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In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions. [1] [2] It is a physiological state with very specific initiating and inhibiting conditions. The mechanism is a means of surviving predictable, unfavorable environmental conditions, such as temperature extremes, drought, or reduced food availability. Diapause is observed in all the life stages of arthropods, especially insects.
Activity levels of diapausing stages can vary considerably among species. Diapause may occur in a completely immobile stage, such as the pupae and eggs, or it may occur in very active stages that undergo extensive migrations, such as the adult monarch butterfly, Danaus plexippus. In cases where the insect remains active, feeding is reduced and reproductive development is slowed or halted.
Embryonic diapause, a somewhat similar phenomenon, occurs in over 130 species of mammals, possibly even in humans, [3] and in the embryos of many of the oviparous species of fish in the order Cyprinodontiformes. [4]
Diapause in insects is a dynamic process consisting of several distinct phases. While diapause varies considerably from one taxon of insects to another, these phases can be characterized by particular sets of metabolic processes and responsiveness of the insect to certain environmental stimuli. [5] For example, Sepsis cynipsea flies primarily use temperature to determine when to enter diapause. [6] Similarly, Chrysoperla plorabunda lacewings regulate their reproductive cycle using daylight length, with adults entering reproductive diapause when there are less than 12-13 hours of daylight. [7] Diapause can occur during any stage of development in arthropods, but each species exhibits diapause in specific phases of development. Reduced oxygen consumption is typical as is reduced movement and feeding. [8] In Polistes exclamans , a social wasp, only the queen is said to be able to undergo diapause. [9]
The sensitive stage is the period when stimulus must occur to trigger diapause in the organism. Examples of sensitive stage/diapause periods in various insects: [10]
Scientific name | Common name | Sensitive stage | Diapause |
Diatraea grandiosella | Southwestern corn borer | early larval | late larval [11] |
Sarcophaga crassipalpis | Flesh fly | early larval | pupa |
Sarcophaga argyrostoma | Flesh fly | mid to late larval | pupa |
Manduca sexta | Tobacco hornworm | late embryonic (egg) to late larval | pupa |
Leptinotarsa decemlineata | Colorado potato beetle | early adult | late adult |
Bombyx mori | Silkworm | late embryonic (egg) to early larval | embryonic |
Lymantria dispar | Spongy moth | late embryonic | late embryonic |
Danaus plexippus | Monarch butterfly | early adulthood | adulthood |
Acronicta rumicis | Knott grass moth | mid larval | mid larval |
Cydia pomonella | Codling moth | early to mid larval | mid larval [12] |
Gynaephora groenlandica | Arctic woolly bear moth | mid larval | mid larval [13] |
Cuterebra fontinella | Mouse botfly | mid larval | pupa [14] |
Nothobranchius furzeri | turquoise killifish | egg | egg |
The induction phase occurs at a genetically predetermined stage of life, and occurs well in advance of the environmental stress. [5] This sensitive stage may occur within the lifetime of the diapausing individual, or in preceding generations, particularly in egg diapause. [15] During this phase, insects are responsive to external cues called token stimuli, which trigger the switch from direct development pathways to diapause pathways. Token stimuli can consist of changes in photoperiod, thermoperiod, or allelochemicals from food plants. These stimuli are not in themselves favourable or unfavourable to development, but they herald an impending change in environmental conditions. [2]
The preparation phase usually follows the induction phase, though insects may go directly from induction to initiation without a preparation phase. [5] During this phase, insects accumulate and store molecules such as lipids, proteins, and carbohydrates. These molecules are used to maintain the insect throughout diapause and to provide fuel for development following diapause termination. Composition of the cuticle may be altered by changing hydrocarbon composition and by adding lipids to reduce water loss, making the organism resistant to desiccation. [16] Diapausing puparia of the flesh fly, Sarcophaga crassipalpis, increase the amount of cuticular hydrocarbons lining the puparium, effectively reducing the ability of water to cross the cuticle. [17]
Photoperiod is the most important stimulus initiating diapause. [10] The initiation phase begins when morphological development ceases. [5] In some cases, this change may be very distinct and can involve moulting into a specific diapause stage, or be accompanied by color change. Enzymatic changes may take place in preparation for cold hardening. For example, only diapausing adults of the fire bug, Pyrrhocoris apterus, have the enzymatic complement that allows them to accumulate polyhydric alcohols, molecules that help to lower their freezing points and thus avoid freezing. [18] Insects may also undergo behavioural changes and begin to aggregate, migrate, or search for suitable overwintering sites.
During the maintenance phase, insects experience lowered metabolism and developmental arrest is maintained. [5] Sensitivity to certain stimuli which act to prevent termination of diapause, such as photoperiod and temperature, is increased. At this stage, insects are unresponsive to changes in the environment that will eventually trigger the end of diapause, but they grow more sensitive to these stimuli as time progresses.
In insects that undergo obligate diapause, termination may occur spontaneously, without any external stimuli. [5] In facultative diapausers, token stimuli must occur to terminate diapause. These stimuli may include chilling, freezing, or contact with water, depending on the environmental conditions being avoided. These stimuli are important in preventing the insect from terminating diapause too soon, for instance in response to warm weather in late fall. In the Edith's checkerspot butterfly, individuals must receive enough sunlight in order to terminate the diapause stage and become a fully grown butterfly. [19] Termination may occur at the height of unfavourable conditions, such as in the middle of winter. Over time, depth of diapause slowly decreases until direct development can resume, if conditions are favourable. Termination can also occur in specific time frames linked to reproductive periods, such as in the beetle Colaphellus bowringi: diapause ends for spring-reproducing beetles between late February and early April and for autumn-reproducing beetles between mid August and early October. [20]
Diapause frequently ends prior to the end of unfavourable conditions and is followed by a state of quiescence from which the insect can arouse and begin direct development, should conditions change to become more favourable. [5] This allows the insect to continue to withstand harsh conditions while being ready to take advantage of good conditions as soon as possible.
Diapause in insects is regulated at several levels. Environmental stimuli interact with genetic pre-programming to affect neuronal signalling, endocrine pathways, and, eventually, metabolic and enzymatic changes.
Environmental regulators of diapause generally display a characteristic seasonal pattern. In temperate regions, photoperiod is the most reliable cues of seasonal change. [15] This informs entry into reproductive diapause for many northern insects, including the fruit fly Drosophila montana. [21] Depending on the season in which diapause occurs, either short or long days can act as token stimuli. Insects may also respond to changing day length as well as relative day length. Temperature may also act as a regulating factor, either by inducing diapause or, more commonly, by modifying the response of the insect to photoperiod. [15] Insects may respond to thermoperiod, the daily fluctuations of warm and cold that correspond with night and day, as well as to absolute or cumulative temperature. This has been observed in many moth species including the Indian mealmoth, where individuals diapause in different developmental stages due to environmental temperature. [22] Food availability and quality may also help regulate diapause. In the desert locust, Schistocerca gregaria, a plant hormone called gibberellin stimulates reproductive development. [23] During the dry season, when their food plants are in senescence and lacking gibberellin, the locusts remain immature and their reproductive tracts do not develop.
The neuroendocrine system of insects consists primarily of neurosecretory cells in the brain, the corpora cardiaca, corpora allata and the prothoracic glands. [2] There are several key hormones involved in the regulation of diapause: juvenile hormone (JH), diapause hormone (DH), and prothoracicotropic hormone (PTTH). [24]
Prothoracicotropic hormone stimulates the prothoracic glands to produce ecdysteroids that are required to promote development. [24] Larval and pupal diapauses are often regulated by an interruption of this connection, either by preventing release of prothoracicotropic hormone from the brain or by failure of the prothoracic glands to respond to prothoracicotropic hormone.
The corpora allata is responsible for the production of juvenile hormone (JH). In the bean bug, Riptortus pedestris, clusters of neurons on the protocerebrum called the pars lateralis maintain reproductive diapause by inhibiting JH production by the corpora allata. [25] Adult diapause is often associated with the absence of JH, while larval diapause is often associated with its presence.
In adults, absence of JH causes degeneration of flight muscles and atrophy or cessation of development of reproductive tissues, and halts mating behaviour. The presence of JH in larvae may prevent moulting to the next larval instar, though successive stationary moults may still occur. [26] In the corn borer, Diatraea gradiosella, JH is required for the accumulation by the fat body of a storage protein that is associated with diapause. [27]
Diapause hormone regulates embryonic diapause in the eggs of the silkworm moth, Bombyx mori. [28] DH is released from the subesophageal ganglion of the mother and triggers trehalase production by the ovaries. This generates high levels of glycogen in the eggs, which is converted into the polyhydric alcohols glycerol and sorbitol. Sorbitol directly inhibits the development of the embryos. Glycerol and sorbitol are reconverted into glycogen at the termination of diapause.
Diapause in the tropics is often initiated in response to biotic rather than abiotic components. [29] For example, food in the form of vertebrate carcasses may be more abundant following dry seasons, or oviposition sites in the form of fallen trees may be more available following rainy seasons. Also, diapause may serve to synchronize mating seasons or reduce competition, rather than to avoid unfavourable climatic conditions.
Diapause in the tropics poses several challenges to insects that are not faced in temperate zones. [29] Insects must reduce their metabolism without the aid of cold temperatures and may be faced with increased water loss due to high temperatures. While cold temperatures inhibit the growth of fungi and bacteria, diapausing tropical insects still have to deal with these pathogens. Also, predators and parasites may still be abundant during the diapause period.
Aggregations are common among diapausing tropical insects, especially in the orders Coleoptera, Lepidoptera, and Hemiptera. [29] Aggregations may be used as protection against predation, since aggregating species are frequently toxic and predators quickly learn to avoid them. They can also serve to reduce water loss, as seen in the fungus beetle, Stenotarsus rotundus, which forms aggregations of up to 70,000 individuals, which may be eight beetles deep. Relative humidity is increased within the aggregations and beetles experience less water loss, probably due to decreased surface area to volume ratios reducing evaporative water loss. [30]
Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insects, jellyfish, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").
Dormancy is a period in an organism's life cycle when growth, development, and physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions. Organisms can synchronize entry to a dormant phase with their environment through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase before the onset of adverse conditions. For example, photoperiod and decreasing temperature are used by many plants to predict the onset of winter. Consequential dormancy occurs when organisms enter a dormant phase after adverse conditions have arisen. This is commonly found in areas with an unpredictable climate. While very sudden changes in conditions may lead to a high mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources.
Helicoverpa zea, commonly known as the corn earworm, is a species in the family Noctuidae. The larva of the moth Helicoverpa zea is a major agricultural pest. Since it is polyphagous during the larval stage, the species has been given many different common names, including the cotton bollworm and the tomato fruitworm. It also consumes a wide variety of other crops.
Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and polyphenisms.
Embryonic diapause (delayed implantation in mammals) is a reproductive strategy used by a number of animal species across different biological classes. In more than 130 types of mammals where this takes place, the process occurs at the blastocyst stage of embryonic development, and is characterized by a dramatic reduction or complete cessation of mitotic activity, arresting most often in the G0 or G1 phase of division.
Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago. Holometabolism is a synapomorphic trait of all insects in the superorder Holometabola. Immature stages of holometabolous insects are very different from the mature stage. In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches. The morphology and behavior of each stage are adapted for different activities. For example, larval traits maximize feeding, growth, and development, while adult traits enable dispersal, mating, and egg laying. Some species of holometabolous insects protect and feed their offspring. Other insect developmental strategies include ametabolism and hemimetabolism.
Photoperiod is the change of day length around the seasons. The rotation of the earth around its axis produces 24 hour changes in light (day) and dark (night) cycles on earth. The length of the light and dark in each phase varies across the seasons due to the tilt of the earth around its axis. The photoperiod defines the length of the light, for example a summer day the length of light could be 16 hours while the dark is 8 hours, whereas a winter day the length of day could be 8 hours, whereas the dark is 16 hours. Importantly, the seasons are different in the northern hemisphere than the southern hemisphere.
The varied carpet beetle is a 3 mm-long beetle belonging to the family Dermestidae, positioned in subgenus Nathrenus. They are a common species, often considered a pest of domestic houses and, particularly, natural history museums, where the larvae may damage natural fibers and can damage carpets, furniture, clothing, and insect collections. A. verbasci was also the first insect to be shown to have an annual behavioral rhythm and to date remains a classic example of circannual cycles in animals.
Hippodamia convergens, commonly known as the convergent lady beetle, is one of the most common lady beetles in North America and is found throughout the continent. They tend to live a variety of habitats, including grasslands and forests.
The European corn borer, also known as the European corn worm or European high-flyer, is a moth of the family Crambidae. It is a pest of grain, particularly maize. The insect is native to Europe, originally infesting varieties of millet, including broom corn. The European corn borer was first reported in North America in 1917 in Massachusetts, but was probably introduced from Europe several years earlier. Since its initial discovery in the Americas, the insect has spread into Canada and westwards across the United States to the Rocky Mountains.
Agrotis ipsilon, the dark sword-grass, black cutworm, greasy cutworm, floodplain cutworm or ipsilon dart, is a small noctuid moth found worldwide. The moth gets its scientific name from black markings on its forewings shaped like the letter "Y" or the Greek letter upsilon. The larvae are known as "cutworms" because they cut plants and other crops. The larvae are serious agricultural pests and feed on nearly all varieties of vegetables and many important grains.
Insect physiology includes the physiology and biochemistry of insect organ systems.
Oncopeltus fasciatus, known as the large milkweed bug, is a medium-sized hemipteran of the family Lygaeidae. It is distributed throughout North America, from Central America through Mexico and the Caribbean to southern areas in Canada. Costa Rica represents this insect's southern limit. It inhabits disturbed areas, roadsides, and open pastures. Due to this widespread geographic distribution, this insect exhibits varying life history trade-offs depending on the population location, including differences in wing length and other traits based on location.
The southwestern corn borer, Diatraea grandiosella, is a moth belonging to the sub-order Heterocera. Like most moths, The southwestern corn borer undergoes complete metamorphosis developing as an egg, larva (caterpillar), pupa and adult. It is capable of entering diapause in its larva stage and under the conditions of a precise photoperiod. Growth and development are regulated by juvenile hormones. The southwestern corn borer has an extensive range. It occurs in Mexico and in Alabama, Arizona, Arkansas, Colorado, Illinois, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, New Mexico, Oklahoma, Tennessee, and Texas.
Sarcophaga crassipalpis is a species of flesh flies (insects in the family Sarcophagidae.
Bruchus rufimanus, commonly known as the broadbean weevil, broadbean beetle, or broadbean seed beetle is a leaf beetle which inhabits crops and fields, as well as some homes. It is a pest of faba beans. The adult beetles feed on pollen, while their larvae tunnel in seeds destroying crops and moving on to new ones once they dry out. The adult beetle, being one of the biggest of its genus, ranges from 3 to 5 mm in length.
In chronobiology, the circannual cycle is characterized by biological processes and behaviors recurring on an approximate annual basis, spanning a period of about one year. This term is particularly relevant in the analysis of seasonal environmental changes and their influence on the physiology, behavior, and life cycles of organisms. Adaptations observed in response to these circannual rhythms include fur color transformation, molting, migration, breeding, fattening and hibernation, all of which are inherently driven and synchronized with external environmental changes.
Stanley Dwight Beck was an American entomologist who specialized in insect physiology and host plant resistance. He became a university researcher after a stint in the navy during World War II. Paralyzed by polio and reliant on a wheelchair from 1952 he continued his research. He worked on insect photoperiodism and wrote two major books Animal Photoperiodism (1963) and Insect Photoperiodism (1968). Among his contributions was the successful development of artificial diets for lepidopteran larvae which enabled controlled laboratory experimentation and research on several pest species. After his retirement he published Two in the Game, a novel.
Chrysoperla plorabunda, also known as a green lacewing, is an insect belonging to the cryptic carnea complex of the genus. Species in the complex are nearly identical in morphology, with differences in substrate-borne vibrational songs being the only identifying factor. C. plorabunda has a widespread distribution across North America, ranging from coast to coast and from northern Canada down to Mexico. Within that range, they are typically found in open habitats such as grasslands and cultivated fields, as well as on the edges of deciduous forests and within coniferous forests. Adults feed predominately on nectar and honeydew, while larvae feed on other soft-bodied arthropods.
Colaphellus bowringi is a species of leaf beetle. It was first characterized by English entomologist Joseph Sugar Baly in 1865 and is primarily found in southeastern China, as well as other areas of Asia.
Annual aplocheiloid killifish embryos possess a rare ability among vertebrates to enter stages of developmental arrest (diapause) when subjected to adverse environmental conditions.