Desiccation Tolerance
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Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or
drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, ...
-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or behavioral
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...
s to withstand these periods are necessary to ensure survival. In particular,
insects Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed ...
occupy a wide range of ecologically diverse niches and, so, exhibit a variety of strategies to avoid desiccation. In general, desiccation resistance in insects is measured by the change in mass during dry conditions. The overall mass difference between measurements before and after aridity exposure is attributed to body water loss, as respiratory water loss is generally considered negligible.


Desiccation and plants

Desiccation tolerant plants include ''
Craterostigma plantagineum ''Craterostigma plantagineum'', is a resurrection plant species in the genus '' Craterostigma''. It is a dwarf growing plant and can be found (in ideal conditions) to make a 'carpet' across the ground, with blooms in shades of blue and purple. It ...
'', ''Lindernia brevidens'' and '' Ramonda serbica''. Desiccation sensitive plants include members of ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model organ ...
'' genus, ''Lindernia subracemosa'', ''
Gossypium hirsutum ''Gossypium hirsutum'', also known as upland cotton or Mexican cotton, is the most widely planted species of cotton in the world. Globally, about 90% of all cotton production is of cultivars derived from this species. In the United States, the wo ...
'', '' Triticum aestivum'' and ''
Zea mays Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...
''.


Types of desiccation resistance

There are three main ways in which insects can increase their tolerance to desiccation: by increasing their total body water content; by reducing the rate of body water loss; and by tolerating a larger proportion of overall water loss from the body. Survival time is determined by initial water content, and can be calculated by dividing water loss tolerance (the maximum amount of water that may be removed without resulting in death) by water loss rate.


Increasing body water content

Insects with a higher initial body water content have better survival rates during arid conditions than insects with a lower initial body water content. Higher amounts of internal body water lengthen the time necessary to remove the amount of water required to kill the organism. The way in which body water content is increased may differ depending on the
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. The accumulation of
glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body. Glycogen functions as one of three regularly used forms ...
during the insect larval stage has been linked to increased body water content and is likely a source of metabolic water during dry conditions. Glycogen, a
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...
, acts as an oxidative energy source during times of physiological stress. Because it binds up to five times its weight in bulk water, insects with increased levels of body glycogen also have higher amounts of internal water. In general, insects selected for desiccation resistance also exhibit longer larval stages than those sensitive to desiccation. This increase in development time is likely a response to the environment, allowing larvae more time to accumulate glycogen, and therefore more water before eclosion. Another possible source contributing to higher levels of initial body water in insects is hemolymph volume. The insect equivalent to blood, hemolymph is the fluid found within the
hemocoel In vertebrates, the circulatory system is a organ system, system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of ...
, and is the largest pool of extracellular water within the insect body. In the fruit-fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'', flies selected for desiccation resistance also yielded higher amounts of hemolymph. Higher hemolymph volume is linked to an increase in
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
s, in particular trehalose, a common sugar found in many plants and animals with high desiccation resistance. ''Drosophila melanogaster'' flies selected for desiccation resistance show a 300% increase in hemolymph volume compared to control flies, correlating to a similar increase in trehalose levels. During periods of aridity, cells dehydrate and draw upon hemolymph stores to replenish intracellular water; therefore, insects with higher levels of this fluid are less prone to desiccation. Insects may also increase body water content by simply feeding more often. Because sugar is slowly absorbed into the hemolymph at each meal, increasing the frequency at which the insect ingests a sugar source also increases its desiccation tolerance. Furthermore, the
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, Fiber, fibre, or fuel. When plants of the same spe ...
may also act not only to store food prior to digestion but to provide an additional reservoir for water and sugar.


Reducing rate of water loss

Another strategy used to reduce the risk of death by dehydration is to reduce the rate at which water is lost. The three main ways through which insects can lose water are (1) the surface of the body (
integument In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, Exoskeleton, shell, germ or Peel (fruit), rind. Etymology The term is derived from ''integumentum'', which is Latin for "a coverin ...
); (2) the tracheae (respiration); and (3)
excretion Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, Kidney (vertebrates), kidneys, and skin. This is in contrast with secretion, where the substa ...
, or waste products. The important feature in reducing water loss in
land snail A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. ''Land snail'' is the common name for terrestrial molluscs, terrestrial gastropod mollusks that have gastropod shell, shel ...
s during inactivity is an epiphragm.Machin J. 1968
''The Permeability of the Epiphragm of Terrestrial Snails to Water Vapor''
Biological Bulletin, Vol. 134, No. 1 (Feb., 1968), pp. 87-95


Integument

The
exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
or integument of insects acts as an impermeable, protective layer against desiccation. It is composed of an outer
epicuticle Arthropods are covered with a tough, resilient integument, cuticle or exoskeleton of chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. T ...
, underlain by a
procuticle Arthropods are covered with a tough, resilient integument, cuticle or exoskeleton of chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. T ...
that itself may be further divided into an exo- and endocuticle. The endocuticle provides the insect with toughness and flexibility and the hard exocuticle serves to protect vulnerable body parts. However, the outer cuticular layer (epicuticle) is a protein-polyphenol complex made up of lipoproteins,
fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...
s, and waxy molecules, and is the insect's primary defense against water loss. Many insect orders secrete an additional cement layer over their wax layer, likely to protect against the abrasion or removal of waxy molecules. This layer is composed of lipids and proteins held together by
polyphenol Polyphenols () are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as ...
ic compounds and is secreted by the
dermal The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
glands. In general, the rate of water loss in insects is low at moderate temperatures. Once a species-specific critical temperature (Tc) is reached, as temperatures continue to increase, rapid water loss occurs. The “
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
melting model” is used to explain this sudden increase in the rate of water loss. The lipid melting model states that increased cuticular water loss is directly related to the melting of surface lipids. Insects already adapted to more arid environments have a higher Tc; that is, their cuticular properties change and lipid structures melt at a higher critical temperature. In some insects, the rate of cuticular water loss is controlled to some extent by the
neuroendocrine Neuroendocrine cells are cells that receive neuronal input (through neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release messenger molecules ( hormones) into the blood. In this way they b ...
system. Immediately following head removal, decapitated cockroaches exhibit a large increase in
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, c ...
across the cuticle, leading to severe dehydration. Injection of brain
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...
s into freshly separated bodies results in a sharp reduction in cuticular water loss.


Tracheae

In general, insects adapted to arid environments also have an impermeable cuticular membrane that prevents water loss. Therefore, a majority of water lost to the atmosphere occurs via the air-filled tracheae. To help reduce water loss, many insects have outer coverings to their tracheae, or spiracles, which shut when open respiration is unnecessary and prevent water from escaping. Insects at a greater risk for water loss face the challenge of either a depleted oxygen supply or desiccation, leading to an adaptive increase in tracheal volume in order to receive more oxygen.


Excretion

Following feeding, most insects retain enough water to completely hydrate their bodies, excreting the remainder. However, the amount of water excreted differs between species, and depends on the relative humidity and dryness of the environment. For example, Tsetse flies maintained at a high relative humidity, and thus non-arid conditions, excrete fecal matter with approximately 75% water content, whereas Tsetse flies maintained at a low relative humidity, and thus dry conditions, excrete fecal matter with only 35% water content. This adaptation helps minimize water loss in unfavorable conditions and increase chances of survival.


Tolerating greater water loss

Most insects can tolerate a 30-50% loss of body water; however, insects adapted to dry environments can tolerate a 40-60% loss of body water. Initial body size also plays a large role in how much water loss can be tolerated, and, in general, larger insects can tolerate a larger percentage of body water loss than smaller insects. The female beetle ''Alphitobius diaperinus'', for example, is larger than its male counterpart and can thus tolerate 4% more water loss. It is hypothesized that larger insects have increased lipid reserves, preventing dehydration and desiccation.


Behavior modification

In addition to physiological adaptations that increase desiccation resistance, behavioral responses of insects to arid environments significantly decrease dehydration potential. ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'' fruit flies, for example, will actively move to areas with higher atmospheric water content when placed in dry environments. Also, the dung beetle buries food in underground chambers, thereby ensuring water and energy sources during periodically dry conditions. Feeding location may also be altered to ensure body hydration. Some caterpillars preferentially feed on the underside of leaves, where microclimate has higher relative humidity. In a highly time-consuming activity such as feeding, these insects significantly reduce their chances of desiccation.


Cryptobiosis (Anhydrobiosis)

Cryptobiosis Cryptobiosis or anabiosis is a metabolic state in extremophilic organisms in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all measurable metabolic processes sto ...
refers to the state of an organism that has no detectable metabolic activity, resulting from extreme and unfavorable environmental conditions; anhydrobiosis refers to the state of surviving the loss of (almost) all body water. Although this state is commonly observed in invertebrates, only one insect is known to be cryptobiotic (anhydrobiotic), the African chironomid '' Polypedilum vanderplanki''. ''Polypedilum vanderplanki'' undergoes anhydrobiosis, a cryptobiotic state wherein the body is completely dehydrated. The larvae of ''P. vanderplanki'' inhabit rock pools that commonly dry out completely. In response, ''P. vanderplanki'' larvae enter an anhydrobiotic state, during which changes in body osmolarity trigger the production of large amounts of trehalose. Due to its capacity for water replacement and
vitrification Vitrification (, via French ') is the full or partial transformation of a substance into a glass, that is to say, a non- crystalline or amorphous solid. Glasses differ from liquids structurally and glasses possess a higher degree of connectivity ...
, the accumulation of trehalose prevents the death of the larvae from water loss.


References

{{Reflist, 2 Droughts Physiology