'Futile cycle' enzymes in the flight muscles of North American bumblebees

KC. Heat exposure limits pentose phosphate pathway activity in bumblebees

Bumblebee populations across the globe are experiencing substantial declines due to climate change, with major consequences for pollination services in both natural and agricultural settings. Using an economically important species, Bombus impatiens, we explored the physiological mechanisms that may cause susceptibility to extreme heat events. We tested the hypothesis that heat exposure limits the activity of the pentose phosphate pathway (PPP)-a parallel pathway to glycolysis that can use nectar sugar to generate antioxidant potential and combat oxidative stress. Using isotopically labelled glucose, we tracked PPP activity in B. impatiens at rest, during exercise and during a post-exercise recovery period under two different temperature regimes (22°C and 32°C). We found that the PPP is routinely used by B. impatiens at moderate temperatures, but that its activity is markedly reduced when ATP demands are high, such as during periods of exercise and heat exposure. We also exposed B. impatiens to either 22°C or 32°C for 5 hours and assessed levels of oxidative damage (lipid peroxidation, protein carbonyls) and antioxidant potential [reduced (GSH) and oxidized (GSSG) glutathione concentrations]. Interestingly, bees exhibited little oxidative damage after the thermal exposure, but we found a lower GSH:GSSG ratio in 32°C-exposed bees, reflecting lower antioxidant potential. Overall, our study demonstrates that acute heat stress severely limits PPP activity and may constrain antioxidant potential in B. impatiens. The repeated attenuation of this pathway in a warming climate may have more severe physiological consequences for this species, with potential implications for pollination services across North America.

Handling heatwaves: balancing thermoregulation, foraging and bumblebee colony success

Climate changes pose risks for bumblebee populations, which have declined relative to the growing frequency and severity of warmer temperature extremes. Bumblebees might mitigate the effects of such extreme weather through colonial behaviours. In particular, fanning behaviour to dissipate heat is an important mechanism that could reduce exposure of thermally sensitive offspring to detrimental nest temperatures (Tn). The allocation of workers towards fanning over prolonged periods could impact foraging activity that is essential for colony-sustaining resource gathering. Colony maintenance and growth could suffer as a result of nutritional and high ambient temperature (Ta) thermal stress. It remains uncertain whether a compromise occurs between thermoregulation and foraging under chronic, sublethal heat events and how colony success is impacted as a result. This study held colonies of Bombus impatiens at constant high Ta (25°C, 30°C or 35°C) for 2 weeks while quantifying the percentage of foragers, fanning incidence, nest temperature (Tn) and other metrics of colony success such as the percentage of adult emergence and offspring production. We found that foraging and adult emergence were not significantly affected by Ta, but that thermoregulation was unsuccessful at maintaining Tn despite increased fanning at 35°C. Furthermore, 35°C resulted in workers abandoning the colony and fewer offspring being produced. Our findings imply that heatwave events that exceed 30°C can negatively impact colony success through failed thermoregulation and reduced workforce production.

High temperature sensitivity of bumblebee castes and the colony-level costs of thermoregulation in Bombus impatiens

Physiological thermal limits often reflect species distribution, but the role that ambient temperature (Ta) plays in limiting species within their thermal environment remains unclear. Climate change-linked declines in bumblebees, an important pollinator group, leave questions regarding which aspect of their physiology is hindered under high Ta. As a eusocial species, bumblebees utilize their ability to thermoregulate as a superorganism to maintain nest temperature (Tn) within a narrow thermal window to buffer developing larvae from developmental defects. Thermoregulatory behaviours, such as thermogenesis to warm up and fanning to cool down the nest, are energetically expensive and it is uncertain how successful large colonies are at maintaining Tn within its optimal range. Using a common bumblebee species, Bombus impatiens, our study first established the critical thermal limits (CTmax) of workers, queens, drones and larvae to determine which caste is most thermally sensitive to heat. We found that larvae had significantly lower heat tolerance than adults, highlighting the importance of colonial thermoregulation. We then measured the energy expenditure of large colonies under acute thermal stress (5-40 °C) using flow-through respirometry while simultaneously quantifying Tn. Colonies that experienced Ta at or below optimal Tn (≤30 °C) were successful at thermoregulation. At 35 °C and above, however, Tn increased despite high energetic costs to the colony. Together our results demonstrate that high Ta poses a risk to colonies that fail to buffer thermally sensitive larvae from changes in Tn.

Overwintering in North American domesticated honeybees (Apis mellifera) causes mitochondrial reprogramming while enhancing cellular immunity

Many factors negatively impact domesticated honeybee (Apis mellifera) health causing a global decrease in their population year after year with major losses occurring during winter, and the cause remains thus far unknown. Here, we monitored for 12 months North American colonies of honeybees enduring important temperature variations throughout the year, to assess the metabolism and immune system of honeybees of summer and winter individuals. Our results show that in flight muscle, mitochondrial respiration via complex I during winter is drastically reduced compared to summer. However, the capacity for succinate and glycerol-3-phosphate (G3P) oxidation by mitochondria is increased during winter, resulting in higher mitochondrial oxygen consumption when complex I substrates, succinate and G3P were assessed altogether. Pyruvate kinase, lactate dehydrogenase, aspartate aminotransferase, citrate synthase and malate dehydrogenase tend to have reduced activity levels in winter unlike hexokinase, NADH dehydrogenase and pyruvate dehydrogenase. Transcript abundance of highly important immunity proteins like Vitellogenin and Defensin-1 were also increased in winter bees, and a stronger phagocytic response as well as a better hemocyte viability was observed during winter. Thus, a reorganization of substrate utilization favoring succinate and G3P while negatively affecting complex I of the ETS is occurring during winter. We suggest that this might be due to complex I transitioning to a dormant conformation through post-translational modification. Winter bees also have an increased response for antibacterial elimination in honeybees. Overall, this study highlights previously unknown cellular mechanisms between summer and winter honeybees that further our knowledge about this important species.

Octopamine drives honeybee thermogenesis

In times of environmental change species have two options to survive: they either relocate to a new habitat or they adapt to the altered environment. Adaptation requires physiological plasticity and provides a selection benefit. In this regard, the Western honeybee (Apis mellifera) protrudes with its thermoregulatory capabilities, which enables a nearly worldwide distribution. Especially in the cold, shivering thermogenesis enables foraging as well as proper brood development and thus survival. In this study, we present octopamine signaling as a neurochemical prerequisite for honeybee thermogenesis: we were able to induce hypothermia by depleting octopamine in the flight muscles. Additionally, we could restore the ability to increase body temperature by administering octopamine. Thus, we conclude that octopamine signaling in the flight muscles is necessary for thermogenesis. Moreover, we show that these effects are mediated by β octopamine receptors. The significance of our results is highlighted by the fact the respective receptor genes underlie enormous selective pressure due to adaptation to cold climates. Finally, octopamine signaling in the service of thermogenesis might be a key strategy to survive in a changing environment.

Transcriptome and proteome dynamics of cervical remodeling in the mouse during pregnancy

During gestation, the female reproductive tract must maintain pregnancy while concurrently preparing for parturition. Here, we explore the transitions in gene expression and protein turnover (fractional synthesis rates [FSR]) by which the cervix implements a transition from rigid to compliant. Shifts in gene transcription to achieve immune tolerance and alter epithelial cell programs begin in early pregnancy. Subsequently, in mid-to-late pregnancy transcriptional programs emerge that promote structural reorganization of the extracellular matrix (ECM). Stable isotope labeling revealed a striking slowdown of overall FSRs across the proteome on gestation day 6 that reverses in mid-to-late pregnancy. An exception was soluble fibrillar collagens and proteins of collagen assembly, which exhibit high turnover in non-pregnant cervix compared to other tissues and FSRs that continue throughout pregnancy. This finding provides a mechanism to explain how cross-linked collagen is replaced by newly synthesized, less-cross-linked collagens, which allows increased tissue compliance during parturition. The rapid transition requires a reservoir of newly synthesized, less cross-linked collagens, which is assured by the high FSR of soluble collagens in the cervix. These findings suggest a previously unrecognized form of "metabolic flexibility" for ECM in the cervix that underlies rapid transformation in compliance to allow parturition.

Adaptations in energy metabolism and gene family expansions revealed by comparative transcriptomics of three Chagas disease triatomine vectors

Background

Chagas disease is a parasitic infection caused by Trypanosoma cruzi. It is an important public health problem affecting around seven to eight million people in the Americas. A large number of hematophagous triatomine insect species, occupying diverse natural and human-modified ecological niches transmit this disease. Triatomines are long-living hemipterans that have evolved to explode different habitats to associate with their vertebrate hosts. Understanding the molecular basis of the extreme physiological conditions including starvation tolerance and longevity could provide insights for developing novel control strategies. We describe the normalized cDNA, full body transcriptome analysis of three main vectors in North, Central and South America, Triatoma pallidipennis, T. dimidiata and T. infestans.

Results

Two-thirds of the de novo assembled transcriptomes map to the Rhodnius prolixus genome and proteome. A Triatoma expansion of the calycin family and two types of protease inhibitors, pacifastins and cystatins were identified. A high number of transcriptionally active class I transposable elements was documented in T. infestans, compared with T. dimidiata and T. pallidipennis. Sequence identity in Triatoma-R. prolixus 1:1 orthologs revealed high sequence divergence in four enzymes participating in gluconeogenesis, glycogen synthesis and the pentose phosphate pathway, indicating high evolutionary rates of these genes. Also, molecular evidence suggesting positive selection was found for several genes of the oxidative phosphorylation I, III and V complexes.

Conclusions

Protease inhibitors and calycin-coding gene expansions provide insights into rapidly evolving processes of protease regulation and haematophagy. Higher evolutionary rates in enzymes that exert metabolic flux control towards anabolism and evidence for positive selection in oxidative phosphorylation complexes might represent genetic adaptations, possibly related to prolonged starvation, oxidative stress tolerance, longevity, and hematophagy and flight reduction. Overall, this work generated novel hypothesis related to biological adaptations to extreme physiological conditions and diverse ecological niches that sustain Chagas disease transmission.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4696-8) contains supplementary material, which is available to authorized users.

The effect of dietary neonicotinoid pesticides on non-flight thermogenesis in worker bumble bees (Bombus terrestris)

For bumble bees (genus Bombus), the capacity for non-flight thermogenesis is essential for two fundamental processes undertaken by adult workers, namely recovery from torpor after chilling and brood incubation. Farmland bees can be widely exposed to dietary residues of neurotoxic neonicotinoid insecticides that appear in the nectar and pollen of treated bee-attractive crops, which may harm them. An earlier study shows that dietary neonicotinoids cause complex alterations to thermoregulation in honey bees, but their effect on the thermogenic capabilities of individual bumble bees has been untested previously. We therefore conducted laboratory trials involving separate dietary exposures of bumble bees to two neonicotinoids, imidacloprid and thiamethoxam, and we measured their effects on the thoracic temperatures of bees during recovery from chilling. Specifically, we used thermal imaging to measure the rates of rewarming by individual bees after chill-induced torpor and to quantify their equilibrated thoracic temperatures post-recovery. We found that both toxicants caused dose-dependent decreases in the rates of rewarming and in the equilibrated thoracic temperatures. As previously found in honey bees, the dose-response relationship for imidacloprid exhibited a biphasic hormesis with low-dose stimulation and high-dose inhibition, for which we propose a mechanism. Our present study is among the first to detect ecologically relevant effects on bees in neonicotinoid exposures involving dietary concentrations below 5 ppb. If the effects on thoracic temperatures that we observed over a short period were sustained, they could have ecologically significant impacts on farmland bumble bees.

Mitochondrial glycerol 3-phosphate facilitates bumblebee pre-flight thermogenesis

Bumblebees (Bombus terrestris) fly at low ambient temperatures where other insects cannot, and to do so they must pre-warm their flight muscles. While some have proposed mechanisms, none fully explain how pre-flight thermogenesis occurs. Here, we present a novel hypothesis based on the less studied mitochondrial glycerol 3-phosphate dehydrogenase pathway (mGPDH). Using calorimetry, and high resolution respirometry coupled with fluorimetry, we report substrate oxidation by mGPDH in permeabilised flight muscles operates, in vitro, at a high flux, even in the absence of ADP. This may be facilitated by an endogenous, mGPDH-mediated uncoupling of mitochondria. This uncoupling increases ETS activity, which results in increased heat release. Furthermore, passive regulation of this mechanism is achieved via dampened temperature sensitivity of mGPDH relative to other respiratory pathways, and subsequent consumption of its substrate, glycerol 3-phosphate (G3P), at low temperatures. Mitochondrial GPDH may therefore facilitate pre-flight thermogenesis through poor mitochondrial coupling. We calculate this can occur at a sufficient rate to warm flight muscles until shivering commences, and until flight muscle function is adequate for bumblebees to fly in the cold.

Sarcolipin and uncoupling protein 1 play distinct roles in diet-induced thermogenesis and do not compensate for one another

OBJECTIVE

It is well known that uncoupling protein 1 (UCP1) in brown adipose tissue plays an important role in diet-induced thermogenesis. In this study, whether sarcolipin (SLN), a regulator of sarco/endoplasmic reticulum Ca(2+) -ATPase pump in muscle, is also an important player of diet-induced thermogenesis was investigated, as well as whether loss of SLN could be compensated by increased UCP1 expression and vice versa.

METHODS

Age- and sex-matched UCP1(-/-) , SLN(-/-) , and double knockout for both UCP1 and SLN mice maintained in C57Bl/6J background were challenged to high-fat diet for 12 weeks and then analyzed for weight gain, alterations in serum metabolites, and changes in thermogenic protein expression.

RESULTS

Loss of either SLN or UCP1 alone was sufficient to cause diet-induced obesity. No compensatory upregulation of UCP1 in SLN(-/-) mice or vice versa was found. Paradoxically, loss of both mechanisms failed to exacerbate the obesity phenotype.

CONCLUSIONS

Data suggest that both SLN- and UCP1-based adaptive thermogenic mechanisms were essential for achieving maximal diet-induced thermogenesis. When both mechanisms were absent, less efficient thermogenic mechanisms were activated to counter energy imbalance.

Seasonal cues induce phenotypic plasticity of Drosophila suzukii to enhance winter survival

BACKGROUND

As global climate change and exponential human population growth intensifies pressure on agricultural systems, the need to effectively manage invasive insect pests is becoming increasingly important to global food security. Drosophila suzukii is an invasive pest that drastically expanded its global range in a very short time since 2008, spreading to most areas in North America and many countries in Europe and South America. Preliminary ecological modeling predicted a more restricted distribution and, for this reason, the invasion of D. suzukii to northern temperate regions is especially unexpected. Investigating D. suzukii phenology and seasonal adaptations can lead to a better understanding of the mechanisms through which insects express phenotypic plasticity, which likely enables invasive species to successfully colonize a wide range of environments.

RESULTS

We describe seasonal phenotypic plasticity in field populations of D. suzukii. Specifically, we observed a trend of higher proportions of flies with the winter morph phenotype, characterized by darker pigmentation and longer wing length, as summer progresses to winter. A laboratory-simulated winter photoperiod and temperature (12:12 L:D and 10 °C) were sufficient to induce the winter morph phenotype in D. suzukii. This winter morph is associated with increased survival at 1 °C when compared to the summer morph, thus explaining the ability of D. suzukii to survive cold winters. We then used RNA sequencing to identify gene expression differences underlying seasonal differences in D. suzukii physiology. Winter morph gene expression is consistent with known mechanisms of cold-hardening such as adjustments to ion transport and up-regulation of carbohydrate metabolism. In addition, transcripts involved in oogenesis and DNA replication were down-regulated in the winter morph, providing the first molecular evidence of a reproductive diapause in D. suzukii.

CONCLUSIONS

To date, D. suzukii cold resistance studies suggest that this species cannot overwinter in northern locations, e.g. Canada, even though they are established pests in these regions. Combining physiological investigations with RNA sequencing, we present potential mechanisms by which D. suzukii can overwinter in these regions. This work may contribute to more accurate population models that incorporate seasonal variation in physiological parameters, leading to development of better management strategies.

Unique PFK regulatory property from some mosquito vectors of disease, and from Drosophila melanogaster

BACKGROUND

Arthropod-borne diseases are some of the most rapidly spreading diseases. Reducing the vector population is currently the only effective way to reduce case numbers. Central metabolic pathways are potential targets to control vector populations, but have not been well explored to this aim. The information available on energy metabolism, as a way to control lifespan and dispersion through flight of dipteran vectors, is inadequate.

METHODS

Phosphofructokinase (PFK) activity was measured in the presence of both of its substrates, fructose-6-phosphate (F6P) and ATP, as well as some allosteric effectors: Fructose- 2,6 - bisphosphate (F2, 6BP), citrate and AMP. Aedes aegypti phosphofructokinase sequence (AaPFK) was aligned with many other insects and also vertebrate sequences. A 3D AaPFK model was produced and docking experiments were performed with AMP and citrate.

RESULTS

The kinetic parameters of AaPFK were determined for both substrates: F6P (V = 4.47 ± 0.15 μmol of F1, 6BP/min, K0.5 = 1.48 ± 0.22 mM) and ATP (V = 4.73 ± 0.57 μmol of F1, 6BP/min, K0.5 = 0.43 ± 0.10 mM). F2,6P was a powerful activator of AaPFK, even at low ATP concentrations. AaPFK inhibition by ATP was not enhanced by citrate, consistent with observations in other insects. After examining the sequence alignment of insect and non-insect PFKs, the hypothesis is that a modification of the citrate binding site is responsible for this unique behavior. AMP, a well-known positive effector of PFK, was not capable of reverting ATP inhibition. Aedes, Anopheles and Culex are dengue, malaria and filariasis vectors, respectively, and are shown to have this distinct characteristic in phosphofructokinase control. The alignment of several insect PFKs suggested a difference in the AMP binding site and a significant change in local charges, which introduces a highly negative charge in this part of the protein, making the binding of AMP unlikely. This hypothesis was supported by 3D modeling of PFK with AMP docking, which suggested that the AMP molecule binds in a reverse orientation due to the electrostatic environment. The present findings imply a potential new way to control PFK activity and are a unique feature of these Diptera.

CONCLUSIONS

The present findings provide the first molecular explanation for citrate insensitivity in insect PFKs, as well as demonstrating for the first time AMP insensitivity in dipterans. It also identified a potential target for novel insecticides for the control of arthropod-borne diseases.

Nutrient balancing of the adult worker bumblebee (Bombus terrestris) depends on the dietary source of essential amino acids

Animals carefully regulate the amount of protein that they consume. The quantity of individual essential amino acids (EAAs) obtained from dietary protein depends on the protein source, but how the proportion of EAAs in the diet affects nutrient balancing has rarely been studied. Recent research using the Geometric Framework for Nutrition has revealed that forager honeybees who receive much of their dietary EAAs from floral nectar and not from solid protein have relatively low requirements for dietary EAAs. Here, we examined the nutritional requirements for protein and carbohydrates of foragers of the buff-tailed bumblebee Bombus terrestris. By using protein (sodium caseinate) or an equimolar mixture of the 10 EAAs, we found that the intake target (nutritional optimum) of adult workers depended on the source and proportion of dietary EAAs. When bees consumed caseinate-containing diets in a range of ratios between 1:250 and 1:25 (protein to carbohydrate), they achieved an intake target (IT) of 1:149 (w/w). In contrast to those fed protein, bees fed the EAA diets had an IT more biased towards carbohydrates (1:560 w/w) but also had a greater risk of death than those fed caseinate. We also tested how the dietary source of EAAs affected free AAs in bee haemolymph. Bees fed diets near their IT had similar haemolymph AA profiles, whereas bees fed diets high in caseinate had elevated levels of leucine, threonine, valine and alanine in the haemolymph. We found that like honeybees, bumblebee workers prioritize carbohydrate intake and have a relatively low requirement for protein. The dietary source of EAAs influenced both the ratio of protein/EAA to carbohydrate and the overall amount of carbohydrate eaten. Our data support the idea that EAAs and carbohydrates in haemolymph are important determinants of nutritional state in insects.

Summary: The essential amino acid profile of the bumblebee diet influences regulation of the amount of both protein and carbohydrate consumed.

Lack of variation at phosphoglucose isomerase (pgi) in bumblebees: implications for conservation genetics studies

Assessing genetic variation underlying ecologically important traits is increasingly of interest and importance in population and conservation genetics. For some groups generally useful markers exist for examining the relative role of selection and drift in shaping genetic diversity e.g. the major histocompatibility complex in vertebrates and self-incompatibility loci in plants. For invertebrates there is no such generally useful locus. However, phosphoglucose isomerase (Pgi) has been proposed as a useful functional marker in the conservation genetics of invertebrates. Where thermal microclimate varies, balanced polymorphisms may be maintained due to trade-offs between thermally stable and kinetically advantageous allelic forms. We here report very low levels of Pgi variation in bumblebees rendering this locus to be of little use as an adaptive marker in a conservation genetics context in this group. Potential explanations for this lack of variation are considered.

Differential effects of nutrient-limited primary production on primary, secondary or tertiary consumers

Nutritional imbalances between predator and prey are the rule rather than the exception at the lower end of food webs. We investigated the role of different grazers in the propagation of nutritionally imbalanced primary production by using the same primary producers in a three-trophic-level food chain and a four-trophic-level food chain experimental setup. The three-trophic-level food chain consisted of a classic single-cell primary producer (Rhodomonas salina), a metazoan grazer (the copepod Acartia tonsa) and a top predator (the jellyfish Gonionemus vertens), while we added a protozoan grazer (Oxyrrhis marina) as primary consumer to the food chain to establish the four-trophic-level food chain. This setup allowed us to investigate how nutrient-limitation effects change from one trophic level to another, and to investigate the performance of two components of our experimental food chains in different trophic positions. Stoichiometry and fatty acid profiles of the algae showed significant differences between the nutrient-depleted [no N and no P addition (-P), respectively] and the nutrient-replete (f/2) treatments. The differences in stoichiometry could be traced when O. marina was the first consumer. Copepods feeding on these flagellates were not affected by the nutritional imbalance of their prey in their stoichiometry, their respiration rates nor in their developmental rates. In contrast, when copepods were the primary consumer, those reared on the -P algae showed significantly higher respiration rates along with significantly lower developmental rates. In neither of our two experimental food chains did the signals from the base of the food chains travel up to jelly fish, our top predator.

F-actin, beta-tubulin, aldolase, and fructose-1,6-bisphosphatase in heteropteran ovarioles--I. Immunocytochemical investigations of whole-mounted ovarioles

The distribution of F-actin, beta-tubulin, aldolase, and fructose-1,6-bisphosphatase (FBPase) in ovarioles of four heteropteran species (Ilyocoris cimicoides, Coreus marginatus, Lygus pratensis, and Notostira elongata) was investigated biochemically and immunocytochemically. Aldolase was found to be uniformly distributed in the cytoplasm of trophocytes and follicular cells, with the highest concentration in prefollicular cells. Its concentration in follicular cells increased during differentiation and reached a peak in ovarian follicles at the stage of late choriogenesis. FBPase was observed in the cytoplasm (weak reaction) and on cell borders (strong reaction) of both germ line and somatic cells. No FBPase or aldolase signal was observed on the F-actin trophic core mesh or on stress fibers.

Control and Regulatory Mechanisms Associated with Thermogenesis in Flying Insects and Birds

Most insects and birds are able to fly. The chitin made exoskeleton of insects poses them several constraints, and this is one the reasons they are in general small sized animals. On the other hand, because birds possess an endoskeleton made of bones they may grow much larger when compared to insects. The two taxa are quite different with regards to their general “design” platform, in particular with respect to their respiratory and circulatory systems. However, because they fly, they may share in common several traits, namely those associated with the control and regulatory mechanisms governing thermogenesis. High core temperatures are essential for animal flight irrespective of the taxa they belong to. Birds and insects have thus evolved mechanisms which allowed them to control and regulate high rates of heat fluxes. This article discusses possible convergent thermogenic control and regulatory mechanisms associated with flight in insects and birds.