Diapause development in frozen larvae of the goldenrod gall fly, Eurosta solidaginis fitch (diptera: tephritidae)

Gall-Inducing Tephritid Flies (Diptera: Tephritidae): Evolution and Host–Plant Relations

The molecular-based phylogenetic analysis of the subfamily Tephritinae, the subfamily that contains almost all the cecidogenous species of the family Tephritidae, has reassigned several tribes and groups of genera and modified their concepts based on morphology alone to other tribes and, thus, changed the hypothetical scenarios of evolution of fly/host–plant relations and, in particular, the gall induction in different phylogenetic lineages. Gall induction is shown to arise independently within the Myopitini (in two lineages), Cecidocharini, Tomoplagia group of genera, Eurostini, Eutreta, Tephritis group of genera, Platensinini, Campiglossa group of genera, and Sphenella group of genera independently and more or less synchronously due to the shift to host plants with smaller flower heads and sensitive to larval feeding causing tissue proliferation. This was possibly a result of temporary aridization of the grassy biomes in the Nearctic and Afrotropic regions in the late Miocene or early Pliocene.

Regulation of an important glycolytic enzyme, pyruvate kinase, through phosphorylation in the larvae of a species of freeze-tolerant insect, Eurosta solidaginis

Larvae of the goldenrod gall fly, Eurosta solidaginis, rely on a freeze tolerance strategy to survive the sub-zero temperatures of Canadian winter. Critical to their survival is the accumulation of polyol cryoprotectants and global metabolic rate depression, both of which require the regulation of glycolysis and reorganization of carbohydrate metabolism. This study explored the role that pyruvate kinase (PK) regulation plays in this metabolic reorganization. PK was purified from control (5 °C-acclimated) and frozen (-15 °C-acclimated) larvae and enzyme kinetic properties, structural stability, and post-translational modifications were examined in both enzyme forms. The K_m phosphoenolpyruvate (PEP) of frozen PK was 20% higher than that of control PK, whereas the V_max of frozen PK was up to 50% lower than that of control PK at the lowest assay temperature, suggesting inhibition of the enzyme during the winter. Additionally, the activity and substrate affinity of both forms of PK decreased significantly at low assay temperatures, and both forms were regulated allosterically by a number of metabolites. Pro-Q™ Diamond phosphoprotein staining and immunoblotting experiments demonstrated significantly higher threonine phosphorylation of PK from frozen animals while acetylation and methylation levels remained constant. Together, these results indicate that PK exists in two structurally distinct forms in E. solidaginis. In response to conditions mimicking the transition to winter, PK appears to be regulated to support metabolic rate depression, the accumulation of polyol cryoprotectants, and the need for extended periods of anaerobic carbohydrate metabolism to allow the animal to survive whole-body freezing.

Costs of averting or prematurely terminating diapause associated with slow decline of metabolic rates at low temperature

Diapause, a form of insect dormancy, generally facilitates overwintering by increasing cold tolerance and decreasing energy drain at high temperatures via metabolic rate suppression. Averting or terminating diapause prior to winter is generally assumed to be a lethal phenotype. However, low temperature acclimation can also increase cold tolerance and decrease metabolic rates. Here, we tested the hypothesis that non- and post-diapause individuals in a cold-induced quiescence can achieve a diapause-like phenotype, compensating for the potential costs of averting diapause. We tested this in the apple maggot fly Rhagoletis pomonella, which typically overwinters in the soil as a diapause pupa, but can avert diapause (non-diapause) or terminate diapause early ('weak diapause') when reared at warm temperatures. Metabolic rates were initially higher in non- and post-diapause than diapause pupae at high (25 °C) and low (4 °C) temperatures, but quiescent non- and post-diapause pupae achieved diapause-like metabolic rates slowly over time when incubated at 4 °C for several weeks. We found that diapause and quiescent pupae were freeze-avoidant and had similar tolerance of extreme low temperatures (cooling to c. -18 °C) following 8 weeks acclimation at 4 °C. Despite high tolerance of subzero temperatures, quiescent pupae did not survive well when chilled for prolonged periods (8 weeks or more) at 4 °C. We conclude that cold acclimation can only partially compensate for costs associated with aversion or premature termination of diapause, and that energy drain at low (not just high) temperatures likely contributes to chilling mortality in quiescent insects.

Laboratory acclimation to autumn-like conditions induces freeze tolerance in the spring field cricket Gryllus veletis (Orthoptera: Gryllidae)

Many temperate insects encounter temperatures low enough to freeze their body fluids. Remarkably, some insects are freeze-tolerant, surviving this internal ice formation. However, the mechanisms underlying freeze tolerance are not well-understood, in part due to a lack of tractable model organisms. We describe a novel laboratory model to study insect freeze tolerance, the spring field cricket Gryllus veletis (Orthopera: Gryllidae). Following acclimation to six weeks of decreasing temperature and photoperiod, G. veletis become freeze-tolerant, similar to those exposed to natural autumn conditions in London, Ontario, Canada. Acclimated crickets suppress their metabolic rate by c. 33%, and survive freezing for up to one week at -8 °C, and to temperatures as low as -12 °C. Freeze-tolerant G. veletis protect fat body cells from freeze injury in vivo, and fat body tissue from freeze-tolerant cricket survives brief freeze treatments when frozen ex vivo. Freeze-tolerant crickets freeze at c. -6 °C, which may be initiated by accumulation of ice-nucleating agents in hemolymph or gut tissue. Although we hypothesize that control of ice formation facilitates freeze tolerance, initiating ice formation at high subzero temperatures does not confer freeze tolerance on freeze-intolerant nymphs. Acclimation increases hemolymph osmolality from c. 400 to c. 650 mOsm, which may facilitate freeze tolerance by reducing ice content. Hemolymph ion concentrations do not change with acclimation, and we therefore predict that freeze-tolerant G. veletis elevate hemolymph osmolality by accumulating other molecules. Gryllus veletis is easily reared and manipulated in a controlled laboratory environment, and is therefore a suitable candidate for further investigating the mechanisms underlying freeze tolerance.

Comparative analysis of the transcriptome of the overwintering desert beetle Microdera punctipennis

The cold tolerance mechanisms of insect have been studied extensively on the model species Drosophila and a few other species at the transcriptional level. However studies on insects that inherit strong cold tolerance are limited. Cold hardy Tenebrionid beetle Microdera punctipennis is endemic to Gurbantonggut Desert, northwest of China. However, its genomic information is lacking. To investigate the overwintering mechanisms of M. punctipennis adult, RNA-seq was performed on the winter adults and the control adults that were kept in laboratory at 30 °C. A total of 175,247 unigenes were acquired with an average length of 645 bp. By using DESeq package, we identified 3367 unigenes that were up-regulated and 7988 down-regulated in the winter adults compared with the controls. To further our understanding of these differentially expressed genes (DEGs), Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were performed. Pathway analysis showed that the "ECM-receptor interaction", "PI3K-Akt signaling pathway", "Estrogen signaling pathway", "Tight junction", and "Regulation of actin cytoskeleton", etc. might play important roles in M. punctipennis overwintering. The DEGs results from the RNA-Seq were confirmed partially by qRT-PCR for 13 DEGs, which showed high consistence with a Pearson's correlation coefficient of 0.851. Overall, the sequence data will provide basic information for subsequent bioinformatical analysis and mining of the genes responsible for cold tolerance in M. punctipennis, as well as for understanding the molecular mechanisms of desert beetle overwintering.

Insect Immunity Varies Idiosyncratically During Overwintering

Overwintering insects face multiple stressors, including pathogen and parasite pressures that shift with seasons. However, we know little of how the insect immune system fluctuates with season, particularly in the overwintering period. To understand how immune activity changes across autumn, winter, and spring, we tracked immune activity of three temperate insects that overwinter as larvae: a weevil (Curculio sp., Coleoptera), gallfly (Eurosta solidaginis, Diptera), and larvae of the lepidopteran Pyrrharctia isabella. We measured baseline circulating hemocyte numbers, phenoloxidase activity, and humoral antimicrobial activity, as well as survival of fungal infection and melanization response at 12°C and 25°C to capture any potential plasticity in thermal performance. In Curculio sp. and E. solidaginis, hemocyte concentrations remained unchanged across seasons and antimicrobial activity against Gram-positive bacteria was lowest in autumn; however, Curculio sp. were less likely to survive fungal infection in autumn, whereas E. solidaginis were less likely to survive infection during the winter. Furthermore, hemocyte concentrations and antimicrobial activity decreased in P. isabella overwintering beneath snow cover. Overall, seasonal changes in activity were largely species dependent, thus it may be difficult to create generalizable predictions about the effects of a changing climate on seasonal immune activity in insects. However, we suggest that the relationship between the response to multiple stressors (e.g., cold and pathogens) drives changes in immune activity, and that understanding the physiology underlying these relationships will inform our predictions of the effects of environmental change on insect overwintering success.

Response of Chrysoperla nipponensis (Okamoto) (Neuroptera: Chrysopidae) Under Long and Short Photoperiods

Photoperiod is an important factor influencing many biological processes including population dynamics of many insect species in temperate zones. To determine the population response of Chrysoperla nipponensis under altered conditions (high temperature and short photoperiod) and to test whether the short photoperiod was suitable for artificial storage, the life table data of were collected at 25 °C under a long photoperiod, 15:9 h (L:D), and a short photoperiod, 9:15 h (L:D) and analyzed using the age-stage, two-sex life table approach. We found that developed faster under long photoperiod than under the short photoperiod. The shorter developmental time, higher fecundity, and higher proportion of females found during the long photoperiod resulted in higher intrinsic and net reproductive rates, but a shorter mean generation time and life expectancy compared to those reared during the short photoperiod. Individuals reared under the short photoperiod also had a high reproductive value. Population projection demonstrated that reared at long photoperiod would complete four generations in 150 d, while reared under short photoperiod would just be entering the second generation. Our results demonstrated that the different fitness values obtained for individuals by varying photoperiod lengths, were readily distinguishable when using the age-stage, two-sex life table.

Differential peptide expression in the central nervous system of the land snail Theba pisana, between active and aestivated

Hypometabolism is a physiological state of dormancy entered by many animals in times of environmental stress. There are gaps in our understanding of the molecular components used by animals to achieve this metabolic state. The availability of genomic and transcriptome data can be useful to study the process of hypometabolism at the molecular level. In this study, we use the land snail Theba pisana to identify peptides that may be involved in the hypometabolic state known as aestivation. We found a total of 22 neuropeptides in the central nervous system (CNS) that were differentially produced during activity and aestivation based on mass spectral-based neuropeptidome analysis. Of these, 4 were upregulated in active animals and 18 were upregulated in aestivation. A neuropeptide known to regulate muscle contractions in a variety of molluscs, the small cardioactive peptide A (sCAPA), and a peptide of yet unknown function (termed Aestivation Associated Peptide 12) were chosen for further investigation using temporal and spatial expression analysis of the precursor gene and peptide. Both peptides share expression within regions of the CNS cerebral ganglia and suboesophageal ganglia. Relative transcript abundance suggests that regulation of peptide synthesis and secretion is post-transcriptional. In summary, we provide new insights into the molecular basis of the regulation of aestivation in land snails through CNS peptide control.

De novo transcriptome of the desert beetle Microdera punctipennis (Coleoptera: Tenebrionidae) using Illumina RNA-seq technology

Insects in Tenebrionidae have unique stress adaptations that allow them to survive temperature extremes. We report here a gene expression profiling of Microdera punctipennis, a beetle in desert region, to gain a global view of its environmental adaptations. A total of 48,158,004 reads were obtained by transcriptome sequencing, and the de novo assembly yielded 56,348 unigenes with an average length of 666 bp. Based on similarity searches with a cut-off E-value of 10⁻⁵ against two protein sequence databases, 41,109 of the unigenes (about 72.96 %) were matched to known proteins. An in-depth analysis of the data revealed a large number of genes were associated with environmental stress, including genes that encode heat shock proteins, antifreeze proteins, and enzymes such as chitinase, trehalose, and trehalose-6-phosphate synthase. This study generated a substantial number of M. punctipennis transcript sequences that can be used to discover novel genes associated with stress adaptation. These sequences are a valuable resource for future studies of the desert beetle and other insects in Tenebrionidae. Transcriptome analysis based on Illumina paired-end sequencing is a powerful approach for gene discovery and molecular marker development for non-model species.

The protective effect of rapid cold-hardening develops more quickly in frozen versus supercooled larvae of the Antarctic midge, Belgica antarctica

During the austral summer, larvae of the terrestrial midge, Belgica antarctica (Diptera: Chironomidae), experience highly variable and often unpredictable thermal conditions. In addition to remaining freeze tolerant year-round, larvae are capable of swiftly increasing their cold tolerance through the rapid cold-hardening (RCH) response. The present study compared the induction of RCH in frozen versus supercooled larvae. At the same induction temperature, RCH occurred more rapidly and conferred a greater level of cryoprotection in frozen versus supercooled larvae. Furthermore, RCH in frozen larvae could be induced at temperatures as low as -12°C, which is the lowest temperature reported to induce RCH. Remarkably, as little as 15 min at -5°C significantly enhanced larval cold tolerance. Not only is protection from RCH acquired swiftly, but it is also quickly lost after thawing for 2 h at 2°C. Because the primary difference between frozen and supercooled larvae is cellular dehydration caused by freeze concentration of body fluids, we also compared the effects of acclimation in dehydrated versus frozen larvae. Since slow dehydration without chilling significantly increased larval survival to a subsequent cold exposure, we hypothesize that cellular dehydration caused by freeze concentration promotes the rapid acquisition of cold tolerance in frozen larvae.

Metabolism and cold tolerance of overwintering adult mountain pine beetles (Dendroctonus ponderosae): evidence of facultative diapause?

We sought evidence for a distinct diapause in adult overwintering mountain pine beetles (Dendroctonus ponderosae Hopkins) by measuring metabolic rate and supercooling ability of field collected beetles throughout the year. Metabolic rates measured at 0, 5, and 10°C declined significantly from October through November, then rose slowly, reaching levels as high as those recorded in October by late May. From December to February metabolic rates were not correlated with minimum weekly phloem temperatures (R(2)=0.0%, P=0.592), but were correlated with phloem temperatures as winter advanced to spring (R(2)=44.8%, P=0.010), a pattern consistent with progression through the maintenance and termination phases of diapause. Supercooling points were also significantly lower in winter compared to fall and spring (F((8,143))=32.6, P<0.001) and were closely correlated with metabolic rates (R(2)>79% for all three temperatures). Dry mass declined linearly with winter progression (F((8,150))=8.34, P<0.001), explained by catabolism of metabolic reserves, with a concomitant accumulation of metabolic water (F((8,147))=35.24, P<0.001). The strong mid-winter metabolic suppression correlated with improved supercooling ability, coupled with their lack of response to variation in environmental temperature, are evidence of possible diapause in adult overwintering mountain pine beetles.

Effect of freezing and dehydration on ion and cryoprotectant distribution and hemolymph volume in the goldenrod gall fly, Eurosta solidaginis

Extracellular freezing and dehydration concentrate hemolymph solutes, which can lead to cellular injury due to excessive water loss. Freeze tolerant larvae of the goldenrod gall fly, Eurosta solidaginis, may experience extreme cold and desiccation in winter. To determine whether larvae employ protective mechanisms against excessive cellular water loss we examined the effect of extracellular freezing and dehydration on hemolymph volume, and cryoprotectant and ion levels in the hemolymph. Dehydrated larvae or ones that had been frozen at -5 or -20°C had a significantly smaller proportion of their body water as hemolymph (26.0-27.4%) compared to controls (30.5%). Even with this reduction in water content, hemolymph osmolality was similar or only slightly higher in frozen or dehydrated individuals than controls (908 mOsm kg(-1)), indicating these stresses led to a reduction in hemolymph solutes. Hemolymph and intracellular content of ions remained largely unchanged between treatment groups; although levels of Mg(++) in the hemolymph were lower in larvae subjected to freezing (0.21±0.01 μg mg(-1)drymass) compared to controls (0.29±0.01 μg mg(-1)drymass), while intracellular levels of K(+) were lower in groups exposed to low temperature (8.31±0.21 μg mg(-1)drymass). Whole body glycerol and sorbitol content was similar among all treatment groups, averaging 432±25 mOsm kg(-1) and 549±78 mOsm kg(-1) respectively. However, larvae subjected to dehydration and freezing at -20°C had a much lower relative amount of cryoprotectants in their hemolymph (∼35%) compared to controls (54%) suggesting these solutes moved into intracellular compartments during these stresses. The correlation between reduced hemolymph volume (i.e. increased cellular water content) and intracellular movement of cryoprotectants may represent a link between tolerance of dehydration and cold in this species.

The protective role of aquaporins in the freeze-tolerant insect Eurosta solidaginis: functional characterization and tissue abundance of EsAQP1

The movement of water and small solutes is integral to the survival of freezing and desiccation in insects, yet the underlying mechanisms of these processes are not fully known. Recent evidence suggests that aquaporin (AQP) water channels play critical roles in protecting cells from osmotic damage during freezing and desiccation. Our study sequenced, functionally characterized and measured the tissue abundance of an AQP from freeze-tolerant larvae of the gall fly, Eurosta solidaginis (Diptera: Tephritidae). The newly characterized EsAQP1 contains two NPA motifs and six transmembrane regions, and is phylogenetically related to an AQP from the anhydrobiotic chironomid Polypedilum vanderplanki. Using a Xenopus laevis oocyte swelling assay, we demonstrated that EsAQP1 increases water permeability to nine times that of simple diffusion through the membrane. In contrast to its high water permeability, EsAQP1 was impermeable to both glycerol and urea. The abundance of EsAQP1 increased from October to December in all tissues tested and was most abundant in the brain of winter larvae. Because the nervous system is thought to be the primary site of freezing injury, EsAQP1 may cryoprotect the brain from damage associated with water imbalance. The sequence, phylogenetic relationship, osmotic permeability, tissue distribution and seasonal abundance of EsAQP1 further support the role of AQPs in promoting freezing tolerance.

Changes in abundance of aquaporin-like proteins occurs concomitantly with seasonal acquisition of freeze tolerance in the goldenrod gall fly, Eurosta solidaginis

The accumulation of cryoprotectants and the redistribution of water between body compartments play central roles in the capacity of insects to survive freezing. Aquaporins (AQPs) allow for rapid redistribution of water and small solutes (e.g. glycerol) across the cell membrane and were recently implicated in promoting freeze tolerance. Here, we examined whether aquaporin-like protein abundance correlated with the seasonal acquisition of freezing tolerance in the goldenrod gall fly, Eurosta solidaginis (Diptera: Tephritidae). Through the autumn, larvae became tolerant of freezing at progressively lower temperatures and accumulated the cryoprotectant glycerol. Furthermore, larvae significantly increased the abundance of membrane-bound aquaporin and aquaglyceroporin-like proteins from July through January. Acute exposure of larvae to cold and desiccation resulted in upregulation of the AQP3-like proteins in October, suggesting that their abundance is regulated by environmental cues. The seasonal increase in abundance of both putative aquaporins and aquaglyceroporins supports the hypothesis that these proteins are closely tied to the seasonal acquisition of freeze tolerance, functioning to permit cells to quickly lose water and take-up glycerol during extracellular ice formation, as well as reestablish water and glycerol concentrations upon thawing.

Suppression of Na+K+ -ATPase activity by reversible phosphorylation over the winter in a freeze-tolerant insect

Larvae of the gall fly, Eurosta solidaginis, use the cold hardiness strategy of freeze tolerance as well as entry into a hypometabolic state (diapause) to survive the winter. Cold hardiness strategies have been extensively explored in this species, but the metabolic features of winter hypometabolism have received little attention. A primary consumer of energy in cells is the ATP-dependent sodium-potassium ion pump (Na(+)K(+)-ATPase) so inhibitory controls over transmembrane ion movements could contribute substantially to energy savings over the winter months. Na(+)K(+)-ATPase activity was quantified in larvae sampled between October and April. Activity was high in October (0.56+/-0.13nmol/min/mg) but fell by 85% in November, remained low through midwinter, and then increased strongly in April. To determine whether the seasonal change in Na(+)K(+)-ATPase activity was linked with posttranslational modification of the enzyme, extracts from 15 degrees C-acclimated larvae were incubated under conditions that stimulated protein kinases A, G, or C. The action of all three kinases suppressed Na(+)K(+)-ATPase activity to levels just 3-8% of control values whereas the opposite treatment with alkaline phosphatase had no effect. Hence, the seasonal suppression of Na(+)K(+)-ATPase activity may be linked to enzyme phosphorylation. Furthermore, acute cold (3 degrees C) or hypoxia exposures of 15 degrees C-acclimated larvae did not alter enzyme activity, and freezing at -16 degrees C increased activity, so environmental factors do not appear to directly influence enzyme activity. Rather, it appears that winter suppression of ion motive ATPase activity may be part of a program of winter metabolic suppression.

Plant senescence cues entry into diapause in the gall fly Eurosta solidaginis: resulting metabolic depression is critical for water conservation

Mechanisms and possible cues for seasonal increases in desiccation resistance in larvae of the goldenrod gall fly Eurosta solidaginis, were examined before and after natural and premature plant senescence, or after being removed from their gall and placed in either 100, 95 or 75% relative humidity (RH). Rates of water loss were 8.6-fold lower, averaging 0.7+/-0.2 microg mm(-2) h(-1), in larvae from senescent gall tissue and after all RH treatments than in control larvae from pre-senescent plants. Enhanced desiccation resistance occurred quickly, within 3 days of removal from their gall. Contrary to most previous reports, a large majority of the increased desiccation resistance (approximately 85%) was due to reduced respiratory transpiration with the remainder being the result of a lowered cuticular permeability. Rates of cuticular water loss were reduced by the presence of a vapor pressure gradient between the larval hemolymph and environmental water vapor and were probably due to increases in cuticular lipids and/or production of the cryoprotectant glycerol. Metabolic rate was reduced by over fourfold, averaging 0.07+/-0.01 microl CO2 g(-1) h(-1), in larvae from senescent gall tissue and all RH treatments compared to larvae from pre-senescent plants. The magnitude of the reduction in metabolic rates indicated that these larvae had entered diapause. In addition, larvae entered diapause in response to removal from, or degeneration of, the gall tissue they feed, on rather than seasonal changes in temperature or photoperiod. The low metabolic rates of the diapausing larvae probably allowed them to dramatically reduce their respiratory transpiration and total rate of water loss compared with non-diapausing controls. Thus, diapause, with its associated lowered metabolic rate, may be essential for conserving water in overwintering temperate insects, which may be dormant for six or more months of the year.

Rapid cold-hardening increases the freezing tolerance of the Antarctic midge Belgica antarctica

Rapid cold-hardening (RCH) is well known to increase the tolerance of chilling or cold shock in a diverse array of invertebrate systems at both organismal and cellular levels. Here, we report a novel role for RCH by showing that RCH also increases freezing tolerance in an Antarctic midge, Belgica antarctica (Diptera, Chironomidae). The RCH response of B. antarctica was investigated under two distinct physiological states: summer acclimatized and cold acclimated. Summer-acclimatized larvae were less cold tolerant, as indicated by low survival following exposure to -10 degrees C for 24 h; by contrast, nearly all cold-acclimated larvae survived -10 degrees C, and a significant number could survive -15 degrees C. Cold-acclimated larvae had higher supercooling points than summer larvae. To evaluate the RCH response in summer-acclimatized midges, larvae and adults, maintained at 4 degrees C, were transferred to -5 degrees C for 1 h prior to exposures to -10, -15 or -20 degrees C. RCH significantly increased survival of summer-acclimatized larvae frozen at -10 degrees C for 1 h compared with larvae receiving no cold-hardening treatment, but adults, which live for only a week or so in the austral summer, lacked the capacity for RCH. In cold-acclimated larvae, RCH significantly increased freeze tolerance to both -15 and -20 degrees C. Similarly, RCH significantly increased cellular survival of fat body, Malpighian tubules and gut tissue from cold-acclimated larvae frozen at -20 degrees C for 24 h. These results indicate that RCH not only protects against non-freezing injury but also increases freeze tolerance.

Eco-physiological phases of insect diapause

Insect diapause is a dynamic process consisting of several successive phases. The conception and naming of the phases is unsettled and, sometimes, ambiguous in the literature. In this paper, the ontogeny of diapause was reviewed and the most often used terms and the best substantiated phases were highlighted, explained and re-defined. The aim was to propose relatively simple and generally applicable terminological system. The phases of diapause induction, preparation, initiation, maintenance, termination and post-diapause quiescence were distinguished. The specific progression through diapause phases in each species, population (genotype), or even individual, is based on (thus far largely unknown) physiological processes, the actual expression of which is significantly modified by diverse environmental factors. Thus, such phases are eco-physiological in their nature.

HIF-1α involvement in low temperature and anoxia survival by a freeze tolerant insect

Winter survival for many insect species relies on the ability to endure the freezing of extracellular body fluids. Because freezing impedes oxygen delivery to tissues, one component of natural freeze tolerance is a well-developed anoxia/ischemia resistance. The present study explores the responses of the hypoxia-inducible factor-1alpha (HIF-1alpha) to cold, freezing and anoxia exposures in the freeze tolerant goldenrod gall fly larva, Eurosta solidaginis. Reverse transcription-PCR was used to quantify hif-1alpha transcript levels; transcripts were significantly elevated by approximately 70% in chilled (3 ( composite function)C), frozen (-16 ( composite function)C) and thawed (returned to 3 ( composite function)C) insects, compared with 15 ( composite function)C controls. Transcripts also rose by approximately 3-fold in insects given anoxia exposure under a nitrogen gas atmosphere. Cold and freezing exposure also elevated HIF-1alpha protein content in the larvae and HIF-1alpha levels increased over the winter months in insects sampled from an outdoor population; levels peaked in February at 2.1-fold higher than in September. A partial sequence of HIF-1alpha that covers the bHLH and PAS domains of the protein was obtained from E. solidaginis and sequence analysis revealed that this segment shared 62% identity overall with Drosophila melanogaster HIF-1alpha and higher percent identities within specific domains: 76% within the bHLH domain and 70% within the PAS domain. The data provide the first documentation of a potential role for HIF-1 in regulating the expression of genes that can aid freezing survival in a cold-hardy animal.

Changes in gut and Malpighian tubule transport during seasonal acclimatization and freezing in the gall flyEurosta solidaginis

Since few studies have examined cold tolerance at the organ level in insects, our primary objective was to characterize the functional responses of the gut and Malpighian tubules (MT) to seasonal acclimatization, chilling and freezing in larvae of the goldenrod gall fly Eurosta solidaginisFitch (Diptera, Tephritidae). From September to December, hemolymph osmolality(455-926 mOsmol kg l-1) and freezing tolerance increased markedly in field-collected larvae. Chlorophenol Red was readily transported into the lumen of the foregut, the posterior portion of the midgut, the ureter, the proximal region of the anterior pair of MT, and entire posterior pair of MT. Ouabain and KCN inhibited transport of Chlorophenol Red in the gut and MT. Transport was readily detected at 0°C and the rate of transport was directly related to temperature. The rate of fluid transport by the MT decreased steadily from a monthly high in September (10.7±0.8 nl min-1 for the anterior pair; 12.7±1.0 nl min-1for the posterior pair) until secretion was no longer detectable in December;this decrease parallels entry into diapause for this species. Even in larvae that died following freezing for 40 days at -20°C, individual organ function was retained to a limited extent. Through the autumn, cholesterol concentrations in the hemolymph increased nearly fourfold. In contrast, the ratio of cholesterol to protein content (nmol mg l-1) in the MT membrane remained relatively constant (22∼24 nmol mg l-1protein) during this period. Freezing of larvae for 20 days at -20°C caused a significant decrease in cholesterol levels in the hemolymph and the MT membranes compared to unfrozen controls. These results suggest that cholesterol plays a role in seasonal cold hardening and freeze tolerance in insects.

Partial link between the seasonal acquisition of cold-tolerance and desiccation resistance in the goldenrod gall fly Eurosta solidaginis(Diptera: Tephritidae)

Possible links between seasonal increases in cold-tolerance and desiccation resistance were examined in field-collected larvae of the goldenrod gall fly, Eurosta solidaginis. From 20 September to 30 October 2001, larvae exhibited a gradual increase in cold-tolerance culminating in 100% survival of freezing at –20°C for 24 h. The increase in cold-tolerance was probably due to a concomitant increase in cryoprotectants as measured by hemolymph osmolality (488–695 mOsmol kg–1). In contrast to the gradual increase in cold-tolerance, larvae exhibited two distinct phases of reduced rates of water loss. The first phase was an abrupt sixfold decrease to 0.57 μg mm–2 h–1 between 3 and 16 October. The first phase of reduced rates of water loss was not correlated with changes in cold-tolerance; nor was it correlated with hemolymph osmolality and body water content, which remained constant throughout the study. The reduction in rates of water loss during the first phase were probably the result of decreased respiratory water loss as the larvae entered diapause, and possibly reduced cuticular water loss as larvae increased the amount of their cuticular hydrocarbons. Interestingly, the first phase of reduced water loss was associated with, and may have been cued by, a reduction in the water potential of the gall tissues surrounding the larvae. The second phase was a more subtle fourfold reduction in rates of water loss occurring between 16 October and 11 December. In contrast to the first phase,the second phase of increased desiccation resistance correlated closely with increases in hemolymph osmolality (568–870 mOsmol kg–1). The correlation between seasonal increases in hemolymph osmolality and reduction in rates of water loss may represent a link between desiccation resistance and cold-tolerance in this species.

Diapause in the pea aphid (Acyrthosiphon pisum) is a slowing but not a cessation of development

BACKGROUND

Many insects undergo a period of arrested development, called diapause, to avoid seasonally recurring adverse conditions. Whilst the phenology and endocrinology of insect diapause have been well studied, there has been comparatively little research into the developmental details of diapause. We investigated developmental aspects of diapause in sexually-produced embryos of the pea aphid, Acyrthosiphon pisum.

RESULTS

We found that early stages of embryogenesis progressed at a temperature-independent rate, characteristic of diapause, whereas later stages of embryogenesis progressed at a temperature-dependent rate. However, embryos maintained at very high temperatures during the temperature-independent stage showed severe developmental abnormalities. Under no temperature regime did embryos display a distinct resting stage. Rather, morphological development progressed slowly but continuously throughout embryogenesis.

CONCLUSION

Diapause in the pea aphid, and perhaps in many other insects, is a temperature-independent slowing but not a cessation of morphological development. This suggests that the mechanisms limiting developmental rate during diapause may be the same as those controlling developmental rate at other stages of growth.

Deleterious effects of mild simulated overwintering temperatures on survival and potential fecundity of rose-galling Diplolepis wasps (Hymenoptera: Cynipidae)

Prepupae of the rose galling Diplolepis spinosa from areas with relatively cold winters in southern Canada, and Diplolepis variabilis from a milder locale in western Canada, were used to test the hypothesis that mild winter temperatures are detrimental to the survival and potential fecundity of insects. Prepupae of D. spinosa held within or removed from their galls were exposed to simulated overwintering temperatures (-22, 0, 5, or 10 degrees C) for approximately four months before measuring their survival, body size, and potential fecundity. Similar studies were conducted using prepupae of D. variabilis that were removed from their gall and subjected to 0 degrees C or 10 degrees C treatments. Diplolepis spinosa, with or without their galls, averaged 66% more mortality at 10 degrees C than at 0 degrees C. Female D. spinosa that survived the 10 degrees C treatment had 32% fewer eggs than those held at 0 degrees C. In contrast, there was no difference in survival or numbers of eggs between D. variabilis held at 0 degrees C and 10 degrees C. Body size of adult females and size of eggs did not differ among temperature treatments for either species. We conclude that mild overwintering temperatures may be detrimental for insects by raising their metabolism, and consequently reducing energetic reserves needed for development to the adult stage and subsequent production of eggs the following spring.

Variations in mitochondrial DNA and gene transcription in freezing-tolerant larvae of Eurosta solidaginis (Diptera: Tephritidae) and Gynaephora groenlandica (Lepidoptera: Lymantriidae)

Respiration, mitochondrial (mt)DNA content, and mitochondrial-specific RNA expression in fat body cells from active and cold-adapted larvae of the goldenrod gall fly, Eurosta solidaginis, and the Arctic woolly bear caterpillar, Gynaephora groenlandica, were compared. Reduced amounts of mtDNA were observed in cold-adapted larvae of both E. solidaginis and G. groenlandica collected in fall or winter, compared with summer-collected larvae. mtDNA increased to levels similar to those of summer-collected larvae after incubation at 10 degrees C or 15 degrees C for 5 h. Mitochondrial-specific RNAs (COI and 16S) were observed in fat body cells of both active and cold-adapted E. solidaginis larvae. Our results suggest that mitochondrial proteins required for respiration may be restored rapidly from stable RNAs present in overwintering larvae.

Acquisition of freezing tolerance in early autumn and seasonal changes in gall water content influence inoculative freezing of gall fly larvae, Eurosta solidaginis (Diptera, Tephritidae)

We examined seasonal changes in freeze tolerance and the susceptibility of larvae of the gall fly, Eurosta solidaginis to inoculative freezing within the goldenrod gall (Solidago sp.). In late September, when the water content of the galls was high (approximately 55%), more than half of the larvae froze within their galls when held at -2.5 degrees C for 24 h, and nearly all larvae froze at -4 or -6 degrees C. At this time, most larvae survived freezing at > or = -4 degrees C. By October plants had senesced, and their water content had decreased to 33%. Correspondingly, the number of larvae that froze by inoculation at -4 and -6 degrees C also decreased, however the proportion of larvae that survived freezing increased markedly. Gall water content reached its lowest value (10%) in November, when few larvae froze during exposure to subzero temperatures > or = -6 degrees C. In winter, rain and melting snow transiently increased gall water content to values as high as 64% causing many larvae to freeze when exposed to temperatures as high as -4 degrees C. However, in the absence of precipitation, gall tissues dried and, as before, larvae were not likely to freeze by inoculation. Consequently, in nature larvae freeze earlier in the autumn and/or at higher temperatures than would be predicted based on the temperature of crystallization (T(c)) of isolated larvae. However, even in early September when environmental temperatures are relatively high, larvae exhibited limited levels of freezing tolerance sufficient to protect them if they did freeze.

Energy and water conservation in frozen vs. supercooled larvae of the goldenrod gall fly, Eurosta solidaginis (fitch) (Diptera: Tephritidae)

Insects that tolerate severe cold during winter may either supercool or tolerate ice forming within the tissues of the body. To compare the relative advantages of freezing and supercooling, we measured rates of CO(2) production and water loss in frozen and supercooled goldenrod gall fly larvae (Eurosta solidaginis). As an important first step, we measured the time required for ice content and metabolic rate to stabilize upon freezing. Ice content stabilized after only three hours of freezing at -5 degrees C, whereas CO(2) production required 12 hours to stabilize. Subsequent experiments found that freezing greatly reduced both water loss and metabolic rate. Comparisons of supercooled and frozen larvae at -5 degrees C indicated that CO(2) production fell 47% with freezing and water loss decreased 35%. As temperature decreased to -10 and -15 degrees C, CO(2) production fell exponentially and was no longer detectable at -20 degrees C with our measurement system. Our results demonstrate that freezing significantly reduces energy consumption during the winter and may therefore improve winter survival and spring fecundity. The advantages of freezing over supercooling would drive selection toward insect freeze tolerance and also toward higher supercooling points to increase the duration of freezing each winter.