several avenues for manipulating cold performance exist and RCH might be more effective as it only triggers fewer genes and affects the general metabolisms less. These observations provide some appropriate methods to improve cold tolerance of C. megacephala, and hold promise for developing an extended use of mass-reared C. megacephala with better cold performance as a pollinator of crops at low temperatures.

Transposable elements contribute to the genomic response to insecticides in Drosophila melanogaster

Most of the genotype–phenotype analyses to date have largely centred attention on single nucleotide polymorphisms. However, transposable element (TE) insertions have arisen as a plausible addition to the study of the genotypic–phenotypic link because of to their role in genome function and evolution. In this work, we investigate the contribution of TE insertions to the regulation of gene expression in response to insecticides. We exposed four Drosophila melanogaster strains to malathion, a commonly used organophosphate insecticide. By combining information from different approaches, including RNA-seq and ATAC-seq, we found that TEs can contribute to the regulation of gene expression under insecticide exposure by rewiring cis-regulatory networks.

This article is part of a discussion meeting issue ‘Crossroads between transposons and gene regulation’.

Listening to your gut: immune challenge to the gut sensitizes body wall nociception in the caterpillar Manduca sexta

Immune-nociceptor connections are found in animals across phyla. Local inflammation and/or damage results in increased nociceptive sensitivity of the affected area. However, in mammals, immune responses far from peripheral nociceptors, such as immune responses in the gut, produce a general increase in peripheral nociceptive sensitivity. This phenomenon has not, to our knowledge, been found in other animal groups. We found that consuming heat-killed pathogens reduced the tactile force needed to induce a defensive strike in the caterpillar Manduca sexta. This increase in the nociceptive sensitivity of the body wall is probably part of the reconfiguration of behaviour and physiology that occurs during an immune response (e.g. sickness behaviour). This increase may help enhance anti-predator behaviour as molecular resources are shifted towards the immune system. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.