The term diapause has been used for developmental delays that allow organisms to survive unfavorable conditions and to synchronize growth and development with favorable conditions.* Many insects diapause to time their life cycle in the face of seasonality. Extreme temperature, predators, and change in photoperiod are factors that elicit diapause. Diapause, coupled with physiological changes, may ensure survival through unfavorable environmental conditions. Depending on species, diapause occurs in one or sometimes several developmental stages: as eggs, larvae, pupae, or adults. Adult diapause is well characterized in potato beetles, monarch butterflies, several grasshoppers, and Drosophila. Apart from insects, diapause also happens in some invertebrate organisms such as rotifers, nematodes, earthworms, crustaceans, and terrestrial gastropods.*
Physioecological experiments of life history have accumulated many observations in this aspect.
Diapause¡ªA profound, endogenously, and centrally mediated interruption that routes the developmental program away from direct morphogenesis into an alternative diapause program of succession of physiological events; the start of diapause usually precedes the advent of adverse conditions, but the end of diapause need not coincide with the end of adversity.*
Diapause is a genetically programmed developmental response that occurs at a specific stage for each species. Species having an obligate diapause arrest development at the same point in the lifecycle of every generation regardless of the environmental conditions, whereas in species with a facultative diapause the environment experienced by individuals, or sometimes their parents, determines whether an individual will diapause.*
Diapause does not entail a complete cessation of development. As evidenced by characteristic temporal patterns of gas exchange, nutrient metabolism, stress resistance, and gene expression, diapause is a dynamic process. Diapausing insects pass through a graded series of physiologically distinct developmental stages including induction, preparation, initiation, maintenance, termination, and sometimes post-diapause quiescence.*
Cold hardiness and diapause are essential components of winter survival for most insects in the temperate zone. The former is often associated with the latter in many species such as Sarcophaga bullata and S. crassipalpis, Bombyx mori, and Hyalophora cecropia.
A close relationship between diapause and cold hardiness is evident in the flesh flies Sarcophaga crassipalpis and S. bullata. In these species, nondiapausing pupae were consistently intolerant of low temperature (-10¡ãC), showing some initial survival after three days exposure but no survival to eclosion. Diapausing pupae of the same age, however, readily survived to eclosion after twenty-five days of exposure to -10¡ãC. Diapause-destined larvae were also more cold tolerant than nondiapause-destined larvae of the same age. Thus the diapause program seems to confer a clear additional cold tolerance.*
Accumulation of low-molecular weight carbohydrates, polyols and sugars, has been found in overwintering insects. These compounds provide colligative depression of freezing point. They may also stabilize cell membrane structure and the native state of proteins at low temperature. In freeze-tolerant species, these compounds regulate cell volume during extracellular ice formation without injurious effects.*
Zhudong Liu et al. reported a summer diapause in the cotton bollworm Helicoverpa armigera (H¨¹bner). The host plants of the pest are usually available in summer, and the only environmental factor that is known to induce summer diapause is high temperature. The cotton bollworm's summer diapause occurs at pupae stage. The weight loss and lipid and glycogen metabolism curves indicate that the summer-diapausing pupae's metabolism is very low. Their research has also confirmed that the offspring of parents which had experienced summer diapause had higher survival rates compared to those whose parents had not experienced summer diapause, and they weighed significantly more than those from non-summer-diapausing parents. Therefore, summer diapause of the cotton bollworm is a mechanism that confers advantages to the offspring. In H. armigera, a portion of the population is dormant during the period of high temperature, but the rest continues to develop, adopting a bet-hedging strategy to protect the species from unpredictable risks due to a fluctuating environment.*
All results reveal that summer diapause can serve as a bet-hedging strategy against unpredictable risks due to fluctuating environments or as a feedback mechanism to synchronize the period of autumn emergence. In fact, the host plant S. chinensis of the zygaenid moth in its natural habitats is often faced with the threat of an arid summer, which restricts supply of fresh leaves. Therefore, when part of the population enters summer diapause, it mitigates the problem of insufficient food resources caused by seasonal drought.*
Coincident with developmental arrest, diapause is characterized by decreased intermediary and respiratory metabolism. Metabolic depression involves shutting down or vastly decreasing the activity of energetically expensive biochemical and physiological systems. In addition to physiological alterations that decrease metabolic rate, careful selection of a diapause site can affect diapause energetics and costs.*
Many metabolic pathways, such as the anabolic pathways leading to cell growth and proliferation, are down-regulated during diapause. Other pathways involved in basic cellular maintenance remain operational at reduced levels during diapause, and some metabolic pathways are up-regulated during diapause. Most obviously up-regulated are stress-resistance pathways leading to cryoprotectant and heat shock protein synthesis.*
Rapid environmentally induced shifts in intermediary metabolism and substrate utilization are also well documented in diapausing insects. Many species convert glycogen stores to cryoprotectants in direct response to low temperature. Therefore, conversion of glycogen into cryoprotectants and reconversion of cryoprotectants back into glycogen or other metabolic substrates is a dynamic process that can occur multiple times during diapause.*
Insects in diapause characteristically feed very little or not at all, thus they are largely or totally dependent on energy reserves sequestered prior to the entry into diapause. Fats are the dominant reserve used during this period, but nonfat reserves are also important for some species, especially during certain phases of diapause. Metabolic depression, coupled with the low temperatures of winter, facilitates the economic utilization of reserves during the many months typical of most diapauses.
Diapausing and direct-developing insects both store metabolic reserves of the same three major groups of macronutrients: lipids, carbohydrates, and amino acids. The primary carbohydrate reserve in both diapausing and nondiapausing insects is the polysaccharide glycogen, while trehalose is the primary blood sugar. While some diapausing insects may have greater trehalose concentrations than their nondiapausing counterparts, this increase in blood sugar is generally thought to function in cold and desiccation resistance rather than directly in storage.
Some diapausing insects also have increased amino acid concentrations in the blood, but, like the sugars, these additional amino acids likely function primarily in cold and desiccation resistance. However, many diapausing insects do store amino acids in specialized proteins. These proteins, which can become very abundant, were initially termed diapause proteins, but later analysis revealed that the majority belong to the storage hexamerin family of insect proteins. Storage proteins are typically accumulated prior to diapause and their constituent amino acids may be used to supply both intermediary and respiratory maintenance metabolism during diapause and post-diapause functions such as resumption of development.*
Diapause is programmed by environmental cues well before the onset of arrest, and diapause-destined insects often alter their physiology during the prediapause preparatory period. Changes in body size and the accumulation of energetic reserves are among the most conspicuous alterations that occur during the prediapause period. Increased body mass is typically correlated with increased nutrient reserves, therefore larger body sizes in diapausing individuals are generally considered to be adaptive because of their greater reserves. Even though size differences may not be apparent, diapausing individuals of many species accumulate greater fat, glycogen, or protein reserves than nondiapause individuals as part of the diapause preparatory program.*
From an adaptive standpoint, it makes good functional sense for diapause-destined individuals to increase their reserves as a strategy to deal with the energetic demands of the diapause period. However, not all species show diapause-associated increases in reserves or body size. In some cases, diapausing individuals are even smaller than nondiapausing individuals.*
Florence N. Munyiri et al. compared how the initial larval weight of Psacothea hilaris larvae at the onset of starvation influences survival and the potential to enter diapause in three different instars. Their finding suggests the presence of a threshold weight (about 600 mg). P. hilaris larvae exceeding a threshold weight of 600 mg could enter diapause, while those weighing less either died or survived as quiescent larvae.*
Diapause is generally believed to entail costs that manifest themselves as decreased survival, rate of development, and/or reproduction after diapause completion.
Annemarie Kroon investigated such diapause costs in the spider mite Tetranychus urticae and found that most evident were the negative correlations between diapause duration and rate of oviposition, peak rate of oviposition, and total egg production. These phenotypic correlations suggest that there may be a trade-off between diapause duration and post-diapause reproduction.*
Diapause allows insects to cope with adverse weather conditions but also poses substantial fitness costs, for example, through reduced survival of the diapausing stage or sub-lethal effects in the following season.
Jacintha Ellers et al. evaluated the energetic costs of diapause in females of the parasitoid Asobara tabida Nees using experimental manipulation of diapause duration. They found that an increase in diapause length not only led to higher mortality among diapausing pupae, but also caused a significant decrease in egg load, fat reserves, and dry weight of the emerging adult females. Only larvae with sufficient resources were able to survive the entire diapause period, and there was a trade-off between the metabolic costs of diapause and adult fitness components. In contrast, the size (tibia length) of emerging females increased with increasing diapause duration.
In spite of its obvious advantages, diapause is a metabolically expensive life-history strategy. Costs of diapause are commonly reflected in lower post-diapause survival and reduced fecundity. The two most likely physiological mechanisms underlying these costs are damage due to diapause-associated stresses, such as desiccation or cold shock, and the depletion of metabolic reserves that could contribute to a decrease in post-diapause fitness.
* For further details, please refer to Deep Structure Studies 7: Experimental Reviews and References I.