David L. Denlinger

Insects at low temperature

Physiology of Insect Cold Hardiness.- 1. A Tribute to R. W. Salt.- 2. Principles of Insect Low Temperature Tolerance.- 3. The Water Relations of Overwintering Insects.- 4. Biochemistry of Cryoprotectants.- 5. Hemolymph Proteins Involved in Insect Subzero-Temperature Tolerance: Ice Nucleators and Antifreeze Proteins.- Impact on Development and Survival.- 6. Cold Shock and Heat Shock.- 7. Effects of Cold on Morphogenesis.- 8. Relationship between Cold Hardiness and Diapause.- 9. Thermoperiodism.- Species Adaptations.- 10. Winter Habitats and Ecological Adaptations for Winter Survival.- 11. Freezing Tolerance in the Goldenrod Gall Fly (Eurosta solidaginis).- 12. Behavioral and Physiological Adaptations to Cold in a Freeze-Tolerant Arctic Insect.- 13. Comparative Invertebrate Cold Hardiness.- 14. Adaptations to Alpine and Polar Environments in Insects and Other Terrestrial Arthropods.- 15. Overwintering of Freshwater Benthic Macroinvertebrates.- Practical Applications.- 16. Cryopreservation of Insect Germplasm: Cells, Tissues, and Organisms.- 17. Cryobiology of Drosophila melanogaster Embryos.- 18. Silkworm Eggs at Low Temperatures: Implications for Sericulture.- 19. Overwintering in Honey Bees: Implications for Apiculture.- 20. Implications of Cold Hardiness for Pest Management.- Taxonomic Index.- Contributors.

10 – Hormonal Control of Diapause

Publisher Summary This chapter briefly discusses hormonal control of diapause. Diapause is an arrest in development accompanied by a major shutdown in metabolic activity. The evolution of diapause is arguably one of the most critical events bolstering the success of insects. The capacity to periodically shut down development has enabled insects and their arthropod relatives to invade environments that are seasonally hostile. Indeed, very few environments permit continuous insect development. Even in the tropics, where temperatures throughout the year may be compatible with ectotherm development, seasonal patterns of rainfall drive cycles of plant growth that favor insects with the ability to periodically become dormant. Unlike a simple quiescence that is an immediate response to an unfavorable environmental condition, diapause is a genetically programmed response that occurs at a specific stage for each species. Sometimes, the insect will enter diapause at this particular stage in each generation regardless of the environmental conditions it receives—a developmental program referred to as obligatory diapause. Much more frequently, the decision to enter diapause is determined by environmental factors, usually day length, received by that individual or its mother at an earlier developmental stage. This is referred to as facultative diapause.

A rapid cold-hardening process in insects

Traditionally studies of cold tolerance in insects have focused on seasonal adaptations related to overwintering that are observed after weeks or months of exposure to low temperature. In contrast, an extremely rapid cold-hardening response was observed in nonoverwintering stages that confers protection against injury due to cold shock at temperatures above the supercooling point. This response was observed in nondiapausing larvae and pharate adults of the flesh fly, Sarcophaga crassipalpis, nondiapausing adults of the elm leaf beetle, Xanthogaleruca luteola, and the milkweed bug, Oncopeltus fasciatus. The rapid hardening response is correlated with the accumulation of glycerol.

Up-regulation of heat shock proteins is essential for cold survival during insect diapause

Diapause, the dormancy common to overwintering insects, evokes a unique pattern of gene expression. In the flesh fly, most, but not all, of the flys heat shock proteins (Hsps) are up-regulated. The diapause up-regulated Hsps include two members of the Hsp70 family, one member of the Hsp60 family (TCP-1), at least four members of the small Hsp family, and a small Hsp pseudogene. Expression of an Hsp70 cognate, Hsc70, is uninfluenced by diapause, and Hsp90 is actually down-regulated during diapause, thus diapause differs from common stress responses that elicit synchronous up-regulation of all Hsps. Up-regulation of the Hsps begins at the onset of diapause, persists throughout the overwintering period, and ceases within hours after the fly receives the signal to reinitiate development. The up-regulation of Hsps appears to be common to diapause in species representing diverse insect orders including Diptera, Lepidoptera, Coleoptera, and Hymenoptera as well as in diapauses that occur in different developmental stages (embryo, larva, pupa, adult). Suppressing expression of Hsp23 and Hsp70 in flies by using RNAi did not alter the decision to enter diapause or the duration of diapause, but it had a profound effect on the pupas ability to survive low temperatures. We thus propose that up-regulation of Hsps during diapause is a major factor contributing to cold-hardiness of overwintering insects.

Energetics of insect diapause.

Managing metabolic resources is critical for insects during diapause when food sources are limited or unavailable. Insects accumulate reserves prior to diapause, and metabolic depression during diapause promotes reserve conservation. Sufficient reserves must be sequestered to both survive the diapause period and enable postdiapause development that may involve metabolically expensive functions such as metamorphosis or long-distance flight. Nutrient utilization during diapause is a dynamic process, and insects appear capable of sensing their energy reserves and using this information to regulate whether to enter diapause and how long to remain in diapause. Overwintering insects on a tight energy budget are likely to be especially vulnerable to increased temperatures associated with climate change. Molecular mechanisms involved in diapause nutrient regulation remain poorly known, but insulin signaling is likely a major player. We also discuss other possible candidates for diapause-associated nutrient regulation including adipokinetic hormone, neuropeptide F, the cGMP-kinase For, and AMPK.

Relationship between Cold Hardiness and Diapause

Cold hardiness and diapause are both essential components of winter survival for most insects of the temperate zone. But, in many cases, it is not clear how these two are related. Are they independent events or is cold hardiness a component of the diapause syndrome? Both independence (Lees, 1955; Salt, 1961; Ring, 1972) and dependence (Asahina, 1969; Mansingh, 1971, 1974) of cold hardiness and diapause have been defended vigorously, and indeed evidence for both possibilities can be found in the literature. In this chapter I argue that cold hardiness can be achieved independently of diapause, but cold hardiness is often a component of the diapause syndrome and the expression of diapause frequently extends the insect’s capacity to cold harden.

INDUCTION AND TERMINATION OF PUPAL DIAPAUSE IN SARCOPHAGA (DIPTERA: SARCOPHAGIDAE)

1. In temperate regions species of Sarcophaga overwinter in pupal diapause. The environmental control of diapause is investigated in Sarcophaga argyrostoma, S. crassipalpis, and three strains of S. bullata. Environmental cues of daylength and temperature, water content of the larval medium, and the sex of the animal determine the induction of diapause. Termination of diapause is temperature dependent.2. Daylength is of primary importance for induction. Diapause is completely averted when adult mothers and larvae are maintained under a long daily photophase or continuous light at 25° C. Short-day exposure of the adults and larvae at 25° induces a high incidence of diapause. However, if short-day is received by only the larvae, diapause is absent, and adult short-day without subsequent larval short-day produces a low diapause incidence. The maximum diapause response is observed when adults are maintained under a short daily photophase at 25°, and larvae are reared at a short daily photophase at 17°.3. Criti...

Low temperature biology of insects

Preface Part I. Physiological and Molecular Responses: 1. A primer on insect cold tolerance Richard E. Lee, Jr, 2. Rapid cold-hardening: ecological significance and underpinning mechanisms Richard E. Lee, Jr and David L. Denlinger 3. Antifreeze and ice nucleator proteins John G. Duman, Kent R. Walters, Todd Sformo, Martin A. Carasco, Philip K. Nickell, Xia Lin and Brian M. Barnes 4. Genomics, proteomics and metabolomics: finding the other players in insect cold tolerance M. Robert Michaud and David L. Denlinger 5. Cell structural modifications in insects at low temperatures Vladimir Kostal 6. Oxygen: stress and adaptation in cold hardy insects Kenneth B. Storey and Janet M. Storey 7. Interactions between cold, desiccation and environmental toxins Martin Holmstrup, Mark Bayley, Sindre A. Pedersen and Karl Erik Zachariassen Part II. Ecological and Evolutionary Responses: 8. The macrophysiology of insect cold hardiness Steven L. Chown and Brent J. Sinclair 9. Evolutionary physiology of insect thermal adaptation to cold environments Raymond B. Huey 10. Insects at not so low temperature: climate change in the temperate zone and its biotic consequences William E. Bradshaw and Christina M. Holzapfel 11. Genetic variability and evolution of cold tolerance Johannes Overgaard, Jesper G. Sorensen and Volker Loeschcke 12. Life history adaptations to polar and alpine environments Peter Convey Part III. Practical Applications: 13. A template for insect cryopreservation Roger A. Leopold and Joseph P. Rinehart 14. Implications of cold tolerance for pest management J. S. Bale Index.

Insulin signaling and FOXO regulate the overwintering diapause of the mosquito Culex pipiens

The short day lengths of late summer program the mosquito Culex pipiens to enter a reproductive diapause characterized by an arrest in ovarian development and the sequestration of huge fat reserves. We suggest that insulin signaling and FOXO (forkhead transcription factor), a downstream molecule in the insulin signaling pathway, mediate the diapause response. When we used RNAi to knock down expression of the insulin receptor in nondiapausing mosquitoes (those reared under long day lengths) the primary follicles were arrested in a stage comparable to diapause. The mosquitoes could be rescued from this developmental arrest with an application of juvenile hormone, an endocrine trigger known to terminate diapause in this species. When dsRNA directed against FOXO was injected into mosquitoes programmed for diapause (reared under short day lengths) fat storage was dramatically reduced and the mosquitos lifespan was shortened, results suggesting that a shutdown of insulin signaling prompts activation of the downstream gene FOXO, leading to the diapause phenotype. Thus, the results are consistent with a role for insulin signaling in the short-day response that ultimately leads to a cessation of juvenile hormone production. The similarity of this response to that observed in the diapause of Drosophila melanogaster and in dauer formation of Caenorhabditis elegans suggests a conserved mechanism regulating dormancy in insects and nematodes.

Cold-shock injury and rapid cold hardening in the flesh fly Sarcophaga crassipalpis

Direct exposure to -10 C, in the absence of tissue freezing, causes high mortality in Sarcophaga crassipalpis: this result suggests that injury is due to cold shock. However, brief acclimation at 0 C enables larvae, pupae, and pharate adults of Sarcophaga crassipalpis to survive -10 C. Chilling for as short a period as 10 min enabled 50% of the flies to survive a 2-h exposure to -10 C. Enhancement of cold tolerance was linear over the first hour of chilling at 0 C. The optimal temperature range eliciting the rapid acclimation response was 6-0 C, but the effect could also be stimulated by high temperature (36 C). The rapid increase in cold tolerance correlates with concomitant increases in hemolymph osmolality and glycerol levels. This response suggests a novel role for glycerol in protecting insects against injury resulting from cold shock, although other unidentified mechanisms may be involved in this response. That both nondiapause- and diapause-programmed flies respond to short-term chilling indicates that this rapid response is not part of the diapause syndrome but probably functions in either type of fly as an adaptation to survive brief periods of low temperature.