Proline as a fuel for insect flight: enhancing carbohydrate oxidation in hymenopterans

Comparative metabolomics reveals how the severity of predation by the invasive insect Cydalima perspectalis modulates the metabolism re-orchestration of native Buxus sempervirens

The recent biological invasion of box tree moth Cydalima perspectalis on Buxus trees has a major impact on European boxwood stands through severe defoliation. This can hinder further regrowth and threaten survival of populations. In a mesocosm approach and controlled larval density over a 2-month period, responses of B. sempervirens essential and specialized metabolites were characterized using metabolomics, combining 1H-NMR and LC-MS/MS approaches. This is the first metabolome depiction of major Buxus responses to boxwood moth invasion. Under severe predation, remaining green leaves accumulate free amino acids (with the noticeable exception of proline). The leaf trans-4-hydroxystachydrine and stachydrine reached 10-13% and 2-3% (DW), while root content was lower but also modulated by predation level. Larval predation promoted triterpenoid and (steroidal) alkaloid synthesis and diversification, while flavonoids did not seem to have a relevant role in Buxus resistance. Our results reveal the concomitant responses of central and specialized metabolism, in relation to severity of predation. They also confirm the potential of metabolic profiling using 1H-NMR and LC-MS to detect re-orchestration of metabolism of native boxwood after severe herbivorous predation by the invasive box-tree moth, and thus their relevance for plant-insect relationships and ecometabolomics.

The effect of pollen monodiets on fat body morphology parameters and energy substrate levels in the fat body and hemolymph of Apis mellifera L. workers

Human activities associated with large-scale farms and the monocultures expose honey bees to one type of food. Moreover, there is an ongoing decline of plant species producing pollen and nectar in Europe. A poorly balanced diet affects a number of processes occurring in a bee's body. The fat body and hemolymph are the tissues that participate in all of them. Therefore, the aim of our study was to determine the effect of hazel, pine, rapeseed, buckwheat, phacelia and goldenrod pollen on the morphological parameters of fat body trophocytes, the diameters of cell nuclei in oenocytes and the concentrations of compounds involved in energy metabolism (glucose, glycogen, triglycerides and protein). In the cage tests, the bees were fed from the first day of life with sugar candy (control group) or candy with a 10% addition of one of the 6 pollen types. Hemolymph and fat body from various locations were collected from 1-, 7- and 14-day-old workers. Pollen produced by plant species such as hazel and pine increased glucose concentrations in the bee tissues, especially in the hemolymph. It can therefore be concluded that they are valuable sources of energy (in the form of simple carbohydrates) which are quickly used by bees. Pollen from plants blooming in the summer and autumn increased the concentrations of proteins, glycogen and triglycerides in the fat body, especially that from the third tergite. The accumulation of these compounds was associated with an increased the length and width of trophocytes as well as with enhanced metabolic activity, which was evidenced in the increasing diameter of oenocyte cell nuclei. It seems a balanced multi-pollen diet is more valuable for bees, but it is important to understand the effects of the particular pollen types in the context of a mono-diet. In the future, this will make it possible to produce mixtures that can ensure homeostasis in the apian body.

Chronic changes in developmental oxygen have little effect on mitochondria and tracheal density in the endothermic moth Manduca sexta

Oxygen availability during development is known to impact the development of insect respiratory and metabolic systems. Drosophila adult tracheal density exhibits developmental plasticity in response to hypoxic or hyperoxic oxygen levels during larval development. Respiratory systems of insects with higher aerobic demands, such as those that are facultative endotherms, may be even more responsive to oxygen levels above or below normoxia during development. The moth Manduca sexta is a large endothermic flying insect that serves as a good study system to start answering questions about developmental plasticity. In this study, we examined the effect of developmental oxygen levels (hypoxia: 10% oxygen, and hyperoxia: 30% oxygen) on the respiratory and metabolic phenotype of adult moths, focusing on morphological and physiological cellular and intercellular changes in phenotype. Mitochondrial respiration rate in permeabilized and isolated flight muscle was measured in adults. We found that permeabilized flight muscle fibers from the hypoxic group had increased mitochondrial oxygen consumption, but this was not replicated in isolated flight muscle mitochondria. Morphological changes in the trachea were examined using confocal imaging. We used transmission electron microscopy to quantify muscle and mitochondrial density in the flight muscle. The respiratory morphology was not significantly different between developmental oxygen groups. These results suggest that the developing M. sexta trachea and mitochondrial respiration have limited developmental plasticity when faced with rearing at 10% or 30% oxygen.

Flies on the rise: acclimation effect on mitochondrial oxidation capacity at normal and high temperatures in Drosophila melanogaster

Increased average temperatures and extreme thermal events (such as heatwaves) brought forth by climate change impose important constraints on aerobic metabolism. Notably, mitochondrial metabolism, which is affected by both long- and short-term temperature changes, has been put forward as an important determinant for thermal tolerance of organisms. This study examined the influence of phenotypic plasticity on metabolic and physiological parameters in Drosophila melanogaster and the link between mitochondrial function and their upper thermal limits. We showed that D. melanogaster acclimated to 15°C have a 0.65°C lower critical thermal maximum (CTmax) compared with those acclimated to 24°C. Drosophila melanogaster acclimated to 15°C exhibited a higher proportion of shorter saturated and monounsaturated fatty acids, concomitant with lower proportions of polyunsaturated fatty acids. No mitochondrial quantitative changes (fractional area and number) were detected between acclimation groups, but changes of mitochondrial oxidation capacities were observed. Specifically, in both 15°C- and 24°C-acclimated flies, complex I-induced respiration was increased when measured between 15 and 24°C, but drastically declined when measured at 40°C. When succinate and glycerol-3-phosphate were added, this decrease was however compensated for in flies acclimated to 24°C, suggesting an important impact of acclimation on mitochondrial function related to thermal tolerance. Our study reveals that the use of oxidative substrates at high temperatures is influenced by acclimation temperature and strongly related to upper thermal tolerance as a difference of 0.65°C in CTmax translates into significant mitochondrial changes.

The Key Role of Amino Acids in Pollen Quality and Honey Bee Physiology-A Review

When studying honey bee nutrition, it is important to pay attention not only to the quantity but also to the quality of pollen for floral visitors. The recommended way to determine the value of pollen is to determine both the protein concentration and the amino acid composition in the insect's hemolymph. In addition, the composition of pollen also includes lipids, sterols and biogenic elements such as carbon, nitrogen, etc. Very high protein concentration is observed in aloe pollen, averaging 51%. Plants with a high protein content, at the level of 27% in Europe, are rapeseed and phacelia. In turn, a plant that is poor in protein (at the level of 11%) is buckwheat. The aforementioned plants are sown over very large areas. Vast acreages in Central and Eastern Europe are occupied by pollen- and nectar-providing invasive plants, such as goldenrod. Therefore, bees are forced to use one food source-a mono diet-which results in their malnutrition. In the absence of natural pollen, beekeepers use other foods for bees; including soy protein, powdered milk, egg yolks, fish meal, etc. However, the colony is the strongest when bees are fed with pollen, as opposed to artificial protein diets. More research is needed on the relationship between bee pollen composition and nutrition, as measured by protein concentration and amino acid composition in apian hemolymph, colony strength, honey yield and good overwintering.

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.

Metabolic Rate Suppression and Maintenance of Flight Muscle Metabolic Capacity during Diapause in Bumble Bee (Bombus impatiens) Queens

AbstractThe common eastern bumble bee (Bombus impatiens) queens endure cold winter months by entering a diapause state. During this overwintering period, these animals use stored energy reserves while maintaining a low metabolic rate. This study investigates changes in the metabolic rate of bumble bee queens during diapause-like laboratory conditions and the potential reorganization of the flight muscle metabolic properties during this period. We first confirmed the hypometabolic state of queens during diapause in the laboratory, which lowered their resting metabolic rate to less than 5% of normal resting values. Body mass decreased during diapause, body composition changed where carbohydrates decreased initially, and later protein declined, with a similar trend for lipid content. Using cellular respirometry, we determined the capacity of the flight muscle cells of bumble bee queens to use various metabolic fuels and whether this capacity changes during the progression of diapause to favor stored lipid-derived substrates. Queens showed a low capacity to oxidize the amino acid proline, compared with workers, and their capacity to oxidize all metabolic substrates did not change during a 4-mo diapause period in the laboratory. We also show no detectable ability to oxidize fatty acid by flight muscle mitochondria in this species. The metabolic properties of flight muscle tissue were further characterized using metabolic enzyme activity profiles showing little change during diapause, indicating that profound metabolic suppression is induced without major changes in muscle metabolic phenotypes. Overall, B. impatiens queens undergo diapause while maintaining flight muscle capacity under the conditions used.

Bombus terrestris Prefer Mixed-Pollen Diets for a Better Colony Performance: A Laboratory Study

Pollen is a major source of proteins and lipids for bumblebees. The nutritional content of pollen may differ from source plants, ultimately affecting colony development. This study investigated the foraging preferences of Bombus terrestris in regard to four pollen species, i.e., oilseed rape, wild apricot, sunflower, and buckwheat, under laboratory conditions. The results show that B. terrestris diversified their preference for pollens; the bumblebees mostly preferred wild apricot pollen, whereas sunflower pollen was the least preferred. The colonies fed on a mixed four-pollen diet, with a protein-lipid ratio of 4.55-4.86, exhibited better development in terms of the number of offspring, individual body size and colony weight. The colonies fed with buckwheat and sunflower pollens produced a significantly lower number of workers and failed to produce queen and male offspring. Moreover, wild apricot pollen had the richest protein content (23.9 g/100 g) of the four pollen species, whereas oilseed rape pollen had the highest lipid content (6.7 g/100 g), as revealed by the P:L ratios of wild apricot, sunflower, buckwheat, and oilseed rape, which were 6.76, 5.52, 3.50, and 3.37, respectively. Generally, B. terrestris showed feeding preferences regarding different pollens and a mixture of pollens, which ultimately resulted in differences in colony development. The findings of this study provide important baseline information to researchers and developers of nutritive pollen diets for bumblebees.

Gustatory responsiveness in Vespula germanica workers: exploring the interplay between sensory perception and task specialization

Workers' task specialization and division of labor are critical features of social insects' ecological success. It has been proposed that the division of labor relies on response threshold models: individuals varying their sensitivity (and responsiveness) to biologically relevant stimuli and performing a specific task when a stimulus exceeds an internal threshold. In this work, we study carbohydrate and protein responsiveness and their relation to worker task specialization in Vespula germanica, an invasive social wasp. The sucrose and peptone responsiveness of two different subcastes, preforagers and foragers, was determined by stimulating the antenna of the wasps with increasing concentrations of the solution and quantifying whether each concentration elicited a licking response. We studied responsiveness in five different ways: (1) response threshold, (2) concentration 50 (concentration to which at least 50% of wasps responded), (3) maximum response, (4) mean scores and (5) median scores. Our results suggest that V. germanica foragers are more sensitive to sucrose (lower thresholds) than preforager workers. However, we found no differences for peptone thresholds (i.e., a protein resource). Nonetheless, this is the first study to investigate response thresholds for protein resources. The intercaste variation in sucrose responsiveness shown in our work contributes to the existing knowledge about response threshold theory as a mechanism for task specialization observed in V. germanica.

The costs of overwintering in paper wasps (Polistes dominula and Polistes gallicus): the use of energy stores

Overwintering insects are facing energetic challenges because of food shortage, low temperature, and desiccation stress. Paper wasps of the genus Polistes overwinter as mated adults (gynes) in hibernacula protecting them from predation, snow, and rain but barely from low environmental temperature. In different climates, they face differing overwintering temperature regimes, and therefore they may differ in their energy use. We investigated how much of energy resources built up until autumn is used during diapause dormancy in natural hibernacula by measuring lipid, glycogen, and free carbohydrate content in autumn and early spring in Polistes dominula from temperate European (Austrian) and warm Mediterranean (Italian) climate and Polistes gallicus from Mediterranean climate. Winter energy consumption amounted to ~ 339 and ~ 310 J per wasp in the Austrian and Italian Polistes dominula populations. The smaller Italian Polistes gallicus consumed ~ 247 J. This amounts to 2.62, 2.35, and 1.79 J per day. Of this, the energy demand was mainly fuelled by lipids (84%, 93%, and 90%, respectively), but glycogen stores contributed also considerably (16%, 6%, and 9%). Free carbohydrates decreased only by 0.7%, 1%, and 0.8%. While fat stores seem still sufficient in spring, the wasps depleted most of their carbohydrates. The energy reserves of 396, 400, and 147 J per wasp remaining in spring in the three populations seem sufficient to fuel rest or simple brood care activities for a whole summer but restrict foraging flights to a few hours (~ 3.5-6 h). Results suggest that energy supply might become challenging in expected future climate scenarios.

Pyrroline-5-carboxylate metabolism protein complex detected in Arabidopsis thaliana leaf mitochondria

Proline dehydrogenase (ProDH) and pyrroline-5-carboxylate (P5C) dehydrogenase (P5CDH) catalyse the oxidation of proline into glutamate via the intermediates P5C and glutamate-semialdehyde (GSA), which spontaneously interconvert. P5C and GSA are also intermediates in the production of glutamate from ornithine and α-ketoglutarate catalysed by ornithine δ-aminotransferase (OAT). ProDH and P5CDH form a fused bifunctional PutA enzyme in Gram-negative bacteria and are associated in a bifunctional substrate-channelling complex in Thermus thermophilus; however, the physical proximity of ProDH and P5CDH in eukaryotes has not been described. Here, we report evidence of physical proximity and interactions between Arabidopsis ProDH, P5CDH, and OAT in the mitochondria of plants during dark-induced leaf senescence when all three enzymes are expressed. Pairwise interactions and localization of the three enzymes were investigated using bimolecular fluorescence complementation with confocal microscopy in tobacco and sub-mitochondrial fractionation in Arabidopsis. Evidence for a complex composed of ProDH, P5CDH, and OAT was revealed by co-migration of the proteins in native conditions upon gel electrophoresis. Co-immunoprecipitation coupled with mass spectrometry analysis confirmed the presence of the P5C metabolism complex in Arabidopsis. Pull-down assays further demonstrated a direct interaction between ProDH1 and P5CDH. P5C metabolism complexes might channel P5C among the constituent enzymes and directly provide electrons to the respiratory electron chain via ProDH.

Inhibition of Trehalose Synthesis in Lepidoptera Reduces Larval Fitness

Trehalose is synthesized in insects through the trehalose 6-phosphate synthase and phosphatase (TPS/TPP) pathway. TPP dephosphorylates trehalose 6-phosphate to release trehalose. Trehalose is involved in metamorphosis, but its relation with body weight, size, and developmental timing is unexplored. The expression and activity of TPS/TPP fluctuate depending on trehalose demand. Thus, TPS/TPP inhibition can highlight the significance of trehalose in insect physiology. TPS/TPP transcript levels are elevated in the pre-pupal and pupal stages in Helicoverpa armigera. The inhibition of recombinantly expressed TPP by N-(phenylthio)phthalimide (NPP), is validated by in vitro assays. In vivo inhibition of trehalose synthesis reduces larval weight and size, hampers metamorphosis, and reduces its overall fitness. Insufficient trehalose leads to a shift in glucose flux, reduced energy, and dysregulated fatty acid oxidation. Metabolomics reaffirms the depletion of trehalose, glucose, glucose 6-phosphate, and suppressed tricarboxylic acid cycle. Reduced trehalose hampers the energy level affecting larval vitality. Through trehalose synthesis inhibition, the importance of trehalose in insect physiology and development is investigated. Also, in two other lepidopterans, TPP inhibition impedes physiology and survival. NPP is also found to be effective as an insecticidal formulation. Overall, trehalose levels affect the larval size, weight, and metabolic homeostasis for larval-pupal transition in lepidoptera.

Effect of amino acid enriched diets on hemolymph amino acid composition in honey bees

Amino acids (AAs) are an abundant class of nectar solutes, and they are involved in the nectar attractiveness to flower visitors. Among the various AAs, proline is the most abundant proteogenic AA, and γ-amino butyric acid (GABA) and β-alanine are the two most abundant non-proteogenic AAs. These three AAs are known to affect insect physiology, being involved in flight metabolism and neurotransmission. The aim of this study was to investigate the effects of artificial diets enriched with either β-alanine, GABA, or proline on consumption, survival, and hemolymph composition in honey bees belonging to two different ages and with different metabolism (i.e., newly emerged and foragers). Differences in feed intake among diets were not observed, while a diet enriched with β-alanine improved the survival rate of newly emerged honey bees compared to the control group. Variations in the hemolymph AA concentrations occurred only in newly emerged honey bees, according to the diet and the time of hemolymph sampling. A greater susceptibility of young honey bees to enriched diets than older honey bees was observed. The variations in the concentrations of hemolymph AAs reflect either the accumulation of dietary AAs or the existence of metabolic pathways that may lead to the conversion of dietary AAs into different ones. This investigation could be an initial contribution to studying the complex dynamics that regulate hemolymph AA composition and its effect on honey bee physiology.

Metabolomic analysis of Drosophila melanogaster larvae lacking pyruvate kinase

Pyruvate kinase (Pyk) is a rate-limiting enzyme that catalyzes the final metabolic reaction in glycolysis. The importance of this enzyme, however, extends far beyond ATP production, as Pyk is also known to regulate tissue growth, cell proliferation, and development. Studies of this enzyme in Drosophila melanogaster are complicated by the fact that the fly genome encodes 6 Pyk paralogs whose functions remain poorly defined. To address this issue, we used sequence distance and phylogenetic approaches to demonstrate that the gene Pyk encodes the enzyme most similar to the mammalian Pyk orthologs, while the other 5 Drosophila Pyk paralogs have significantly diverged from the canonical enzyme. Consistent with this observation, metabolomic studies of 2 different Pyk mutant strains revealed that larvae lacking Pyk exhibit a severe block in glycolysis, with a buildup of glycolytic intermediates upstream of pyruvate. However, our analysis also unexpectedly reveals that pyruvate levels are unchanged in Pyk mutants, indicating that larval metabolism maintains pyruvate pool size despite severe metabolic limitations. Consistent with our metabolomic findings, a complementary RNA-seq analysis revealed that genes involved in lipid metabolism and protease activity are elevated in Pyk mutants, again indicating that loss of this glycolytic enzyme induces compensatory changes in other aspects of metabolism. Overall, our study provides both insight into how Drosophila larval metabolism adapts to disruption of glycolytic metabolism as well as immediate clinical relevance, considering that Pyk deficiency is the most common congenital enzymatic defect in humans.

Origin and Function of Amino Acids in Nectar and Nectaries of Pitcairnia Species with Particular Emphasis on Alanine and Glutamine

Floral nectar contains sugars and numerous other compounds, including amino acids, but little is known about their function and origin in nectar. Therefore, the amino acid, sugar, and inorganic ion concentrations, as well as the activity of alanine aminotransferase (AlaAT) and glutamine synthetase (GS) in nectar, nectaries, and leaves were analyzed in 30 Pitcairnia species. These data were compared with various floral traits, the pollinator type, and the phylogenetic relationships of the species to find possible causes for the high amino acid concentrations in the nectar of some species. The highest concentrations of amino acids (especially alanine) in nectar were found in species with reddish flowers. Furthermore, the concentration of amino acids in nectar and nectaries is determined through analyzing flower color/pollination type rather than phylogenetic relations. This study provides new insights into the origin of amino acids in nectar. The presence of almost all amino acids in nectar is mainly due to their transport in the phloem to the nectaries, with the exception of alanine, which is partially produced in nectaries. In addition, active regulatory mechanisms are required in nectaries that retain most of the amino acids and allow the selective secretion of specific amino acids, such as alanine.

How to eliminate pathogen without killing oneself? Immunometabolism of encapsulation and melanization in Drosophila

Cellular encapsulation associated with melanization is a crucial component of the immune response in insects, particularly against larger pathogens. The infection of a Drosophila larva by parasitoid wasps, like Leptopilina boulardi, is the most extensively studied example. In this case, the encapsulation and melanization of the parasitoid embryo is linked to the activation of plasmatocytes that attach to the surface of the parasitoid. Additionally, the differentiation of lamellocytes that encapsulate the parasitoid, along with crystal cells, is accountable for the melanization process. Encapsulation and melanization lead to the production of toxic molecules that are concentrated in the capsule around the parasitoid and, at the same time, protect the host from this toxic immune response. Thus, cellular encapsulation and melanization represent primarily a metabolic process involving the metabolism of immune cell activation and differentiation, the production of toxic radicals, but also the production of melanin and antioxidants. As such, it has significant implications for host physiology and systemic metabolism. Proper regulation of metabolism within immune cells, as well as at the level of the entire organism, is therefore essential for an efficient immune response and also impacts the health and overall fitness of the organism that survives. The purpose of this "perspective" article is to map what we know about the metabolism of this type of immune response, place it in the context of possible implications for host physiology, and highlight open questions related to the metabolism of this important insect immune response.

Metabolomic analysis of Drosophila melanogaster larvae lacking Pyruvate kinase

Pyruvate kinase (Pyk) is a rate-limiting enzyme that catalyzes the final metabolic reaction in glycolysis. The importance of this enzyme, however, extends far beyond ATP production, as Pyk is also known to regulate tissue growth, cell proliferation, and development. Studies of this enzyme in Drosophila melanogaster , however, are complicated by the fact that the fly genome encodes six Pyk paralogs whose functions remain poorly defined. To address this issue, we used sequence distance and phylogenetic approaches to demonstrate that the gene Pyk encodes the enzyme most similar to the mammalian Pyk orthologs, while the other five Drosophila Pyk paralogs have significantly diverged from the canonical enzyme. Consistent with this observation, metabolomic studies of two different Pyk mutant backgrounds revealed that larvae lacking Pyk exhibit a severe block in glycolysis, with a buildup of glycolytic intermediates upstream of pyruvate. However, our analysis also unexpectedly reveals that steady state pyruvate levels are unchanged in Pyk mutants, indicating that larval metabolism maintains pyruvate pool size despite severe metabolic limitations. Consistent with our metabolomic findings, a complementary RNA-seq analysis revealed that genes involved in lipid metabolism and peptidase activity are elevated in Pyk mutants, again indicating that loss of this glycolytic enzyme induces compensatory changes in other aspects of metabolism. Overall, our study provides both insight into how Drosophila larval metabolism adapts to disruption of glycolytic metabolism as well as immediate clinical relevance, considering that Pyk deficiency is the most common congenital enzymatic defect in humans.

Chilled, starved or frozen: Insect mitochondrial adaptations to overcome the cold

Physiological adaptations to tackle cold exposure are crucial for insects living in temperate and arctic environments and here we review how cold adaptation is manifested in terms of mitochondrial function. Cold challenges are diverse, and different insect species have evolved metabolic and mitochondrial adaptations to: i) energize homeostatic regulation at low temperature, ii) stretch energy reserves during prolonged cold exposure, and iii) preserve structural organization of organelles following extracellular freezing. While the literature is still sparse, our review suggests that cold-adapted insects preserve ATP production at low temperatures by maintaining preferred mitochondrial substrate oxidation, which is otherwise challenged in cold-sensitive species. Chronic cold exposure and metabolic depression during dormancy is linked to reduced mitochondrial metabolism and may involve mitochondrial degradation. Finally, adaptation to extracellular freezing could be associated with superior structural integrity of the mitochondrial inner membrane following freezing which is linked to cellular and organismal survival.

Solving the conundrum of intra-specific variation in metabolic rate: A multidisciplinary conceptual and methodological toolkit: New technical developments are opening the door to an understanding of why metabolic rate varies among individual animals of a species: New technical developments are opening the door to an understanding of why metabolic rate varies among individual animals of a species

Researchers from diverse disciplines, including organismal and cellular physiology, sports science, human nutrition, evolution and ecology, have sought to understand the causes and consequences of the surprising variation in metabolic rate found among and within individual animals of the same species. Research in this area has been hampered by differences in approach, terminology and methodology, and the context in which measurements are made. Recent advances provide important opportunities to identify and address the key questions in the field. By bringing together researchers from different areas of biology and biomedicine, we describe and evaluate these developments and the insights they could yield, highlighting the need for more standardisation across disciplines. We conclude with a list of important questions that can now be addressed by developing a common conceptual and methodological toolkit for studies on metabolic variation in animals.

Changes in resource perception throughout the foraging visit contribute to task specialization in the honey bee Apis mellifera

Division of labor is central to the ecological success of social insects. Among foragers of the honey bee, specialization for collecting nectar or pollen correlates with their sensitivity to sucrose. So far, differences in gustatory perception have been mostly studied in bees returning to the hive, but not during foraging. Here, we showed that the phase of the foraging visit (i.e. beginning or end) interacts with foraging specialization (i.e. predisposition to collect pollen or nectar) to modulate sucrose and pollen sensitivity in foragers. In concordance with previous studies, pollen foragers presented higher sucrose responsiveness than nectar foragers at the end of the foraging visit. On the contrary, pollen foragers were less responsive than nectar foragers at the beginning of the visit. Consistently, free-flying foragers accepted less concentrated sucrose solution during pollen gathering than immediately after entering the hive. Pollen perception also changes throughout foraging, as pollen foragers captured at the beginning of the visit learned and retained memories better when they were conditioned with pollen + sucrose as reward than when we used sucrose alone. Altogether, our results support the idea that changes in foragers' perception throughout the foraging visit contributes to task specialization.

In Which Way Do the Flower Properties of the Specialist Orchid Goodyera repens Meet the Requirements of Its Generalist Pollinators?

This article is the next part of a series of studies documenting the influence of flower traits on the reproductive success (RS) of orchids. Knowledge of factors influencing RS helps to understand the mechanisms and processes crucial for shaping plant-pollinator interactions. The aim of the present study was to determine the role of flower structure and nectar composition in shaping the RS of the specialist orchid Goodyea repens, which is pollinated by generalist bumblebees. We found a high level of pollinaria removal (PR) and female reproductive success (fruiting, FRS) as well as a high level of variation between populations, although in certain populations pollination efficiency was low. Floral display traits, mainly inflorescence length, influenced FRS in certain populations. Among the flower traits, only the height of flowers was correlated with FRS in one population, suggesting that the flower structure of this orchid is well adapted to pollination by bumblebees. The nectar of G. repens is diluted and dominated by hexoses. Sugars were less important in shaping RS than amino acids. At the species level, twenty proteogenic and six non-proteogenic AAs were noted, along with their differentiated amounts and participation in particular populations. We found that distinct AAs or their groups mainly shaped PR, especially when correlations were considered at the species level. Our results suggest that both the individual nectar components and the ratios between them have an impact on G. repens RS. Because different nectar components influence the RS parameters in different ways (i.e., negatively or positively), we suggest that different Bombus species play the role of main pollinators in distinct populations.

Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar of Melianthus spp

The black nectar produced by Melianthus flowers is thought to serve as a visual attractant to bird pollinators, but the chemical identity and synthesis of the black pigment are unknown. A combination of analytical biochemistry, transcriptomics, proteomics, and enzyme assays was used to identify the pigment that gives Melianthus nectar its black color and how it is synthesized. Visual modeling of pollinators was also used to infer a potential function of the black coloration. High concentrations of ellagic acid and iron give the nectar its dark black color, which can be recapitulated through synthetic solutions containing only ellagic acid and iron(iii). The nectar also contains a peroxidase that oxidizes gallic acid to form ellagic acid. In vitro reactions containing the nectar peroxidase, gallic acid, hydrogen peroxide, and iron(iii) fully recreate the black color of the nectar. Visual modeling indicates that the black color is highly conspicuous to avian pollinators within the context of the flower. Melianthus nectar contains a natural analog of iron-gall ink, which humans have used since at least medieval times. This pigment is derived from an ellagic acid-Fe complex synthesized in the nectar and is likely involved in the attraction of passerine pollinators endemic to southern Africa.

To each their own! Nectar plasticity within a flower mediates distinct ecological interactions

Nuptial and extranuptial nectaries are involved in interactions with different animal functional groups. Nectar traits involved in pollination mutualisms are well known. However, we know little about those traits involved in other mutualisms, such as ant-plant interactions, especially when both types of nectaries are in the same plant organ, the flower. Here we investigated if when two types of nectaries are exploited by distinct functional groups of floral visitors, even being within the same plant organ, the nectar secreted presents distinct features that fit animal requirements. We compared nectar secretion dynamics, floral visitors and nectar chemical composition of both nuptial and extranuptial nectaries in natural populations of the liana Amphilophium mansoanum (Bignoniaceae). For that we characterized nectar sugar, amino acid and specialized metabolite composition by high-performance liquid chromatography. Nuptial nectaries were visited by three medium- and large-sized bee species and extranuptial nectaries were visited mainly by ants, but also by cockroaches, wasps and flies. Nuptial and extranuptial nectar differed regarding volume, concentration, milligrams of sugars per flower and secretion dynamics. Nuptial nectar was sucrose-dominated, with high amounts of γ-aminobutyric acid and β-aminobutyric acid and with theophylline-like alkaloid, which were all exclusive of nuptial nectar. Whereas extranuptial nectar was hexose-rich, had a richer and less variable amino acid chemical profile, with high amounts of serine and alanine amino acids and with higher amounts of the specialized metabolite tyramine. The nectar traits from nuptial and extranuptial nectaries differ in energy amount and nutritional value, as well as in neuroactive specialized metabolites. These differences seem to match floral visitors' requirements, since they exclusively consume one of the two nectar types and may be exerting selective pressures on the composition of the respective resources of interest.

Hemolymph Metabolism Analysis of Honey Bee (Apis mellifera L.) Response to Different Bee Pollens

Pollen is essential to the development of honey bees. The nutrients in bee pollen vary greatly among plant species. Here, we analyzed the differences in the amino acid compositions of pear (Pyrus bretschneideri), rape (Brassica napus), and apricot (Armeniaca sibirica) pollens and investigated the variation in hemolymph metabolites and metabolic pathways through untargeted metabolomics in caged adult bees at days 7 and 14. The results showed that the levels of five essential amino acids (isoleucine, phenylalanine, lysine, methionine, and histidine) were the highest in pear pollen, and the levels of four amino acids (isoleucine: 50.75 ± 1.93 mg/kg, phenylalanine: 87.25 ± 2.66 mg/kg, methionine: 16.00 ± 0.71 mg/kg and histidine: 647.50 ± 24.80 mg/kg) were significantly higher in pear pollen than in the other two kinds of bee pollen (p < 0.05). The number of metabolites in bee hemolymph on day 14 (615) was significantly lower than that on day 7 (1466). The key metabolic pathways of bees, namely, “sphingolipid metabolism (p = 0.0091)”, “tryptophan metabolism (p = 0.0245)”, and “cysteine and methionine metabolism (p = 0.0277)”, were significantly affected on day 7. There was no meaningful pathway enrichment on day 14. In conclusion, pear pollen had higher nutritional value among the three bee pollens in terms of amino acid level, followed by rape and apricot pollen, and the difference in amino acid composition among bee pollens was reflected in the lipid and amino acid metabolism pathways of early adult honey bee hemolymph. This study provides new insights into the physiological and metabolic functions of different bee pollens in bees.

Microbiome-Driven Proline Biogenesis in Plants under Stress: Perspectives for Balanced Diet to Minimize Depression Disorders in Humans

According to the World Health Organization (WHO), depression is a leading cause of disability worldwide and a major contributor to the overall global burden of mental disorders. An increasing number of studies have revealed that among 20 different amino acids, high proline consumption is a dietary factor with the strongest impact on depression in humans and animals, including insects. Recent studies acknowledged that gut microbiota play a key role in proline-related pathophysiology of depression. In addition, the multi-omics approach has alleged that a high level of metabolite proline is directly linked to depression severity, while variations in levels of circulating proline are dependent on microbiome composition. The gut–brain axis proline analysis is a gut microbiome model of studying depression, highlighting the critical importance of diet, but nothing is known about the role of the plant microbiome–food axis in determining proline concentration in the diet and thus about preventing excessive proline intake through food consumption. In this paper, we discuss the protocooperative potential of a holistic study approach combining the microbiota–gut–brain axis with the microbiota–plant–food–diet axis, as both are involved in proline biogenesis and metabolism and thus on in its effect on mood and cognitive function. In preharvest agriculture, the main scientific focus must be directed towards plant symbiotic endophytes, as scavengers of abiotic stresses in plants and modulators of high proline concentration in crops/legumes/vegetables under climate change. It is also implied that postharvest agriculture—including industrial food processing—may be critical in designing a proline-balanced diet, especially if corroborated with microbiome-based preharvest agriculture, within a circular agrifood system. The microbiome is suggested as a target for selecting beneficial plant endophytes in aiming for a balanced dietary proline content, as it is involved in the physiology and energy metabolism of eukaryotic plant/human/animal/insect hosts, i.e., in core aspects of this amino acid network, while opening new venues for an efficient treatment of depression that can be adapted to vast groups of consumers and patients. In that regard, the use of artificial intelligence (AI) and molecular biomarkers combined with rapid and non-destructive imaging technologies were also discussed in the scope of enhancing integrative science outcomes, agricultural efficiencies, and diagnostic medical precisions.

The insect perspective on Z-disc structure and biology

Myofibrils are the intracellular structures formed by actin and myosin filaments. They are paracrystalline contractile cables with unusually well-defined dimensions. The sliding of actin past myosin filaments powers contractions, and the entire system is held in place by a structure called the Z-disc, which anchors the actin filaments. Myosin filaments, in turn, are anchored to another structure called the M-line. Most of the complex architecture of myofibrils can be reduced to studying the Z-disc, and recently, important advances regarding the arrangement and function of Z-discs in insects have been published. On a very small scale, we have detailed protein structure information. At the medium scale, we have cryo-electron microscopy maps, super-resolution microscopy and protein-protein interaction networks, while at the functional scale, phenotypic data are available from precise genetic manipulations. All these data aim to answer how the Z-disc works and how it is assembled. Here, we summarize recent data from insects and explore how it fits into our view of the Z-disc, myofibrils and, ultimately, muscles.

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.

Genome-wide transcriptomic changes reveal the genetic pathways involved in insect migration

Insects are capable of extraordinary feats of long‐distance movement that have profound impacts on the function of terrestrial ecosystems. The ability to undertake these movements arose multiple times through the evolution of a suite of traits that make up the migratory syndrome, however the underlying genetic pathways involved remain poorly understood. Migratory hoverflies (Diptera: Syrphidae) are an emerging model group for studies of migration. They undertake seasonal movements in huge numbers across large parts of the globe and are important pollinators, biological control agents and decomposers. Here, we assembled a high‐quality draft genome of the marmalade hoverfly (Episyrphus balteatus). We leveraged this genomic resource to undertake a genome‐wide transcriptomic comparison of actively migrating Episyrphus, captured from a high mountain pass as they flew south to overwinter, with the transcriptomes of summer forms which were non‐migratory. We identified 1543 genes with very strong evidence for differential expression. Interrogation of this gene set reveals a remarkable range of roles in metabolism, muscle structure and function, hormonal regulation, immunity, stress resistance, flight and feeding behaviour, longevity, reproductive diapause and sensory perception. These features of the migrant phenotype have arisen by the integration and modification of pathways such as insulin signalling for diapause and longevity, JAK/SAT for immunity, and those leading to octopamine production and fuelling to boost flight capabilities. Our results provide a powerful genomic resource for future research, and paint a comprehensive picture of global expression changes in an actively migrating insect, identifying key genomic components involved in this important life‐history strategy.

Sweet solutions: nectar chemistry and quality

Nectar, the main floral reward for pollinators, varies greatly in composition and concentration. The assumption that nectar quality is equivalent to its sugar (energy) concentration is too simple. Diverse non-sugar components, especially amino acids and secondary metabolites, play various roles in nutrition and health of pollinators. Many nectar compounds have indirect effects by altering the foraging behaviour of pollinators or protecting them from disease. This review also emphasizes the water component of nectar, often ignored because of evaporative losses and difficulties in sampling small nectar volumes. Nectar properties vary with environmental factors, pollinator visits and microbial contamination. Pollination mutualisms depend on the ability of insect and vertebrate pollinators to cope with and benefit from the variation and diversity in nectar chemistry. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Malaria mosquitoes acquire and allocate cattle urine to enhance life history traits

Background

Nutrient acquisition and allocation integrate foraging and life-history traits in insects. To compensate for the lack of a particular nutrient at different life stages, insects may acquire these through supplementary feeding, for example, on vertebrate secretions, in a process known as puddling. The mosquito Anopheles arabiensis emerges undernourished, and as such, requires nutrients for both metabolism and reproduction. The purpose of this study was to assess whether An. arabiensis engage in puddling on cattle urine to obtain nutrients to improve life history traits.

Methods

To determine whether An. arabiensis are attracted to the odour of fresh, 24 h, 72 h and 168 h aged cattle urine, host-seeking and blood-fed (48 h post-blood meal) females were assayed in a Y-tube olfactometer, and gravid females assessed in an oviposition assay. Combined chemical and electrophysiological analyses were subsequently used to identify the bioactive compounds in all four age classes of cattle urine. Synthetic blends of bioactive compounds were evaluated in both Y-tube and field assays. To investigate the cattle urine, and its main nitrogenous compound, urea, as a potential supplementary diet for malaria vectors, feeding parameters and life history traits were measured. The proportion of female mosquitoes and the amount of cattle urine and urea imbibed, were assessed. Following feeding, females were evaluated for survival, tethered flight and reproduction.

Results

Host-seeking and blood-fed An. arabiensis were attracted to the natural and synthetic odour of fresh and aged cattle urine in both laboratory and field studies. Gravid females were indifferent in their response to cattle urine presence at oviposition sites. Host-seeking and blood-fed females actively imbibed cattle urine and urea, and allocated these resources according to life history trade-offs to flight, survival or reproduction, as a function of physiological state.

Conclusions

Anopheles arabiensis acquire and allocate cattle urine to improve life history traits. Supplementary feeding on cattle urine affects vectorial capacity directly by increasing daily survival and vector density, as well as indirectly by altering flight activity, and thus should be considered in future models.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04179-6.

Flexible Thermal Sensitivity of Mitochondrial Oxygen Consumption and Substrate Oxidation in Flying Insect Species

Mitochondria have been suggested to be paramount for temperature adaptation in insects. Considering the large range of environments colonized by this taxon, we hypothesized that species surviving large temperature changes would be those with the most flexible mitochondria. We thus investigated the responses of mitochondrial oxidative phosphorylation (OXPHOS) to temperature in three flying insects: the honeybee (Apis mellifera carnica), the fruit fly (Drosophila melanogaster) and the Colorado potato beetle (Leptinotarsa decemlineata). Specifically, we measured oxygen consumption in permeabilized flight muscles of these species at 6, 12, 18, 24, 30, 36, 42 and 45°C, sequentially using complex I substrates, proline, succinate, and glycerol-3-phosphate (G3P). Complex I respiration rates (CI-OXPHOS) were very sensitive to temperature in honeybees and fruit flies with high oxygen consumption at mid-range temperatures but a sharp decline at high temperatures. Proline oxidation triggers a major increase in respiration only in potato beetles, following the same pattern as CI-OXPHOS for honeybees and fruit flies. Moreover, both succinate and G3P oxidation allowed an important increase in respiration at high temperatures in honeybees and fruit flies (and to a lesser extent in potato beetles). However, when reaching 45°C, this G3P-induced respiration rate dropped dramatically in fruit flies. These results demonstrate that mitochondrial functions are more resilient to high temperatures in honeybees compared to fruit flies. They also indicate an important but species-specific mitochondrial flexibility for substrate oxidation to sustain high oxygen consumption levels at high temperatures and suggest previously unknown adaptive mechanisms of flying insects’ mitochondria to temperature.

The effects of dietary proline, β-alanine, and γ-aminobutyric acid (GABA) on the nest construction behavior in the Oriental hornet (Vespa orientalis)

Adult wasps primary food resource is larval saliva. This liquid secretion consists mainly of amino acids and carbohydrates processed from the prey brought to the colony by the foragers. However, adults also regularly consume floral nectar. The nectar's most abundant proteinogenic amino acid is proline, and the two most abundant non-proteinogenic amino acids are β-alanine and GABA. These three amino acids are also common in larval saliva. Here, we study the effect of these dietary amino acids on the physiology and nest construction behavior of the Oriental hornet. Our results reveal their deleterious effects, especially at high concentrations: β-alanine and GABA consumption reduced the hornets' lifespan and completely inhibited their construction behavior; while proline induced a similar but more moderate effect. At low concentrations, these amino acids had no effect on hornet survival but did slow down the nest construction process. Using carbon isotopically labeled amino acids, we show that, unlike proline, β-alanine is stored in most body tissues (brain, muscles, and fat body), suggesting that it is rapidly metabolized after consumption. Our findings demonstrate how a single amino acid can impact the fitness of a nectarivore insect.

Exploring Thermal Sensitivities and Adaptations of Oxidative Phosphorylation Pathways

Temperature shifts are a major challenge to animals; they drive adaptations in organisms and species, and affect all physiological functions in ectothermic organisms. Understanding the origin and mechanisms of these adaptations is critical for determining whether ectothermic organisms will be able to survive when faced with global climate change. Mitochondrial oxidative phosphorylation is thought to be an important metabolic player in this regard, since the capacity of the mitochondria to produce energy greatly varies according to temperature. However, organism survival and fitness depend not only on how much energy is produced, but, more precisely, on how oxidative phosphorylation is affected and which step of the process dictates thermal sensitivity. These questions need to be addressed from a new perspective involving a complex view of mitochondrial oxidative phosphorylation and its related pathways. In this review, we examine the effect of temperature on the commonly measured pathways, but mainly focus on the potential impact of lesser-studied pathways and related steps, including the electron-transferring flavoprotein pathway, glycerophosphate dehydrogenase, dihydroorotate dehydrogenase, choline dehydrogenase, proline dehydrogenase, and sulfide:quinone oxidoreductase. Our objective is to reveal new avenues of research that can address the impact of temperature on oxidative phosphorylation in all its complexity to better portray the limitations and the potential adaptations of aerobic metabolism.

Metabolic Changes in Larvae of Predator Chrysopa sinica Fed on Azadirachtin-Treated Plutella xylostella Larvae

Biological control is a key component of integrated pest management (IPM). To suppress pests in a certain threshold, chemical control is used in combination with biological and other control methods. An essential premise for using pesticides in IPM is to ascertain their compatibility with beneficial insects. Chrysopa sinica (Neuroptera: Chrysopidae) is an important predator of various pests and used for pest management. This study was intended to analyze metabolic changes in C. sinica larvae after feeding on azadirachtin-treated Plutella xylostella (Lepidoptera, Plutellidae) larvae through a non-targeted LC–MS (Liquid chromatography–mass spectrometry) based metabolomics analysis. Results showed that C. sinica larvae did not die after consuming P. xylostella larvae treated with azadirachtin. However, their pupation and eclosion were adversely affected, resulting in an impairment in the completion of their life cycle. Feeding C. sinica larvae with azadirachtin-treated P. xylostella larvae affected over 10,000 metabolites across more than 20 pathways, including the metabolism of amino acids, carbohydrates, lipid, cofactors, and vitamins in C. sinica larvae, of which changes in amnio acid metabolism were particularly pronounced. A working model was proposed to illustrate differential changes in 20 metabolites related to some amino acid metabolisms. Among them, 15 were markedly reduced and only five were elevated. Our results suggest that azadirachtin application may not be exclusively compatible with the use of the predator C. sinica for control of P. xylostella. It is recommended that the compatibility should be evaluated not only based on the survival of the predatory insects but also by the metabolic changes and the resultant detrimental effects on their development.

Nutrient Utilization during Male Maturation and Protein Digestion in the Oriental Hornet

Males of social Hymenoptera spend the first days following eclosion inside the nest before dispersing to find a young queen to mate with. During this period, they must acquire enough nutrients to enable their sexual maturation and store energy to sustain them through their nuptial journey. It was previously argued that adult hornets are unable to process dietary proteins and rely on the larvae to supply them with free amino acids and carbohydrates that they secrete via trophallaxis. Using isotopically enriched diets, we examined nutrient allocation and protein turnover in newly-emerged males of the Oriental hornet during their maturation period and tested the protein digestion capability in the presence and absence of larvae in both males and worker hornets. The results indicated that protein turnover in males occurs during the first days following eclosion, while carbohydrates are incorporated into body tissues at higher rates towards the end of the maturation period. Additionally, we found that males cannot digest protein and depend on larval secretions as a source of nutrition, while workers, in contrast to previous reports, can metabolize protein independently. Our findings demonstrate the contribution of adult male nutrition and larval secretions to colony fitness.

Made for Each Other: Vector-Pathogen Interfaces in the Huanglongbing Pathosystem

Citrus greening, or huanglongbing (HLB), currently is the most destructive disease of citrus. HLB disease is putatively caused by the phloem-restricted α-proteobacterium 'Candidatus Liberibacter asiaticus'. This bacterium is transmitted primarily by the Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae). Most animal pathogens are considered pathogenic to their insect vectors, whereas the relationships between plant pathogens and their insect vectors are variable. Lately, the relationship of 'Ca. L. asiaticus' with its insect vector, D. citri, has been well investigated at the molecular, biochemical, and biological levels in many studies. Herein, the findings concerning this relationship are discussed and molecular features of the acquisition of 'Ca. L. asiaticus' from the plant host and its growth and circulation within D. citri, as well as its transmission to plants, are presented. In addition, the effects of 'Ca. L. asiaticus' on the energy metabolism (respiration, tricarboxylic acid cycle, and adenosine triphosphate production), metabolic pathways, immune system, endosymbionts, and detoxification enzymes of D. citri are discussed together with other impacts such as shorter lifespan, altered feeding behavior, and higher fecundity. Overall, although 'Ca. L. asiaticus' has significant negative effects on its insect vector, it increases its vector fitness, indicating that it develops a mutualistic relationship with its vector. This review will help in understanding the specific interactions between 'Ca. L. asiaticus' and its psyllid vector in order to design innovative management strategies.

Allocation and metabolism of naturally occurring dietary amino acids in the Oriental hornet

Oriental hornet (Vespa orientalis) foragers are strong, long-distance flyers exhibiting a high metabolic rate. Accordingly, they feed on carbohydrate-rich diets, such as floral nectar and larval secretions. These nutritional sources, in addition to carbohydrates, also contain free amino acids (AAs). Leucine, glycine, and proline are three common AAs in the diet of social wasps. Using diets enriched with carbon-specific (¹³C₁) isotopically labeled leucine, glycine, and proline, and a cavity ring-down spectroscope (CRDS) stable carbon isotope analyzer, we examined the metabolism of these AAs, their allocation in the hornets' respiration during rest and activity, and their incorporation into the body tissues. In hornets that consumed ¹³C proline, we detected the heavy isotope only in the exhaled CO₂, suggesting that proline was utilized solely as a metabolic fuel and was not incorporated into their body (i.e., as protein). Labeled carbons from glycine and leucine, in contrast, were found in all the examined tissues (i.e., muscles, brain, fat bodies, ovaries, and venom glands), and were also utilized as a metabolic fuel, but mostly during rest. Using AAs labeled with a specific stable carbon isotope, we demonstrate the compatibility between the hornet's metabolic requirements and AA use, in both the living organism as a whole and in its different body tissues.

Acute Imidacloprid Exposure Alters Mitochondrial Function in Bumblebee Flight Muscle and Brain

Mitochondria are intracellular organelles responsible for cellular respiration with one of their major roles in the production of energy in the form of ATP. Activities with increased energetic demand are especially dependent on efficient ATP production, hence sufficient mitochondrial function is fundamental. In bees, flight muscle and the brain have particularly high densities of mitochondria to facilitate the substantial ATP production required for flight activity and neuronal signalling. Neonicotinoids are systemic synthetic insecticides that are widely utilised against crop herbivores but have been reported to cause, by unknown mechanisms, mitochondrial dysfunction, decreasing cognitive function and flight activity among pollinating bees. Here we explore, using high-resolution respirometry, how the neonicotinoid imidacloprid may affect oxidative phosphorylation in the brain and flight muscle of the buff-tailed bumblebee, Bombus terrestris. We find that acute exposure increases routine oxygen consumption in the flight muscle of worker bees. This provides a candidate explanation for prior reports of early declines in flight activity following acute exposure. We further find that imidacloprid increases the maximum electron transport capacity in the brain, with a trend towards increased overall oxygen consumption. However, intra-individual variability is high, limiting the extent to which apparent effects of imidacloprid on brain mitochondria are shown conclusively. Overall, our results highlight the necessity to examine tissue-specific effects of imidacloprid on respiration and energy production.

The Multifaceted Roles of Proline in Cell Behavior

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: (i) A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); (ii) an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; (iii) an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); (iv) a neural metabotoxin associated with schizophrenia; (v) a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; (vi) an epigenetic modifier able to promote DNA and histone hypermethylation; (vii) an inducer of proliferation of stem and tumor cells; and (viii) a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.

Proline as a Sparker Metabolite of Oxidative Metabolism during the Flight of the Bumblebee, Bombus impatiens

Several insect species use the amino acid proline as a major energy substrate. Although initially thought to be limited to blood-feeding dipterans, studies have revealed this capability is more widespread. Recent work with isolated flight muscle showed that the bumblebee Bombus impatiens can oxidize proline at a high rate. However, its role as a metabolic fuel to power flight is unclear. To elucidate the extent to which proline is oxidized to power flight and how its contribution changes during flight, we profiled 14 metabolites central to energy and proline metabolism at key time points in flight muscle and abdominal tissues. Ultra-high performance liquid chromatography-electrospray ionization-quadrupole time of flight mass spectrometry (UPLC-ESI-QTOF MS) analysis revealed that proline is likely used as a sparker metabolite of the tricarboxylic acid cycle at the onset of flight, whereby it supplements the intermediates of the cycle. Carbohydrates are the major energy substrates, which is evidenced by marked decreases in abdominal glycogen stores and a lack of alanine accumulation to replenish flight muscle proline. The time course of fuel stores and metabolites changes during flight highlights homeostatic regulation of energy substrates and patterns of changes in metabolic intermediates within pathways. This study clarifies the role of proline and carbohydrate metabolism during flight in hymenopterans, such as B. impatiens.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

Nectar biosynthesis is conserved among floral and extrafloral nectaries

Nectar is a primary reward mediating plant-animal mutualisms to improve plant fitness and reproductive success. Four distinct trichomatic nectaries develop in cotton (Gossypium hirsutum), one floral and three extrafloral, and the nectars they secrete serve different purposes. Floral nectar attracts bees for promoting pollination, while extrafloral nectar attracts predatory insects as a means of indirect protection from herbivores. Cotton therefore provides an ideal system for contrasting mechanisms of nectar production and nectar composition between different nectary types. Here, we report the transcriptome and ultrastructure of the four cotton nectary types throughout development and compare these with the metabolomes of secreted nectars. Integration of these datasets supports specialization among nectary types to fulfill their ecological niche, while conserving parallel coordination of the merocrine-based and eccrine-based models of nectar biosynthesis. Nectary ultrastructures indicate an abundance of rough endoplasmic reticulum positioned parallel to the cell walls and a profusion of vesicles fusing to the plasma membranes, supporting the merocrine model of nectar biosynthesis. The eccrine-based model of nectar biosynthesis is supported by global transcriptomics data, which indicate a progression from starch biosynthesis to starch degradation and sucrose biosynthesis and secretion. Moreover, our nectary global transcriptomics data provide evidence for novel metabolic processes supporting de novo biosynthesis of amino acids secreted in trace quantities in nectars. Collectively, these data demonstrate the conservation of nectar-producing models among trichomatic and extrafloral nectaries.

Dramatic changes in mitochondrial substrate use at critically high temperatures: a comparative study using Drosophila

Ectotherm thermal tolerance is critical to species distribution, but at present the physiological underpinnings of heat tolerance remain poorly understood. Mitochondrial function is perturbed at critically high temperatures in some ectotherms, including insects, suggesting that heat tolerance of these animals is linked to failure of oxidative phosphorylation (OXPHOS) and/or ATP production. To test this hypothesis, we measured mitochondrial oxygen consumption rate in six Drosophila species with different heat tolerance using high-resolution respirometry. Using a substrate-uncoupler-inhibitor titration protocol, we examined specific steps of the electron transport system to study how temperatures below, bracketing and above organismal heat limits affect mitochondrial function and substrate oxidation. At benign temperatures (19 and 30°C), complex I-supported respiration (CI-OXPHOS) was the most significant contributor to maximal OXPHOS. At higher temperatures (34, 38, 42 and 46°C), CI-OXPHOS decreased considerably, ultimately to very low levels at 42 and 46°C. The enzymatic catalytic capacity of complex I was intact across all temperatures and accordingly the decreased CI-OXPHOS is unlikely to be caused directly by hyperthermic denaturation/inactivation of complex I. Despite the reduction in CI-OXPHOS, maximal OXPHOS capacity was maintained in all species, through oxidation of alternative substrates - proline, succinate and, particularly, glycerol-3-phosphate - suggesting important mitochondrial flexibility at temperatures exceeding the organismal heat limit. Interestingly, this failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures that correlated with species heat tolerance, such that heat-tolerant species could defend 'normal' mitochondrial function at higher temperatures than sensitive species. Future studies should investigate why CI-OXPHOS is perturbed and how this potentially affects ATP production rates.

The Importance of Time and Place: Nutrient Composition and Utilization of Seasonal Pollens by European Honey Bees (Apis mellifera L.)

Simple Summary

Honey bees rely on pollen and nectar to provide nutrients to support their yearly colony cycle. Specifics of the cycle differ among geographic regions as do the species of flowering plants and the nutrients they provide. We examined responses of honey bees from two different queen lines fed pollens from locations that differed in floral species composition and yearly colony cycles. We detected differences between the queen lines in the amount of pollen they consumed and the size of their hypopharyngeal glands (HPG). There were also seasonal differences between the nutrient composition of pollens. Spring pollens collected from colonies in both locations had higher amino and fatty acid concentrations than fall pollens. There also were seasonal differences in responses to the pollens consumed by bees from both queen lines. Bees consumed more spring than fall pollen, but digested less of it so that bees consumed more protein from fall pollens. Though protein consumption was higher with fall pollen, HPG were larger in spring bees.

Abstract

Honey bee colonies have a yearly cycle that is supported nutritionally by the seasonal progression of flowering plants. In the spring, colonies grow by rearing brood, but in the fall, brood rearing declines in preparation for overwintering. Depending on where colonies are located, the yearly cycle can differ especially in overwintering activities. In temperate climates of Europe and North America, colonies reduce or end brood rearing in the fall while in warmer climates bees can rear brood and forage throughout the year. To test the hypothesis that nutrients available in seasonal pollens and honey bee responses to them can differ we analyzed pollen in the spring and fall collected by colonies in environments where brood rearing either stops in the fall (Iowa) or continues through the winter (Arizona). We fed both types of pollen to worker offspring of queens that emerged and open mated in each type of environment. We measured physiological responses to test if they differed depending on the location and season when the pollen was collected and the queen line of the workers that consumed it. Specifically, we measured pollen and protein consumption, gene expression levels (hex 70, hex 110, and vg) and hypopharyngeal gland (HPG) development. We found differences in macronutrient content and amino and fatty acids between spring and fall pollens from the same location and differences in nutrient content between locations during the same season. We also detected queen type and seasonal effects in HPG size and differences in gene expression between bees consuming spring vs. fall pollen with larger HPG and higher gene expression levels in those consuming spring pollen. The effects might have emerged from the seasonal differences in nutritional content of the pollens and genetic factors associated with the queen lines we used.

Cytochrome c Oxidase at Full Thrust: Regulation and Biological Consequences to Flying Insects

Flight dispersal represents a key aspect of the evolutionary and ecological success of insects, allowing escape from predators, mating, and colonization of new niches. The huge energy demand posed by flight activity is essentially met by oxidative phosphorylation (OXPHOS) in flight muscle mitochondria. In insects, mitochondrial ATP supply and oxidant production are regulated by several factors, including the energy demand exerted by changes in adenylate balance. Indeed, adenylate directly regulates OXPHOS by targeting both chemiosmotic ATP production and the activities of specific mitochondrial enzymes. In several organisms, cytochrome c oxidase (COX) is regulated at transcriptional, post-translational, and allosteric levels, impacting mitochondrial energy metabolism, and redox balance. This review will present the concepts on how COX function contributes to flying insect biology, focusing on the existing examples in the literature where its structure and activity are regulated not only by physiological and environmental factors but also how changes in its activity impacts insect biology. We also performed in silico sequence analyses and determined the structure models of three COX subunits (IV, VIa, and VIc) from different insect species to compare with mammalian orthologs. We observed that the sequences and structure models of COXIV, COXVIa, and COXVIc were quite similar to their mammalian counterparts. Remarkably, specific substitutions to phosphomimetic amino acids at critical phosphorylation sites emerge as hallmarks on insect COX sequences, suggesting a new regulatory mechanism of COX activity. Therefore, by providing a physiological and bioenergetic framework of COX regulation in such metabolically extreme models, we hope to expand the knowledge of this critical enzyme complex and the potential consequences for insect dispersal.

Preference and detrimental effects of high fat, sugar, and salt diet in wild-caught Drosophila simulans are reversed by flight exercise

High saturated fat, sugar, and salt contents are a staple of a Western diet (WD), contributing to obesity, metabolic syndrome, and a plethora of other health risks. However, the combinatorial effects of these ingredients have not been fully evaluated. Here, using the wild-caught Drosophila simulans, we show that a diet enriched with saturated fat, sugar, and salt is more detrimental than each ingredient separately, resulting in a significantly decreased lifespan, locomotor activity, sleep, reproductive function, and mitochondrial function. These detrimental effects were more pronounced in female than in male flies. Adding regular flight exercise to flies on the WD markedly negated the adverse effects of a WD. At the molecular level, the WD significantly increased levels of triglycerides and caused mitochondrial dysfunction, while exercise counterbalanced these effects. Interestingly, fruit flies developed a preference for the WD after pre-exposure, which was averted by flight exercise. The results demonstrate that regular aerobic exercise can mitigate adverse dietary effects on fly mitochondrial function, physiology, and feeding behavior. Our data establish Drosophila simulans as a novel model of diet-exercise interaction that bears a strong similarity to the pathophysiology of obesity and eating disorders in humans.

Climate Change-Induced Stress Reduce Quantity and Alter Composition of Nectar and Pollen From a Bee-Pollinated Species (Borago officinalis, Boraginaceae)

In temperate ecosystems, elevated temperatures, and drought occur especially during spring and summer, which are crucial periods for flowering, pollination, and reproduction of a majority of temperate plants. While many mechanisms may underlie pollinator decline in the wake of climate change, the interactive effects of temperature and water stress on the quantity and quality of floral nectar and pollen resources remain poorly studied. We investigated the impact of temperature rise (+3 and +6°C) and water stress (soil humidity lower than 15%) on the floral resources produced by the bee-pollinated species Borago officinalis. Nectar volume decreased with both temperature rise and water stress (6.1 ± 0.5 μl per flower under control conditions, 0.8 ± 0.1 μl per flower under high temperature and water stress conditions), resulting in a 60% decrease in the total quantity of nectar sugars (mg) produced per flower. Temperature rise but not water stress also induced a 50% decrease in pollen weight per flower but a 65% increase in pollen polypeptide concentration. Both temperature rise and water stress increased the total amino acid concentration and the essential amino acid percentage in nectar but not in pollen. In both pollen and nectar, the relative percentage of the different amino acids were modified under stresses. We discuss these modifications in floral resources in regards to plant-pollinator interactions and consequences on plant pollination success and on insect nutritional needs.

Mechanical Permeabilization as a New Method for Assessment of Mitochondrial Function in Insect Tissues

Respirometry analysis is an effective technique to assess mitochondrial physiology. Insects are valuable biochemical models to understand metabolism and human diseases. Insect flight muscle and brain have been extensively used to explore mitochondrial function due to dissection feasibility and the low sample effort to allow oxygen consumption measurements. However, adequate plasma membrane permeabilization is required for substrates/modulators to reach mitochondria. Here, we describe a new method for study of mitochondrial physiology in insect tissues based on mechanical permeabilization as a fast and reliable method that do not require the use of detergents for chemical permeabilization of plasma membrane, while preserves mitochondrial integrity.